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PostgreSQL 7.1 Tutorial

1. zjH Qo Uuz gives the result SNO SNAME CITY Jones Paris 3 Adams Vienna 1 4 2 Data Definition There is a set of commands used for data definition included in the SQL language 1 4 2 1 Create Table The most fundamental command for data definition is the one that creates a new relation a new table The syntax of the CREATE TABLE command is CREATE TABLE table name name of attr 1 type of attr 1 name of attr 2 type of attr 2 ls 112 Example 1 11 Table Creation To create the tables defined in The Suppliers and Parts Database the following SQL statements are used CREATE TABLE SUPPLIER SNO INTEGER SNAME VARCHAR 20 CITY VARCHAR 20 CREATE TABLE PART PNO INTEGER PNAME VARCHAR 20 PRICE DECIMAL 4 2 CREATE TABLE SELLS SNO INTEGER PNO INTEGER 1 4 2 2 Data Types in SQL The following is a list of some data types that are supported by SQL INTEGER signed fullword binary integer 31 bits precision SMALLINT signed halfword binary integer 15 bits precision DECIMAL gl q signed packed decimal number of up to p digits with q digits to the right of the decimal point If y is omitted it is assumed to be 0 FLOAT signed doubleword floating point number CHAR n fixed length character string of length n VARC
2. S SNO S SNAME SE PNO 1 Smith 1 2 2 Jones 4 3 Adams I 3 4 Blake 2 3 4 In our example we got four groups and now we can apply the aggregate operator COUNT to every group leading to the final result of the query given above Note that for a query using GROUP BY and aggregate operators to make sense the target list can only refer directly to the attributes being grouped by Other attributes may only be used inside the argument of an aggregate function Otherwise there would not be a unique value to associate with the other attributes Also observe that it makes no sense to ask for an aggregate of an aggregate e g AVG MAX sno because a SELECT only does one pass of grouping and aggregation You can get a result of this kind by using a temporary table or a sub SELECT in the FROM clause to do the first level of aggregation 13 1 4 1 5 Having The HAVING clause works much like the WHERE clause and is used to consider only those groups satisfying the qualification given in the HAVING clause Essentially WHERE filters out unwanted input rows before grouping and aggregation are done whereas HAVING filters out unwanted group rows post GROUP Therefore WHERE cannot refer to the results of aggregate functions On the other hand there s no point in writing a HAVING condition that doesn t involve an aggregate function If your condition doesn t involve aggregates you might as well write it in WHERE and thereby avoid the computation
3. Yu and Chen 1995 A Yu and J Chen The POSTGRES Group Sept 5 1995 University of California Berkeley CA 38 Proceedings and Articles Partial indexing in POSTGRES research project Olson 1993 Nels Olson 1993 UCB Engin T7 49 1993 O676 University of California Berkeley CA A Unified Framework for Version Modeling Using Production Rules in a Database System Ong and Goh 1990 L Ong and J Goh April 1990 ERL Technical Memorandum M90 33 University of California Berkeley CA The Postgres Data Model Rowe and Stonebraker 1987 L Rowe and M Stonebraker Sept 1987 VLDB Conference Brighton England 1987 Generalized partial indexes http simon cs cornell edu home praveen papers partindex de95 ps Z Seshardri 1995 P Seshadri and A Swami March 1995 Eleventh International Conference on Data Engineering 1995 Cat No 95CH35724 IEEE Computer Society Press The Design of Postgres Stonebraker and Rowe 1986 M Stonebraker and L Rowe May 1986 Conference on Management of Data Washington DC ACM SIGMOD 1986 The Design of the Postgres Rules System Stonebraker Hanson Hong 1987 M Stonebraker E Hanson and C H Hong Feb 1987 Conference on Data Engineering Los Angeles CA IEEE 1987 The Postgres Storage System Stonebraker 1987 M Stonebraker Sept 1987 VLDB Conference Brighton England 1987 A Commentary on the Postgres Rules System Stonebraker et al 1989 M Stonebraker
4. 1 4 1 4 Aggregation by Groups SQL allows one to partition the tuples of a table into groups Then the aggregate operators described above can be applied to the groups i e the value of the aggregate operator is no longer calculated over all the values of the specified column but over all values of a group Thus the aggregate operator is evaluated separately for every group The partitioning of the tuples into groups is done by using the keywords GROUP BY followed by a list of attributes that define the groups If we have GROUP BY Aj Ax we partition the relation into groups such that two tuples are in the same group if and only if they agree on all the attributes Au Ax Example 1 6 Aggregates If we want to know how many parts are sold by every supplier we formulate the query SELECT S SNO S SNAME COUNT SE PNO FROM SUPPLIER S SELLS SE WHERE S SNO SE SNO GROUP BY S SNO S SNAME 12 and get SNO SNAME COUNT 1 Smith 2 2 Jones d 3 Adams 2 4 Blake 3 Now let s have a look of what is happening here First the join of the tables SUPPLIER and SELLS is derived S SNO S SNAME SE PNO Smith Smith Jones Adams Adams Blake Blake Blake Dob WB uN ap BwWwNY WP BNE Next we partition the tuples into groups by putting all tuples together that agree on both attributes S SNO and S SNAME
5. 2 INHERITS cities In this case a row of capitals inherits all columns name population and altitude from its parent cities The type of the column name is text a native Postgres type for variable length ASCII strings The type of the column population is real a type for single precision floating point numbers State capitals have an extra column state that shows their state In Postgres a table can inherit from zero or more other tables and a query can reference either all rows of a table or all rows of a tables plus all of its descendants Note The inheritance hierarchy is a directed acyclic graph For example the following query finds the names of all cities including state capitals that are located at an altitude over 500ft SELECT name altitude FROM cities WHERE altitude 500 which returns name altitude Las Vegas 2174 Mariposa 1 953 Madison 845 34 On the other hand the following query finds all the cities that are not state capitals and are situated at an altitude of 500ft or higher SELECT name altitude FROM ONLY cities WHERE altitude 500 name altitude Las Vegas 2174 Mariposa 1953 Here the ONLY before cities indicates that the query should be run over only the cities table and not tables below cities in the inheritance hierarchy Many of the commands that we have already discussed SE
6. Cam 25 The tables PART and SUPPLIER may be regarded as entities and SELLS may be regarded as a relationship between a particular part and a particular supplier As we will see later SQL operates on tables like the ones just defined but before that we will study the theory of the relational model 1 2 Relational Data Model Formalities The mathematical concept underlying the relational model is the set theoretic relation which is a subset of the Cartesian product of a list of domains This set theoretic relation gives the model its name do not confuse it with the relationship from the Entity Relationship model Formally a domain is simply a set of values For example the set of integers is a domain Also the set of character strings of length 20 and the real numbers are examples of domains The Cartesian product of domains D 5 p written Di x D2 x X Dx is the set of all k tuples Vi V2 Vx Such that vi Di v2 D2 Vk Dx For example when we have kz2 Di 0 1 and Dz a b c then D x D2 is 0 a 0 5 0 c 1 a 1 b 1 c A Relation is any subset of the Cartesian product of one or more domains R C D X D2 X X Dx For example 0 a 0 5 1 a is a relation it is in fact a subset of D x Dz mentioned above The members of a relation are called tuples Each relation of some Cartesian product D x D2 x X Dx is said to have arity k and is therefore a set of k tuples A relation ca
7. Z PNO y PNO z PNAME Screw Evaluating the query against the tables from The Suppliers and Parts Database again leads to the same result as in A Query Using Relational Algebra 1 3 4 Relational Algebra vs Relational Calculus The relational algebra and the relational calculus have the same expressive power i e all queries that can be formulated using relational algebra can also be formulated using the relational calculus and vice versa This was first proved by E F Codd in 1972 This proof is based on an algorithm Codd s reduction algorithm by which an arbitrary expression of the relational calculus can be reduced to a semantically equivalent expression of relational algebra For a more detailed discussion on that refer to Date 1994 and Ullman 1986 It is sometimes said that languages based on the relational calculus are higher level or more declarative than languages based on relational algebra because the algebra partially specifies the order of operations while the calculus leaves it to a compiler or interpreter to determine the most efficient order of evaluation 1 4 The SQL Language As is the case with most modern relational languages SQL is based on the tuple relational calculus As a result every query that can be formulated using the tuple relational calculus or equivalently relational algebra can also be formulated using SQL There are however capabilities beyond the scope of relational alge
8. mid size databases etc since this information is too inexact to be of use The output you got Please do not say that it didn t work or crashed If there is an error message show it even if you do not understand it If the program terminates with an operating system error say which If nothing at all happens say so Even if the result of your test case is a program crash or otherwise obvious it might not happen on our platform The easiest thing is to copy the output from the terminal if possible Note In case of fatal errors the error message provided by the client might not contain all the information available In that case also look at the log output of the database server If you do not keep your server output this would be a good time to start doing so xii The output you expected is very important to state If you just write This command gives me that output or This is not what I expected we might run it ourselves scan the output and think it looks okay and is exactly what we expected We should not have to spend the time to decode the exact semantics behind your commands Especially refrain from merely saying that This is not what SQL says Oracle does Digging out the correct behavior from SQL is not a fun undertaking nor do we all know how all the other relational databases out there behave If your problem is a program crash you can obviously omit this item Any command line options and other start up options
9. 24 3 3 1 Creating a Database Let s say you want to create a database named mydb You can do this with the following command createdb mydb If you do not have the privileges required to create a database you will see the following createdb mydb NOTICE user your username is not allowed to create destroy databases createdb database creation failed on mydb Postgres allows you to create any number of databases at a given site and you automatically become the database administrator of the database you just created Database names must have an alphabetic first character and are limited to 32 characters in length Not every user has authorization to become a database administrator If Postgres refuses to create databases for you then the site administrator needs to grant you permission to create databases Consult your site administrator if this occurs 3 3 2 Accessing a Database Once you have constructed a database you can access it by Running the Postgres terminal monitor programs e g psql which allows you to interactively enter edit and execute SQL commands Using an existing native frontend tool like pgaccess or ApplixWare via ODBC to create and manipulate a database Using a language like perl or tcl which has a supported interface for Postgres Some of these languages also have convenient and powerful GUI toolkits which can help you construct custom applications pgaccess mentioned above is one such applicat
10. IBM ORACLE Oracle Corp DG SQL Data General Corp and SYBASE Sybase Inc SQL is also an official standard now In 1982 the American National Standards Institute ANSI chartered its Database Committee X3H2 to develop a proposal for a standard relational language This proposal was ratified in 1986 and consisted essentially of the IBM dialect of SQL In 1987 this ANSI standard was also accepted as an international standard by the International Organization for Standardization ISO This original standard version of SQL is often referred to informally as SQL 86 In 1989 the original standard was extended and this new standard is often again informally referred to as SQL 89 Also in 1989 a related standard called Database Language Embedded SQL ESQL was developed The ISO and ANSI committees have been working for many years on the definition of a greatly expanded version of the original standard referred to informally as SQL2 or SQL 92 This version became a ratified standard International Standard ISO IEC 9075 1992 Database Language SQL in late 1992 SQL 92 is the version normally meant when people refer to the SQL standard A detailed description of SQL 92 is given in Date and Darwen 1997 At the time of writing this document a new standard informally referred to as SOL3 is under development It is planned to make SQL a Turing complete language i e all computable queries e g recursive queries will be possible This is
11. RI EFE I EA S MER CETERI e iii Tisto Figures cti ente sas aon Se acd Dine Re xebt e deer Sen iex iue i dex emus aute bres v Listo Examples 2 verbu heuer A A Le S aane are nee s vi Preface euentu hence aeui mae tema via edd Dre aeui eo do e ates aues vii 1 Whatis PostgreSOL comarca eer yr ema xe RIS e RR ERR Ra 9 vii 2 A Short History of Postgres o oooooooocrococrcrcrcrrrrrrrrrrms rs hr nn vii 2 1 The Berkeley Postgres Project cc cece cece eee cece cece eee cence Viii 2 2 ROSES IN erat a cin daa ast x ER RR RN ENIRO Re ease hd ROCA San Vues e oe uda viii 2 3 PostereSQL RA EE x RU US UR red Beal RT Cu NT ix 3 Documentation Resources cece cece cece ee ree cece hh n nnn ix 4 Terminology and Notation cece cece eee c eee c eee cere hh hh nnn xi 5 Bug Reporting Guidelines 0 cece cece cc ccc cece cere reece hn nnn xi 5 1 Identifying BUSS 000 ou ee E ex ER RR I UPC Ee ataca xi 5 2 What to report okelce exe cR eem xe alae ails dc ea mace xe e ecd xii 5 3 Where to report bugs cece cece cece cere eh hn n htt nn xiii 6 Y2K Statement 1 1 a Sao ooo diia DERE xiv Chapter LSO ter sitar tiere erected Medo TEE eda eso eir iere e 1 1 1 The Relational Data Model 0 ccc ccc cee eee cee ehh nnn 1 1 2 Relational Data Model Formalities 0 ccc cece ccc c ec cece reece ees 2 1 2 1 Domains vs Data Types ccc ccc cece cece reece Hh 3 1 3 Ope
12. USING sno JOIN part USING pno giving again sname pname Smith Screw Adams Screw Smith Nut Blake Nut Adams Bolt Blake Bolt Jones Cam Blake Cam 8 rows A joined table created using JOIN syntax is a table reference list item that occurs in a FROM clause and before any WHERE GROUP BY or HAVING clause Other table references including table names or other JOIN clauses may be included in the FROM clause if separated by commas JOINed tables are logically like any other table listed in the FROM clause SQL JOINs come in two main types CROSS JOINs unqualified joins and qualified JOINs Qualified joins can be further subdivided based on the way in which the join condition is specified ON USING or NATURAL and the way in which it is applied INNER or OUTER join Join Types CROSS JOIN 71 CROSS JOIN 72 A cross join takes two tables T1 and T2 having N and M rows respectively and returns a joined table containing all N M possible joined rows For each row R1 of T1 each row R2 of T2 is joined with R1 to yield a joined table row JR consisting of all fields in R1 and R2 A CROSS JOIN is equivalent to an INNER JOIN ON TRUE Qualified JOINs T1 NATURAL INNER LEFT RIGHT FULL OUTER JOIN 72 ON search conditionlUSING join column list A qualified JOIN must specify its join condition by providing one and only one of NATURAL ON or USING The ON clause takes a search condit
13. With the assignment of a data type we also have selected a domain for an attribute The domain of SNAME is the set of all character strings of length lt 20 the domain of SNO is the set of all integer numbers 1 3 Operations in the Relational Data Model In the previous section Relational Data Model Formalities we defined the mathematical notion of the relational model Now we know how the data can be stored using a relational data model but we do not know what to do with all these tables to retrieve something from the database yet For example somebody could ask for the names of all suppliers that sell the part Screw Therefore two rather different kinds of notations for expressing operations on relations have been defined The Relational Algebra which is an algebraic notation where queries are expressed by applying specialized operators to the relations The Relational Calculus which is a logical notation where queries are expressed by formulating some logical restrictions that the tuples in the answer must satisfy 1 3 1 Relational Algebra The Relational Algebra was introduced by E F Codd in 1972 It consists of a set of operations on relations SELECT 6 extracts tuples from a relation that satisfy a given restriction Let R be a table that contains an attribute A 6a R te R t A a where t denotes a tuple of R and t A denotes the value of attribute A of tuple t PROJECT 6 extracts
14. a database from some application that is written in the host language e g a ticket reservation system with a graphical user interface is written in C and the information about which tickets are still left is stored in a database that can be accessed using embedded SQL A program using embedded SQL in a host language consists of statements of the host language and of embedded SQL ESQL statements Every ESQL statement begins with the keywords EXEC SQL The ESQL statements are transformed to statements of the host language by a precompiler which usually inserts calls to library routines that perform the various SQL commands When we look at the examples throughout Select we realize that the result of the queries is very often a set of tuples Most host languages are not designed to operate on sets so we need a mechanism to access every single tuple of the set of tuples returned by a SELECT statement This mechanism can be provided by declaring a cursor After that we can use the FETCH command to retrieve a tuple and set the cursor to the next tuple For a detailed discussion on embedded SQL refer to Date and Darwen 1997 Date 1994 or Ullman 1968 21 Chapter 2 Architecture 2 1 Postgres Architectural Concepts Before we begin you should understand the basic Postgres system architecture Understanding how the parts of Postgres interact will make the next chapter somewhat clearer In database jargon Postgres uses a simple process
15. added a SQL language interpreter to Postgres Postgres95 was subsequently released to the Web to find its own way in the world as an open source descendant of the original Postgres Berkeley code Postgres95 code was completely ANSI C and trimmed in size by 25 Many internal changes improved performance and maintainability Postgres95 v1 0 x ran about 30 5096 faster on the Wisconsin Benchmark compared to Postgres v4 2 Apart from bug fixes these were the major enhancements The query language Postquel was replaced with SQL implemented in the server Subqueries were not supported until PostgreSQL see below but they could be imitated in Postgres95 with user defined SQL functions Aggregates were re implemented Support for the GROUP BY query clause was also added The 1ibpq interface remained available for C programs In addition to the monitor program a new program psql was provided for interactive SQL queries using GNU readline A new front end library libpgtcl supported Tcl based clients A sample shell pgtclsh provided new Tcl commands to interface tcl programs with the Postgres95 backend The large object interface was overhauled The Inversion large objects were the only mechanism for storing large objects The Inversion file system was removed viii The instance level rule system was removed Rules were still available as rewrite rules A short tutorial introducing regular SQL features as well as those of Postgres95 was
16. and Simon 1993 and Date and Darwen 1997 You should be aware that some language features are extensions to the standard 4 1 Interactive Monitor In the examples that follow we assume that you have created the mydb database as described in the previous subsection and have started psql Examples in this manual can also be found in source distribution in the directory src tutorial Refer to the README file in that directory for how to use them To start the tutorial do the following cd src tutorial psql s mydb Welcome to the POSTGRESQL interactive sql monitor Please read the file COPYRIGHT for copyright terms of POSTGRESOL type 1 for help on slash commands type Mq to quit type Mg or terminate with semicolon to execute query You are currently connected to the database postgres mydb Mi basics sql The Yi command read in queries from the specified files The s option puts you in single step mode which pauses before sending a query to the backend Queries in this section are in the file basics sql psql has a variety of d commands for showing system information Consult these commands for more details for a listing type V at the psql prompt 4 2 Concepts The fundamental notion in Postgres is that of a table which is a named collection of rows Each row has the same set of named columns and each column is of a specific type Furthermore each row has a permanent object identifier OID that is uniqu
17. can probably copy and paste them from the screen but all too often important details are left out because someone thought it does not matter or the report would be understood anyway The following items should be contained in every bug report The exact sequence of steps from program start up necessary to reproduce the problem This should be self contained it is not enough to send in a bare select statement without the preceding create table and insert statements if the output should depend on the data in the tables We do not have the time to reverse engineer your database schema and if we are supposed to make up our own data we would probably miss the problem The best format for a test case for query language related problems is a file that can be run through the psql frontend that shows the problem Be sure to not have anything in your psqlrc start up file An easy start at this file is to use pg dump to dump out the table declarations and data needed to set the scene then add the problem query You are encouraged to minimize the size of your example but this is not absolutely necessary If the bug is reproduceable we will find it either way If your application uses some other client interface such as PHP then please try to isolate the offending queries We will probably not set up a web server to reproduce your problem In any case remember to provide the exact input files do not guess that the problem happens for large files or
18. correct this by projecting out the duplicate column Example 1 2 An Inner Join Let s have a look at the tables that are produced by evaluating the steps necessary for a join Let the following two tables be given R S AIBIC CIDIE et 11213 3lalb 41516 6lcld 71819 First we calculate the Cartesian product R x s and get Rex Si A B R C Sul D E NN BBP BR co o OO M N 0 0 O0 O0 Q0 W DUO O OD W Qo vo ovo v Qo 00000 After the selection n c sc R x S we get To remove the duplicate column s c we project it out by the following operation OR A RBR CS D S E Or c s c R x S and get p DIVIDE Let R be a table with the attributes A B C and D and let s be a table with the attributes C and D Then we define the division as R S 2 t VtseSdt eR such that t A B tAt C D ts where t x y denotes a tuple of table R that consists only of the components x and y Note that the tuple t only consists of the components A and B of relation R Given the following tables R S A pam par L UJ Q iw Q 9 00 Oo w 90000 200000 OD FQ mh Fh O R S is derived as For a more detailed description and definition of the relational algebra refer to Ullman 1988 or Date 1994 Example 1 3 A Query Using Relational Algebra Recall that we formulated all those relational operators to be able to retrieve data from the da
19. groups and aggregates are computed Thus the WHERE clause may not contain aggregate functions it makes no sense to try to use an aggregate to determine which rows will be inputs to the aggregates On the other hand HAVING clauses always contain aggregate functions Strictly speaking you are allowed to write a HAVING clause that doesn t use aggregates but it s wasteful the same condition could be used more efficiently at the WHERE stage As an example we can find the highest low temperature reading anywhere with SELECT max temp lo FROM weather If we want to know what city or cities that reading occurred in we might try SELECT city FROM weather WHERE temp lo max temp 10 but this will not work since the aggregate max can t be used in WHERE However as is often the case the query can be restated to accomplish the intended result here by using a subselect SELECT city FROM weather WHERE temp lo SELECT max temp lo FROM weather This is OK because the sub select is an independent computation that computes its own aggregate separately from what s happening in the outer select Aggregates are also very useful in combination with GROUP BY clauses For example we can get 32 the maximum low temperature observed in each city with H SE ECT city max temp 10 FROM weather GROUP BY city which gives us one output row per city We can
20. of aggregates for groups that you re just going to throw away anyway Example 1 7 Having If we want only those suppliers selling more than one part we use the query SELECT S SNO S SNAME COUNT SE PNO FROM SUPPLIER S SELLS SE WHERE S SNO SE SNO GROUP BY S SNO S SNAME HAVING COUNT SE PNO 1 and get SNO SNAME 1 Smith 3 Adams 4 Blake 1 4 1 6 Subqueries In the WHERE and HAVING clauses the use of subqueries subselects is allowed in every place where a value is expected In this case the value must be derived by evaluating the subquery first The usage of subqueries extends the expressive power of SQL Example 1 8 Subselect If we want to know all parts having a greater price than the part named Screw we use the query SELECT FROM PART WHERE PRICE ECT PRICE FROM PART WHERE PNAME Screw v The result is PNO PNAME PRICE 3 Bolt 15 4 Cam 25 When we look at the above query we can see the keyword SELECT two times The first one at the beginning of the query we will refer to it as outer SELECT and the one in the WHERE clause 14 which begins a nested query we will refer to it as inner SELECT For every tuple of the outer SELECT the inner SELECT has to be evaluated After every evaluation we know the price of the tuple nam
21. per user client server model A Postgres session consists of the following cooperating Unix processes programs A supervisory daemon process postmaster the user s frontend application e g the psql program and the one or more backend database servers the postgres process itself A single postmaster manages a given collection of databases on a single host Such a collection of databases is called a cluster of databases Frontend applications that wish to access a given database within a cluster make calls to the library The library sends user requests over the network to the postmaster Figure 2 1 which in turn starts a new backend server process Figure 2 1 How a connection is established POSTMASTER SERVER server host and connects the frontend process to the new server From that point on the frontend process and the backend server communicate without intervention by the postmaster Hence the postmaster is always running waiting for requests whereas frontend and backend processes come and go The libpg library allows a single frontend to make multiple connections to backend processes However the frontend application is still a single threaded process Multithreaded frontend backend connections are not currently supported in 1ibpq One implication of this architecture is that the postmaster and the backend always run on the same machine the database server while the frontend application may run anywhere You sho
22. specified attributes columns from a relation Let R be a relation that contains an attribute X x R t X te R where t X denotes the value of attribute X of tuple t PRODUCT x builds the Cartesian product of two relations Let R be a table with arity k1 and let S be a table with arity k2 R x S is the set of all k k2 tuples whose first ki components form a tuple in R and whose last k components form a tuple in s UNION U builds the set theoretic union of two tables Given the tables R and s both must have the same arity the union R U S is the set of tuples that are in R or S or both INTERSECT 0 builds the set theoretic intersection of two tables Given the tables R and S R f S is the set of tuples that are in R and in s We again require that R and s have the same arity DIFFERENCE or builds the set difference of two tables Let R and S again be two tables with the same arity R S is the set of tuples in R but not in S JOIN TD connects two tables by their common attributes Let R be a table with the attributes A B and C and let S be a table with the attributes C D and E There is one attribute common to both relations the attribute c R S nARsEAcspsE nc sc R x S What are we doing here We first calculate the Cartesian product R x s Then we select those tuples whose values for the common attribute C are equal r c sc Now we have a table that contains the attribute C two times and we
23. temp hi columns of each WEATHER row to the temp lo and temp hi columns of all other WEATHER columns Note This is only a conceptual model The actual join may be performed in a more efficient manner but this is invisible to the user We can do this with the following query SELECT Wl city Wl temp lo AS low Wl temp hi AS high W2 city W2 temp lo AS low W2 temp hi AS high FROM weather W1 weather W2 WHERE Wl temp lo W2 temp lo AND Wl temp hi gt W2 temp hi city low high city low high San Francisco 43 57 San Francisco 46 50 San Francisco 37 54 San Francisco 46 50 Note The semantics of such a join are that the qualification is a truth expression defined for the Cartesian product of the tables indicated in the query For those rows in the Cartesian product for which the qualification is true Postgres computes and returns the values specified in the target list Postgres SQL does not assign any meaning to duplicate values in such expressions This means that Postgres sometimes recomputes the same target list several times this frequently happens when Boolean expressions are connected with an or To remove such duplicates you must use the SELECT DISTINCT statement In this case both w1 and w2 are surrogates for a row of the table weather and both range over all rows of the table In the terminology of most database systems w1 and w2 are known as range variables A query c
24. to accept valid input as defined in the documentation A program accepts invalid input without a notice or error message Keep in mind that your idea of invalid input might be our idea of an extension or compatibility with traditional practice PostgreSQL fails to compile build or install according to the instructions on supported platforms Here program refers to any executable not only the backend server Being slow or resource hogging is not necessarily a bug Read the documentation or ask on one of the mailing lists for help in tuning your applications Failing to comply to SQL is not a bug unless compliance for the specific feature is explicitly claimed Before you continue check on the TODO list and in the FAQ to see if your bug is already known If you cannot decode the information on the TODO list report your problem The least we can do is make the TODO list clearer 5 2 What to report The most important thing to remember about bug reporting is to state all the facts and only facts Do not speculate what you think went wrong what it seemed to do or which part of the program has a fault If you are not familiar with the implementation you would probably guess wrong and not help us a bit And even if you are educated explanations are a great supplement to but no substitute for facts If we are going to fix the bug we still have to see it happen for ourselves first Reporting the bare facts is relatively straightforward you
25. 9 language support and other modern features POSTGRES pioneered many of the object relational concepts now becoming available in some commercial databases Traditional relational database management systems RDBMS support a data model consisting of a collection of named relations containing attributes of a specific type In current commercial systems possible types include floating point numbers integers character strings money and dates It is commonly recognized that this model is inadequate for future data processing applications The relational model successfully replaced previous models in part because of its Spartan simplicity However as mentioned this simplicity often makes the implementation of certain applications very difficult Postgres offers substantial additional power by incorporating the following additional concepts in such a way that users can easily extend the system inheritance data types functions Other features provide additional power and flexibility constraints triggers rules transaction integrity These features put Postgres into the category of databases referred to as object relational Note that this is distinct from those referred to as object oriented which in general are not as well suited to supporting the traditional relational database languages So although Postgres has some object oriented features it is firmly in the relational database world In fact some commercial databases have rece
26. HAR n varying length character string of maximum length n 17 1 4 2 3 Create Index Indices are used to speed up access to a relation If a relation R has an index on attribute A then we can retrieve all tuples t having t A a in time roughly proportional to the number of such tuples t rather than in time proportional to the size of R To create an index in SQL the CREATE INDEX command is used The syntax is CREATE INDEX index name ON table name name of attribute Example 1 12 Create Index To create an index named I on attribute SNAME of relation SUPPLIER we use the following statement CREATE INDEX I ON SUPPLIER SNAME The created index is maintained automatically i e whenever a new tuple is inserted into the relation SUPPLIER the index I is adapted Note that the only changes a user can perceive when an index is present are increased speed for SELECT and decreases in speed of updates 1 4 2 4 Create View A view may be regarded as a virtual table i e a table that does not physically exist in the database but looks to the user as if it does By contrast when we talk of a base table there is really a physically stored counterpart of each row of the table somewhere in the physical storage Views do not have their own physically separate distinguishable stored data Instead the system stores the definition of the view i e the rules about how to access physically stored base tabl
27. LECT UPDATE and DELETE support this ONLY notation Deprecated In previous versions of Postgres the default was not to get access to child tables This was found to be error prone and is also in violation of SQL99 Under the old syntax to get the sub tables you append to the table name For example SELECT from cities You can still explicitly specify scanning child tables by appending as well as explicitly specify not scanning child tables by writing ONLY But beginning in version 7 1 the default behavior for an undecorated table name is to scan its child tables too whereas before the default was not to do so To get the old default behavior set the configuration option SQL Inheritance to off e g SET SQL Inheritance TO OFF or add a line in your postgresq1 conf file 5 2 Non Atomic Values One of the tenets of the relational model is that the columns of a table are atomic Postgres does not have this restriction columns can themselves contain sub values that can be accessed from the query language For example you can create columns that are arrays of base types 5 2 1 Arrays Postgres allows columns of a row to be defined as fixed length or variable length multi dimensional arrays Arrays of any base type or user defined type can be created To illustrate their use we first create a table with arrays of base types CREATE TABLE SAL EMP name text pay by quarter integer Schedule te
28. M Hearst and S Potamianos Sept 1989 Record 18 3 SIGMOD 1989 The case for partial indexes DBMS http s2k ftp CS Berkeley EDU 8000 postgres papers ERL M89 17 pdf Stonebraker M 1989b M Stonebraker Dec 1989 Record 18 no 4 4 11 SIGMOD 1989 The Implementation of Postgres Stonebraker Rowe Hirohama 1990 M Stonebraker L A Rowe and M Hirohama March 1990 Transactions on Knowledge and Data Engineering 2 1 IEEE On Rules Procedures Caching and Views in Database Systems Stonebraker et al ACM 1990 M Stonebraker and et al June 1990 Conference on Management of Data ACM SIGMOD 39
29. PART P SELLS SE WHERE S SNO SE SNO AND P PNO SE PNO GROUP BY S SNO subtable The subquery returns one row per supplier because of its GROUP BY and then we aggregate over those rows in the outer query 1 4 1 8 Union Intersect Except These operations calculate the union intersection and set theoretic difference of the tuples derived by two subqueries 15 Example 1 10 Union Intersect Except The following query is an example for UNION SELECT S SNO S SNAME S CITY FROM SUPPLIER S WHERE S SNAME Jones SELECT S SNO FROM SUPPLIER WHERE S SNA SNAME S CITY nw E Adams gives the result SNO SNAME Jones 3 Adams Here is an example for INTERSECT SELECT S SNO S SNAME S CITY FROM SUPPLIER S HERE S SNO gt 1 SECT ELECT S SNO S SNAM ROM SUPPLIER S HERE S SNO lt 3 INTE E y S CITY SmAanwes gives the result SNO SNAME CITY 2 Jones Paris The only tuple returned by both parts of the query is the one having SNO 2 Finally an example for EXCEPT SELECT S SNO S SNAME S CITY FROM SUPPLIER S HERE S SNO gt 1 ECT S SNO S SNAM ROM SUPPLIER S HERE S SNO gt 3 El y S CITY
30. PostgreSQL 7 1 Tutorial The PostgreSQL Global Development Group PostgreSQL 7 1 Tutorial by The PostgreSQL Global Development Group Copyright O 1996 2001 by PostgreSQL Global Development Group Legal Notice PostgreSQL is Copyright O 1996 2001 by the PostgreSQL Global Development Group and is distributed under the terms of the license of the University of California below Postgres95 is Copyright O 1994 5 by the Regents of the University of California Permission to use copy modify and distribute this software and its documentation for any purpose without fee and without a written agreement is hereby granted provided that the above copyright notice and this paragraph and the following two paragraphs appear in all copies IN NO EVENT SHALL THE UNIVERSITY OF CALIFORNIA BE LIABLE TO ANY PARTY FOR DIRECT INDIRECT SPECIAL INCIDENTAL OR CONSEQUENTIAL DAMAGES INCLUDING LOST PROFITS ARISING OUT OF THE USE OF THIS SOFTWARE AND ITS DOCUMENTATION EVEN IF THE UNIVERSITY OF CALIFORNIA HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGE THE UNIVERSITY OF CALIFORNIA SPECIFICALLY DISCLAIMS ANY WARRANTIES INCLUDING BUT NOT LIMITED TO THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE THE SOFTWARE PROVIDED HEREUNDER IS ON AN AS IS BASIS AND THE UNIVERSITY OF CALIFORNIA HAS NO OBLIGATIONS TO PROVIDE MAINTAINANCE SUPPORT UPDATES ENHANCEMENTS OR MODIFICATIONS Table of Contents Table of Contents A i REL TR
31. a very complex task and therefore the completion of the new standard can not be expected before 1999 1 1 The Relational Data Model As mentioned before SQL is a relational language That means it is based on the relational data model first published by E F Codd in 1970 We will give a formal description of the relational model later in Relational Data Model Formalities but first we want to have a look at it from a more intuitive point of view A relational database is a database that is perceived by its users as a collection of tables and nothing else but tables A table consists of rows and columns where each row represents a record and each column represents an attribute of the records contained in the table The Suppliers and Parts Database shows an example of a database consisting of three tables SUPPLIER is a table storing the number SNO the name SNAME and the city CITY of a supplier PART is a table storing the number PNO the name PNAME and the price PRICE of a part SELLS stores information about which part PNO is sold by which supplier SNO It serves in a sense to connect the other two tables together Example 1 1 The Suppliers and Parts Database SUPPLIER SELLS SNO SNAME CITY SNO PNO T Smith London 1 1 2 Jones Paris ali 2 3 Adams Vienna 2 4 4 Blake Rome 3 1 3 3 4 2 PART 4 3 PNO PNAME PRICE 4 4 al Screw 10 2 Nut 8 3 Bolt 15 4
32. about compilers make etc is also necessary Do not be afraid if your bug report becomes rather lengthy That is a fact of life It is better to report everything the first time than us having to squeeze the facts out of you On the other hand if your input files are huge it is fair to ask first whether somebody is interested in looking into it Do not spend all your time to figure out which changes in the input make the problem go away This will probably not help solving it If it turns out that the bug cannot be fixed right away you will still have time to find and share your work around Also once again do not waste your time guessing why the bug exists We will find that out soon enough When writing a bug report please choose non confusing terminology The software package as such is called PostgreSQL sometimes Postgres for short Sometimes the abbreviation Pgsql is used but don t do that When you are specifically talking about the backend server mention that do not just say Postgres crashes The interactive frontend is called psql and is for all intends and purposes completely separate from the backend 5 3 Where to report bugs In general send bug reports to the bug report mailing list at lt pgsql bugs postgresql org gt You are invited to find a descriptive subject for your email message perhaps parts of the error message Do not send bug reports to any of the user mailing lists such as lt pgsql sql postgresg
33. an contain an arbitrary number of table names and surrogates 4 8 Updates You can update existing rows using the UPDATE command Suppose you discover the temperature readings are all off by 2 degrees as of Nov 28 you may update the data as follow UPDATE weather SET temp hi temp hi 25 temp lo temp lo 2 WHERE date 1994 11 28 3l 4 9 Deletions Deletions are performed using the DELETE command DELETE FROM weather WHERE city Hayward All weather recording belonging to Hayward are removed One should be wary of queries of the form DELETE FROM tablename Without a qualification DELETE will simply remove all rows from the given table leaving it empty The system will not request confirmation before doing this 4 10 Using Aggregate Functions Like most other relational database products PostgreSQL supports aggregate functions An aggregate function computes a single result from multiple input rows For example there are aggregates to compute the count sum avg average max maximum and min minimum over a set of rows It is important to understand the interaction between aggregates and SQL s WHERE and HAVING clauses The fundamental difference between WHERE and HAVING is this WHERE selects input rows before groups and aggregates are computed thus it controls which rows go into the aggregate computation whereas HAVING selects group rows after
34. bra or calculus Here is a list of some additional features provided by SQL that are not part of relational algebra or calculus Commands for insertion deletion or modification of data Arithmetic capability In SQL it is possible to involve arithmetic operations as well as comparisons e g A lt B 3 Note that or other arithmetic operators appear neither in relational algebra nor in relational calculus Assignment and Print Commands It is possible to print a relation constructed by a query and to assign a computed relation to a relation name Aggregate Functions Operations such as average sum max etc can be applied to columns of a relation to obtain a single quantity 1 4 1 Select The most often used command in SQL is the SELECT statement used to retrieve data The syntax is SELECT ALL DISTINCT ON expression 1 expression AS output_name INTO TEMPORARY TEMP TABLE new table FROM from item WHERE condition GROUP BY expression HAVING condition UNION INTERSECT EXCEPT ALL select ORDER BY expression ASC DESC USING operator FOR UPDATE OF class_name LIMIT count ALL OFFSET start Now we will illustrate the complex syntax of the SELECT statement with various examples The tables used for the examples are defined in The Suppliers and Parts Database 1 4 1 1 Simple Selects Her
35. d Alan R Simon 1 55860 245 3 1993 Morgan Kaufmann 1993 Abstract Accessible reference for SQL features Principles of Database and Knowledge Base Systems Ullman 1988 Jeffrey D Ullman 1 Computer Science Press 1988 PostgreSQL Specific Documentation The PostgreSQL Administrator s Guide The Administrator s Guide Edited by Thomas Lockhart 2000 05 01 The PostgreSQL Global Development Group The PostgreSQL Developer s Guide The Developer s Guide Edited by Thomas Lockhart 2000 05 01 The PostgreSQL Global Development Group The PostgreSQL Programmer s Guide The Programmer s Guide Edited by Thomas Lockhart 2000 05 01 The PostgreSQL Global Development Group The PostgreSQL Tutorial Introduction The Tutorial Edited by Thomas Lockhart 2000 05 01 The PostgreSQL Global Development Group The PostgreSQL User s Guide The User s Guide Edited by Thomas Lockhart 2000 05 01 The PostgreSQL Global Development Group Enhancement of the ANSI SOL Implementation of PostgreSQL Simkovics 1998 Stefan Simkovics O Univ Prof Dr Georg Gottlob November 29 1998 Department of Information Systems Vienna University of Technology Discusses SQL history and syntax and describes the addition of INTERSECT and EXCEPT constructs into Postgres Prepared as a Master s Thesis with the support of O Univ Prof Dr Georg Gottlob and Univ Ass Mag Katrin Seyr at Vienna University of Technology The Postgres95 User Manual
36. distributed with the source code GNU make instead of BSD make was used for the build Also Postgres95 could be compiled with an unpatched gcc data alignment of doubles was fixed 2 3 PostgreSQL By 1996 it became clear that the name Postgres95 would not stand the test of time We chose a new name PostgreSQL to reflect the relationship between the original Postgres and the more recent versions with SQL capability At the same time we set the version numbering to start at 6 0 putting the numbers back into the sequence originally begun by the Postgres Project The emphasis during development of Postgres95 was on identifying and understanding existing problems in the backend code With PostgreSQL the emphasis has shifted to augmenting features and capabilities although work continues in all areas Major enhancements in PostgreSQL include Table level locking has been replaced with multi version concurrency control which allows readers to continue reading consistent data during writer activity and enables hot backups from pg dump while the database stays available for queries Important backend features including subselects defaults constraints and triggers have been implemented Additional SQL92 compliant language features have been added including primary keys quoted identifiers literal string type coercion type casting and binary and hexadecimal integer input Built in types have been improved including new wide
37. e Drop Index Drop View To destroy a table including all tuples stored in that table the DROP TABLE command is used DROP TABLE table name To destroy the SUPPLIER table use the following statement DROP TABLE SUPPLIER The DROP INDEX command is used to destroy an index DROP INDEX index name Finally to destroy a given view use the command DROP VIEW DROP VIEW view name 19 1 4 3 Data Manipulation 1 4 3 1 Insert Into Once a table is created see Create Table it can be filled with tuples using the command INSERT INTO The syntax is INSERT INTO table name name of attr 1 name of attr 2 VALUES val attr 1 val attr 2 1 To insert the first tuple into the relation SUPPLIER from The Suppliers and Parts Database we use the following statement INSERT INTO SUPPLIER SNO SNAME CITY VALUES 1 Smith London To insert the first tuple into the relation SELLS we use INSERT INTO SELLS SNO PNO VALUES 1 1 1 4 3 2 Update To change one or more attribute values of tuples in a relation the UPDATE command is used The syntax is UPDATE table name SET name of attr 1 value 1 name of attr k value kj WHERE condition To change the value of attribute PRICE of the part Screw in the relation PART we use UPDATE PART SET PRICE 15 WHERE PNAME Screw The new value
38. e no fee software xiv Chapter 1 SQL This chapter introduces the mathematical concepts behind relational databases It is not required reading so if you bog down or want to get straight to some simple examples feel free to jump ahead to the next chapter and come back when you have more time and patience This stuff is supposed to be fun This material originally appeared as a part of Stefan Simkovics Master s Thesis Simkovics 1998 SQL has become the most popular relational query language The name SQL is an abbreviation for Structured Query Language In 1974 Donald Chamberlin and others defined the language SEQUEL Structured English Query Language at IBM Research This language was first implemented in an IBM prototype called SEQUEL XRM in 1974 75 In 1976 77 a revised version of SEQUEL called SEQUEL 2 was defined and the name was changed to SQL subsequently A new prototype called System R was developed by IBM in 1977 System R implemented a large subset of SEQUEL 2 now SQL and a number of changes were made to SQL during the project System R was installed in a number of user sites both internal IBM sites and also some selected customer sites Thanks to the success and acceptance of System R at those user sites IBM started to develop commercial products that implemented the SQL language based on the System R technology Over the next years IBM and also a number of other vendors announced SQL products such as SQL DS IBM DB2
39. e are some simple examples using a SELECT statement Example 1 4 Simple Query with Qualification To retrieve all tuples from table PART where the attribute PRICE is greater than 10 we formulate the following query SELECT FROM PART WHERE PRICE gt 10 and get the table PNO PNAME PRICE E per eec pl human 3 Bolt 15 4 Cam 25 Using in the SELECT statement will deliver all attributes from the table If we want to retrieve only the attributes PNAME and PRICE from table PART we use the statement SELECT PNAME PRICE FROM PART WHERE PRICE gt 10 In this case the result is PNAME PRICE LS Pesos ows Bolt 15 Cam 25 Note that the SQL SELECT corresponds to the projection in relational algebra not to the selection see Relational Algebra for more details The qualifications in the WHERE clause can also be logically connected using the keywords OR AND and NOT SELECT PNAME PRICE FROM PART WHERE PNAME Bolt AND PRICE 0 OR PRICE lt 15 will lead to the result PNAME PRICE Stet 228 a eee Bolt 15 Arithmetic operations may be used in the target list and in the WHERE clause For example if we want to know how much it would cost if we take two pieces of a part we could use the following query SELECT PNAME PRICE 2 AS DOUBLE FROM PART WHERE PRICE 2 lt 50 and we get PNAME DOUBLE sio cio Screw 20 Nut 16 Bolt 30 Note that the word DOUBLE after the key
40. e commands in the Postgres query language Postgres SQL 3 1 Setting Up Your Environment This section discusses how to set up your own environment so that you can use frontend applications We assume Postgres has already been successfully installed and started refer to the Administrator s Guide and the installation notes for how to install Postgres Postgres is a client server application As a user you only need access to the client portions of the installation an example of a client application is the interactive monitor psql For simplicity we will assume that Postgres has been installed in the directory usr local pgsql Therefore wherever you see the directory usr local pgsql you should substitute the name of the directory where Postgres is actually installed All Postgres commands are installed in the directory usr local pgsql bin Therefore you should add this directory to your shell command path If you use a variant of the Berkeley C shell such as csh or tcsh you would add set path usr local pgsql bin path in the login file in your home directory If you use a variant of the Bourne shell such as sh ksh or bash then you would add PATH usr local pgsql bin PATH export PATH oe oe to the profile file in your home directory From now on we will assume that you have added the Postgres bin directory to your path In addition we will make frequent reference to setting a shell variable or setting an environmen
41. e throughout the database cluster Historially tables have been called classes in Postgres rows are object instances and columns are attributes This makes sense if you consider the object relational aspects of the database system but in this manual we will use the customary SQL terminology As previously discussed tables are grouped into databases and a collection of databases managed by a single postmaster process constitutes a database cluster 28 4 3 Creating a New Table You can create a new table by specifying the table name along with all column names and their types CREATE TABLE weather city varchar 80 temp lo int low temperatur temp hi int high temperature prcp real precipitation date date Note that both keywords and identifiers are case insensitive identifiers can preserve case by surrounding them with double quotes as allowed by SQL92 Postgres SQL supports the usual SQL types int float real smallint char N varchar N date time and timestamp as well as other types of general utility and a rich set of geometric types As we will see later Postgres can be customized with an arbitrary number of user defined data types Consequently type names are not syntactical keywords except where required to support special cases in the SQL92 standard So far the Postgres CREATE command looks exactly like the command used to create a table in a traditional relational system However we will p
42. ed Screw and we can check if the price of the actual tuple is greater Actually in this example the inner query need only be evaluated once since it does not depend on the state of the outer query If we want to know all suppliers that do not sell any part e g to be able to remove these suppliers from the database we use SELECT FROM SUPPLIER S WHERE NOT EXISTS SELECT FROM SELLS SE WHERE SE SNO S SNO In our example the result will be empty because every supplier sells at least one part Note that we use S SNO from the outer SELECT within the WHERE clause of the inner SELECT Here the subquery must be evaluated afresh for each tuple from the outer query i e the value for S SNO is always taken from the current tuple of the outer SELECT 1 4 1 7 Subqueries in FROM A somewhat different way of using subqueries is to put them in the FROM clause This is a useful feature because a subquery of this kind can output multiple columns and rows whereas a subquery used in an expression must deliver just a single result It also lets us get more than one round of grouping aggregation without resorting to a temporary table Example 1 9 Subselect in FROM If we want to know the highest average part price among all our suppliers we can t write MAX AVG PRICE but we can write SELECT MAX subtable avgprice FROM SELECT AVG P PRICE AS avgprice FROM SUPPLIER S
43. eediiigs and Articles 426 eser hk vale aa a ae o RR e e Rn 39 iv List of Figures 2 1 How a connection is established List of Examples 1 1 The Suppliers and Parts Database eese nono nono nooo nooo nora enne 2 1 3 A Query Using Relational Algebra esee eene een eene eene nenne 5 1 4 Simple Query with Qualification oocnocnnonnnonocononononononononnnonnnnnnnnnnnnnnc nono rennen rene enne tene nenne 8 1 5 Aggregates 2i ste See RH be REED OP ee RE PUER ERES 12 1 6 AP Bre Sates cio gU Une E a E UI OR Dire reri RE eh 12 IH IUE n 14 8 Subselect iii RU e EE epe RI npe Data 14 1 9 Subs lect inm FROM teg eR D pe eee petere iia 15 1 10 Union Intersect BXCept stated odon rien RR ORE EAR EHE 16 ETI Table Creation 17 1712 Create ld eut ertet REOR URINE DE ERUNT 18 vi Preface 1 What is PostgreSQL PostgreSQL is an object relational database management system ORDBMS based on POSTGRES Version 4 2 http s2k ftp CS Berkeley EDU 8000 postgres postgres html developed at the University of California at Berkeley Computer Science Department The POSTGRES project led by Professor Michael Stonebraker was sponsored by the Defense Advanced Research Projects Agency DARPA the Army Research Office ARO the National Science Foundation NSF and ESL Inc PostgreSQL is an open source descendant of this original Berkeley code It provides SQL92 SQL9
44. es in order to materialize the view somewhere in the system catalogs see System Catalogs For a discussion on different techniques to implement views refer to SIM98 In SQL the CREATE VIEW command is used to define a view The syntax is CREATE VIEW view name AS select stmt where select stmt is a valid select statement as defined in Select Note that select stmt is not executed when the view is created It is just stored in the system catalogs and is executed whenever a query against the view is made 18 Let the following view definition be given we use the tables from The Suppliers and Parts Database again CREATE VIEW London Suppliers AS SELECT S SNAME P PNAME FROM SUPPLIER S PART P SELLS SE WHERE S SNO SE SNO AND P PNO SE PNO AND S CITY London Now we can use this virtual relation London Suppliers as if it were another base table SELECT FROM London Suppliers WHERE PNAME Screw which will return the following table SNAME PNAME Screw Smith To calculate this result the database system has to do a hidden access to the base tables SUPPLIER SELLS and PART first It does so by executing the query given in the view definition against those base tables After that the additional qualifications given in the query against the view can be applied to obtain the resulting table 1 4 2 5 Drop Tabl
45. f an array subarrays by specifying both lower and upper bounds for each subscript This query retrieves the first item on Bill s schedule for the first two days of the week 36 SELECT SAL EMP schedule 1 2 1 1 FROM SAL EMP WHERE SAL EMP name Bill Schedule meeting n 5 3 More Advanced Features Postgres has many features not touched upon in this tutorial introduction which has been oriented toward newer users of SQL These are discussed in more detail in both the User s and Programmer s Guides 37 Bibliography Selected references and readings for SOL and Postgres Some white papers and technical reports from the original Postgres development team are available at the University of California Berkeley Computer Science Department web site http s2k ftp CS Berkeley EDU 8000 postgres papers SQL Reference Books The Practical SOL Handbook Bowman et al 1996 Using Structured Query Language 3 Judith Bowman Sandra Emerson and Marcy Darnovsky 0 201 44787 8 1996 Addison Wesley 1996 A Guide to the SOL Standard Date and Darwen 1997 A user s guide to the standard database language SQL 4 C J Date and Hugh Darwen 0 201 96426 0 1997 Addison Wesley 1997 An Introduction to Database Systems Date 1994 6 C J Date 1 1994 Addison Wesley 1994 Understanding the New SQL Melton and Simon 1993 A complete guide Jim Melton an
46. filter these grouped rows using HAVING SE E ECT city max temp_lo FROM weather GROUP BY city HAVING min temp_lo lt 0 which gives us the same results for only the cities that have some below zero readings Finally if we only care about cities whose names begin with P we might do SE r4 ECT city max temp 10 FROM weather WHERE city like P GROUP BY city HAVING min temp 1o lt 0 Note that we can apply the city name restriction in WHERE since it needs no aggregate This is more efficient than adding the restriction to HAVING because we avoid doing the grouping and aggregate calculations for all rows that fail the WHERE check 33 Chapter 5 Advanced Postgres SQL Features Having covered the basics of using Postgres SQL to access your data we will now discuss those features of Postgres that distinguish it from conventional data managers These features include inheritance time travel and non atomic data values array and set valued attributes Examples in this section can also be found in advance sq1 in the tutorial directory Refer to Chapter 4 for how to use it 5 1 Inheritance Let s create two tables The capitals table contains state capitals that are also cities Naturally the capitals table should inherit from cities CREATE TABLE cities name text population real altitude int cw FE My CREATE TABLE capitals state char
47. including concerned environment variables or configuration files that you changed from the default Again be exact If you are using a pre packaged distribution that starts the database server at boot time you should try to find out how that is done Anything you did at all differently from the installation instructions The PostgreSQL version You can run the command SELECT version to find out the version of the server you are connected to Most executable programs also support a version option at least postmaster version and psgl version should work If the function or the options do not exist then your version is probably old enough You can also look into the README file in the source directory or at the name of your distribution file or package name If you run a pre packaged version such as RPMs say so including any subversion the package may have If you are talking about a CVS snapshot mention that including its date and time If your version is older than 7 1 we will almost certainly tell you to upgrade There are tons of bug fixes in each new release that is why we make new releases Platform information This includes the kernel name and version C library processor memory information In most cases it is sufficient to report the vendor and version but do not assume everyone knows what exactly Debian contains or that everyone runs on Pentiums If you have installation problems then information
48. ing a Database 0 ccc ccc ccc cece rece eee nnt nn 26 Chapter 4 The Query Language ccc ccc ccc cece cere hh n n n nnn 28 4 1 Interactive Monitor 0 cc ccc ccc ce cece hehe hh hh hh scene 28 A DAC ONCE Pt sie one ic a ores RS eite eeu ere I aM 28 4 3 Creating a New Tables cos soseo ccc ccc ccc cc cece ehh hh hr eens 29 4 4 Populating a Table with Rows ccc cece cece cece cece cece eh hh nn 29 4 5 Querying a Table cinco ainda eger y rea tte sre eR RO Shea Dials Vue e acetone 29 4 6 Redirecting SELECT Queries o oooooocoocnocnoroocrocrornnorcorccnoo o 30 4 7 Joins Between Tables o oooooooocoocroocroccrrrraccoccocco hh nnn 31 AS Updates iile see eere Nt xe ca lee D res e eU te Rec at wae 31 4 9 Deletions iure bee et Dien dei benedi dae ele qmi Led ee 32 4 10 Using Aggregate Functions o ooo ooocoococmoroocroorrrnnorrommo hn 32 Chapter 5 Advanced Postgres SQL Features 0 cc cee cece cece cece cence mo 34 iii 5 1 Inheritance 52 voee a eee Rees Ved Siete 1208 Fa ree Os VR Rede de a voe Rr ce dte 34 5 2 Non Atomic Val S e e s es hh eo m aii e i a e e a Rare SI a SR Ter a LS 35 ABE SV CIC LP 35 5 3 More Advanced Features soies mea ia e cece reece cece E eee e eee scenes 37 hjlitrucidaer cnr 38 SQL Reference Books oll E ree a S ao e a Roda aan 38 PostgreSQL Specific Documentation 0 cece cece cee eee cece cere eee nn 38 Proc
49. ion which is the same as in a WHERE clause The USING clause takes a comma separated list of column names which the joined tables must have in common and joins the tables on equality of those columns NATURAL is shorthand for a USING clause that lists all the common column names of the two tables A side effect of both USING and NATURAL is that only one copy of each joined column is emitted into the result table compare the relational algebra definition of JOIN shown earlier 10 INNER JOIN For each row R1 of T1 the joined table has a row for each row in T2 that satisfies the join condition with R1 Tip The words INNER and OUTER are optional for all JOINs INNER is the default LEFT RIGHT and FULL imply an OUTER JOIN LEFT OUTER JOIN First an INNER JOIN is performed Then for each row in T1 that does not satisfy the join condition with any row in T2 an additional joined row is returned with null fields in the columns from T2 Tip The joined table unconditionally has a row for each row in T1 RIGHT OUTER JOIN First an INNER JOIN is performed Then for each row in T2 that does not satisfy the join condition with any row in T1 an additional joined row is returned with null fields in the columns from T1 Tip The joined table unconditionally has a row for each row in T2 FULL OUTER JOIN First an INNER JOIN is performed Then for each row in T1 that does not satisfy the join condition with any row i
50. ion written in tk tcl and can be used as an example Writing a C program using the LIBPQ subroutine library This allows you to submit SQL commands from C and get answers and status messages back to your program This interface is discussed further in The PostgreSQL Programmer s Guide You might want to start up psql to try out the examples in this manual It can be activated for the mydb database by typing the command o psql mydb 25 You will be greeted with the following message Welcome to the POSTGRESQL interactive sql monitor Please read the file COPYRIGHT for copyright terms of POSTGRESOL type M for help on slash commands type Mq to quit type Mg or terminate with semicolon to execute query You are currently connected to the database templatel mydb gt This prompt indicates that the terminal monitor is listening to you and that you can type SQL queries into a workspace maintained by the terminal monitor The psql program responds to escape codes that begin with the backslash character 1 For example you can get help on the syntax of various Postgres SQL commands by typing mydb gt Ah Once you have finished entering your queries into the workspace you can pass the contents of the workspace to the Postgres server by typing mydb gt Mg This tells the server to process the query If you terminate your query with a semicolon the Ag is not necessary psql will automatically process semicolon
51. l want to confirm that you are choosing an allowed connection option If you get the following error message from a Postgres command such as psql or createdb o psql templatel psqdl connectDBStart connect failed No such file or directory Is the postmaster running locally and accepting connections on Unix socket tmp s PGSQL 5432 or psql h localhost templatel psql PQconnectPoll connect failed Connection refused Is the postmaster running with i at localhost and accepting connections on TCP IP port 5432 it is usually because the postmaster is not running or you are attempting to connect to the wrong server host If you get the following error message FATAL 1 Feb 17 23 19 55 process userid 2360 database owner 268 it means that the site administrator started the postmaster as the wrong user Tell him to restart it as the Postgres superuser 3 3 Managing a Database Now that Postgres is up and running we can create some databases to experiment with Here we describe the basic commands for managing a database Most Postgres applications assume that the database name if not specified is the same as the name on your computer account If your database administrator has set up your account without database creation privileges then she should have told you what the name of your database is If this is the case then you can skip the sections on creating and destroying databases
52. lo 2 AS temp avg date FROM weather Arbitrary Boolean operators AND OR and NOT are allowed in the qualification of any query For example SELECT FROM weather WHERE city San Francisco AND prcp 0 0 results in city temp lo temp hi prcp date San Francisco 46 50 0 25 1994 11 27 As a final note you can specify that the results of a select can be returned in a sorted order or with duplicate rows removed SE I ECT DISTINCT city FROM weather ORDER BY city 4 6 Redirecting SELECT Queries Any SELECT query can be redirected to a new table SE H ECT INTO TABLE temp FROM weather This forms an implicit CREATE command creating a new table temp with the column names and types specified in the target list of the SELECT INTO command We can then of course perform any operations on the resulting table that we can perform on other tables 30 4 7 Joins Between Tables Thus far our queries have only accessed one table at a time Queries can access multiple tables at once or access the same table in such a way that multiple rows of the table are being processed at the same time A query that accesses multiple rows of the same or different tables at one time is called a join query As an example say we wish to find all the records that are in the temperature range of other records In effect we need to compare the temp lo and
53. n 1998 10 22 Updated 2000 03 31 The PostgreSQL Global Development Group provides the PostgreSQL software code tree as a public service without warranty and without liability for its behavior or performance However at the time of writing The author of this statement a volunteer on the Postgres support team since November 1996 is not aware of any problems in the Postgres code base related to time transitions around Jan 1 2000 Y2K The author of this statement is not aware of any reports of Y2K problems uncovered in regression testing or in other field use of recent or current versions of Postgres We might have expected to hear about problems if they existed given the installed base and the active participation of users on the support mailing lists To the best of the author s knowledge the assumptions Postgres makes about dates specified with a two digit year are documented in the current User s Guide in the chapter on data types For two digit years the significant transition year is 1970 not 2000 e g 70 01 01 is interpreted as 1970 01 01 whereas 69 01 01 is interpreted as 2069 01 01 Any Y2K problems in the underlying OS related to obtaining the current time may propagate into apparent Y2K problems in Postgres Refer to The Gnu Project http www gnu org software year2000 html and The Perl Institute http language perl com news y2k html for further discussion of Y2K issues particularly as it relates to open sourc
54. n T2 an additional joined row is returned with null fields in the columns from T2 Also for each row in T2 that does not satisfy the join condition with any row in T1 an additional joined row is returned with null fields in the columns from TI Tip The joined table unconditionally has a row for every row of T1 and a row for every row of T2 JOINs of all types can be chained together or nested where either or both of T1 and T2 may be JOINed tables Parenthesis can be used around JOIN clauses to control the order of JOINs which are otherwise processed left to right 1 4 1 3 Aggregate Operators SQL provides aggregate operators e g AVG COUNT SUM MIN MAX that take an expression as argument The expression is evaluated at each row that satisfies the WHERE clause and the aggregate operator is calculated over this set of input values Normally an aggregate delivers a 11 single result for a whole SELECT statement But if grouping is specified in the query then a separate calculation is done over the rows of each group and an aggregate result is delivered per group see next section Example 1 5 Aggregates If we want to know the average cost of all parts in table PART we use the following query SELECT AVG PRICE AS AVG PRICE FROM PART The result is AVG_PRICE If we want to know how many parts are defined in table PART we use the statement SELECT COUNT PNO FROM PART and get COUNT
55. n be viewed as a table as we already did remember The Suppliers and Parts Database where every tuple is represented by a row and every column corresponds to one component of a tuple Giving names called attributes to the columns leads to the definition of a relation scheme A relation scheme Ris a finite set of attributes Ai Az Ax There is a domain D for each attribute Ai l lt i lt k where the values of the attributes are taken from We often write a relation scheme asR A1 Az Ax Note A relation scheme is just a kind of template whereas a relation is an instance of a relation scheme The relation consists of tuples and can therefore be viewed as a table not so the relation scheme 1 2 1 Domains vs Data Types We often talked about domains in the last section Recall that a domain is formally just a set of values e g the set of integers or the real numbers In terms of database systems we often talk of data types instead of domains When we define a table we have to make a decision about which attributes to include Additionally we have to decide which kind of data is going to be stored as attribute values For example the values of sNAME from the table SUPPLIER will be character strings whereas SNO will store integers We define this by assigning a data type to each attribute The type of SNAME will be VARCHAR 20 this is the SQL type for character strings of length lt 20 the type of sNo will be INTEGER
56. n the latest release upcoming features and other information to make your work or play with PostgreSQL more productive Mailing Lists The lt pgsql general postgresql org gt archive http www postgresql org mhonarc pgsql general mailing list is a good place to have user questions answered Other mailing lists are available consult the User s Lounge http www postgresql org users lounge section of the PostgreSQL web site for details Yourself PostgreSQL is an open source effort As such it depends on the user community for ongoing support As you begin to use PostgreSQL you will rely on others for help either through the documentation or through the mailing lists Consider contributing your knowledge back If you learn something which is not in the documentation write it up and contribute it If you add features to the code contribute it Even those without a lot of experience can provide corrections and minor changes in the documentation and that is a good way to start The lt pgsql docs postgresql org gt archive http www postgresql org mhonarc pgsql docs mailing list is the place to get going 4 Terminology and Notation The terms Postgres and PostgreSQL will be used interchangeably to refer to the software that accompanies this documentation An administrator is generally a person who is in charge of installing and running the server A user could be anyone who is using or wants to use any part of
57. ntly incorporated features pioneered by Postgres 2 A Short History of Postgres The object relational database management system now known as PostgreSQL and briefly called Postgres95 is derived from the Postgres package written at the University of California at Berkeley With over a decade of development behind it PostgreSQL is the most advanced open source database available anywhere offering multi version concurrency control supporting almost all SQL constructs including subselects transactions and user defined types and functions and having a wide range of language bindings available including C C Java Perl Tcl and Python vii 2 1 The Berkeley Postgres Project Implementation of the Postgres DBMS began in 1986 The initial concepts for the system were presented in The Design of Postgres and the definition of the initial data model appeared in The Postgres Data Model The design of the rule system at that time was described in The Design of the Postgres Rules System The rationale and architecture of the storage manager were detailed in The Postgres Storage System Postgres has undergone several major releases since then The first demoware system became operational in 1987 and was shown at the 1988 ACM SIGMOD Conference We released Version 1 described in The Implementation of Postgres to a few external users in June 1989 In response to a critique of the first rule system A Commentary on the Postgres Rules System
58. of attribute PRICE of the tuple whose name is Screw is now 15 20 1 4 3 3 Delete To delete a tuple from a particular table use the command DELETE FROM The syntax is DELE H E FROM table_name WHERE condition To delete the supplier called Smith of the table SUPPLIER the following statement is used DELE H E FROM SUPPLIER WHERE SNAME Smith 1 4 4 System Catalogs In every SQL database system system catalogs are used to keep track of which tables views indexes etc are defined in the database These system catalogs can be queried as if they were normal relations For example there is one catalog used for the definition of views This catalog stores the query from the view definition Whenever a query against a view is made the system first gets the view definition query out of the catalog and materializes the view before proceeding with the user query see Simkovics 1998 for a more detailed description For more information about system catalogs refer to Date 1994 1 4 5 Embedded SQL In this section we will sketch how SQL can be embedded into a host language e g C There are two main reasons why we want to use SQL from a host language There are queries that cannot be formulated using pure SQL i e recursive queries To be able to perform such queries we need a host language with a greater expressive power than SQL We simply want to access
59. ql org gt or lt pgsql general postgresql org gt These mailing lists are for answering user questions and xiii their subscribers normally do not wish to receive bug reports More importantly they are unlikely to fix them Also please do not send reports to the developers mailing list lt pgsql hackers postgresql org gt This list is for discussing the development of PostgreSQL and it would be nice if we could keep the bug reports separate We might choose to take up a discussion about your bug report on it if the bug needs more review If you have a problem with the documentation send email to the documentation mailing list lt pgsql docs postgresql org gt Mention the document chapter and sections in your problem report If your bug is a portability problem on a non supported platform send mail to lt pgsql ports postgresql org gt so we and you can work on porting PostgreSQL to your platform Note Due to the unfortunate amount of spam going around all of the above email addresses are closed mailing lists That is you need to be subscribed to a list to be allowed to post on it If you simply want to send mail but do not want to receive list traffic you can subscribe and set your subscription option to nomail For more information send mail to ma jordomofpostgresql org gt With the single word help in the body of the message 6 Y2K Statement Author Written by Thomas Lockhart lt lockhart alumni caltech edu gt o
60. range date time types and additional geometric type support Overall backend code speed has been increased by approximately 20 40 and backend start up time has decreased 80 since version 6 0 was released 3 Documentation Resources This manual set is organized into several parts Tutorial An introduction for new users Does not cover advanced features User s Guide Documents the SQL query language environment including data types and functions Programmer s Guide Advanced information for application programmers Topics include type and function extensibility library interfaces and application design issues Administrator s Guide Installation and server management information 1X Reference Manual Reference pages for SQL command syntax and client and server programs Developer s Guide Information for Postgres developers This is intended for those who are contributing to the Postgres project application development information should appear in the Programmer s Guide In addition to this manual set there are other resources to help you with Postgres installation and use man pages The Reference Manual s pages in the traditional Unix man format FAQs Frequently Asked Questions FAQ lists document both general issues and some platform specific issues READMEs README files are available for some contributed packages Web Site The PostgreSQL web site http www postgresql org carries details o
61. rations in the Relational Data Model o oooooooocmoomormommo m2 o 3 1 3 1 Relational Algebra oo oooooooomocrocorocrrrrrcrrcrrrromo 3 1 3 2 Relational Calculus o ooooooocooocrornorrororrrrrormom o cn 6 1 3 3 Tuple Relational Calculus oo oooooomocroocrorrormormo m2 6 1 3 4 Relational Algebra vs Relational Calculus o ooo ooomoomoo 6 1 4 The SQL Lang age ook lel RR MS x RR Rr I RE RR be Ren 7 LAS lO ELLE exeibumenE ere ie aas de 7 1 4 2 Data Definition oo oooooooocmoocrorrorsrrnrrrrronsorsonon 17 1 4 3 Data Manipulation o ooo ooocoocnocnorrorocrocncorcormomoo 20 1 4 4 System Catalogs i5 to RT A er Rye TES Ewa RS 21 1 4 5 Embedded SQL 1 luelz e a Re SL oe kn io 21 Chapter 2 Architecture ssas a oa le td ra Te rd RR Ir er a Rae be Ee 22 2 1 Postgres Architectural Concepts 2 ec cee cee reece cece eee hh 22 Chapter 3 Getting Started sino 0 ccc ccc ccc ccc ce cee ree cece ee eee cece ee eee nnn 23 3 1 Setting Up Your Environment 0 ccc ccc cee cece cee eee e nnn 23 3 2 Starting the Interactive Monitor psql 0 cece cece cece cece cere ree eees 24 3 3 Managing a Database cece ccc cc ce cece cece cece cere eee eee sees 24 3 3 1 Creating a Database ccc ccc cee ecw eee enhn nn 25 3 3 2 Accessing a Database 0 ccc ccc ccc cece eee eee e cece htt 25 3 3 3 Destroy
62. resently see that tables have properties that are extensions of the relational model 4 4 Populating a Table with Rows The INSERT statement is used to populate a table with rows INSERT INTO weather VALUES San Francisco 46 50 0 25 1994 11 27 You can also use COPY to load large amounts of data from flat ASCII files This is usually faster because the data is read or written as a single atomic transaction directly to or from the target table An example would be COPY weather FROM home user weather txt USING DELIMITERS where the path name for the source file must be available to the backend server machine not the client since the backend server reads the file directly 4 5 Querying a Table The weather table can be queried with normal relational selection and projection queries A SQL SELECT statement is used to do this The statement is divided into a target list the part that lists the columns to be returned and a qualification the part that specifies any restrictions For example to retrieve all the rows of weather type SE r4 ECT FROM weather 29 and the output should be city temp lo temp hi prcp date San Francisco 46 50 0 25 1994 11 27 San Francisco 43 57 0 1994 11 29 Hayward 37 54 1994 11 29 You may specify any arbitrary expressions in the target list For example you can do SE H ECT city temp_hi temp_
63. t variable throughout this document If you did not fully understand the last paragraph on modifying your search path you should consult the Unix manual pages that describe your shell before going any further If your site administrator has not set things up in the default way you may have some more work to do For example if the database server machine is a remote machine you will need to set the PGHOST environment variable to the name of the database server machine The environment variable PGPORT may also have to be set The bottom line is this if you try to start an application program and it complains that it cannot connect to the postmaster you should immediately consult your site administrator to make sure that your environment is properly set up 23 3 2 Starting the Interactive Monitor psql Assuming that your site administrator has properly started the postmaster process and authorized you to use the database you as a user may begin to start up applications As previously mentioned you should add usr local pgsql bin to your shell search path In most cases this is all you should have to do in terms of preparation Two different styles of connections are supported The site administrator will have chosen to allow TCP IP network connections or will have restricted database access to local same machine socket connections only These choices become significant if you encounter problems in connecting to a database since you wil
64. tabase Let s return to our example from the previous section Operations in the Relational Data Model where someone wanted to know the names of all suppliers that sell the part Screw This question can be answered using relational algebra by the following operation OsuPPLIER SNAME PART PNAME scew SUPPLIER SELLS PART We call such an operation a query If we evaluate the above query against the our example tables The Suppliers and Parts Database we will obtain the following result SNAME 1 3 2 Relational Calculus The relational calculus is based on the first order logic There are two variants of the relational calculus The Domain Relational Calculus DRC where variables stand for components attributes of the tuples The Tuple Relational Calculus TRC where variables stand for tuples We want to discuss the tuple relational calculus only because it is the one underlying the most relational languages For a detailed discussion on DRC and also TRC see Date 1994 or Ullman 1988 1 3 3 Tuple Relational Calculus The queries used in TRC are of the following form x A F x where x is a tuple variable A is a set of attributes and F is a formula The resulting relation consists of all tuples t A that satisfy F t If we want to answer the question from example A Query Using Relational Algebra using TRC we formulate the following query x SNAME x e SUPPLIER dye SELLS 3z e PART y SNO x SNO
65. terminated queries To read queries from a file say myFile instead of entering them interactively type mydb Mi fileName To get out of psql and return to Unix type mydb gt Xq and psql will quit and return you to your command shell For more escape codes type M at the monitor prompt White space 1 e spaces tabs and newlines may be used freely in SQL queries Single line comments are denoted by Everything after the dashes up to the end of the line is ignored Multiple line comments and comments within a line are denoted by 3 3 3 Destroying a Database If you are the database administrator for the database mydb you can destroy it using the following Unix command dropdb mydb 26 This action physically removes all of the Unix files associated with the database and cannot be undone so this should only be done with a great deal of forethought 27 Chapter 4 The Query Language The Postgres query language is a variant of the SQL standard It has many extensions to SQL such as an extensible type system inheritance functions and production rules These are features carried over from the original Postgres query language PostQuel This section provides an overview of how to use Postgres SQL to perform simple operations This manual is only intended to give you an idea of our flavor of SQL and is in no way a complete tutorial on SQL Numerous books have been written on SQL92 including Melton
66. the PostgreSQL system These terms should not be interpreted too narrowly this documentation set does not have fixed presumptions about system administration procedures usr local pgsql is generally used as the root directory of the installation and usr local pgsql data as the directory with the database files These directories may vary on your site details can be derived in the Administrator s Guide In a command synopsis brackets and indicate an optional phrase or keyword Anything in braces and and containing vertical bars indicates that you must choose one Examples will show commands executed from various accounts and programs Commands executed from a Unix shell may be preceeded with a dollar sign Commands executed from particular user accounts such as root or postgres are specially flagged and explained SQL commands may be preceeded with gt or will have no leading prompt depending on the context Note The notation for flagging commands is not universally consistant throughout the documentation set Please report problems to the documentation mailing list lt pgsql docs postgresql org gt 5 Bug Reporting Guidelines When you find a bug in PostgreSQL we want to hear about it Your bug reports play an important part in making PostgreSQL more reliable because even the utmost care cannot guarantee that every part of PostgreSQL will work on every platform under every circumstance The following s
67. the rule system was redesigned On Rules Procedures Caching and Views in Database Systems and Version 2 was released in June 1990 with the new rule system Version 3 appeared in 1991 and added support for multiple storage managers an improved query executor and a rewritten rewrite rule system For the most part releases until Postgres95 see below focused on portability and reliability Postgres has been used to implement many different research and production applications These include a financial data analysis system a jet engine performance monitoring package an asteroid tracking database a medical information database and several geographic information systems Postgres has also been used as an educational tool at several universities Finally Illustra Information Technologies http www illustra com since merged into Informix http www informix com picked up the code and commercialized it Postgres became the primary data manager for the Sequoia 2000 http www sdsc edu O Parts Collabs S2K s2k home html scientific computing project in late 1992 The size of the external user community nearly doubled during 1993 It became increasingly obvious that maintenance of the prototype code and support was taking up large amounts of time that should have been devoted to database research In an effort to reduce this support burden the project officially ended with Version 4 2 2 2 Postgres95 In 1994 Andrew Yu and Jolly Chen
68. uggestions are intended to assist you in forming bug reports that can be handled in an effective fashion No one is required to follow them but it tends to be to everyone s advantage We cannot promise to fix every bug right away If the bug is obvious critical or affects a lot of users chances are good that someone will look into it It could also happen that we tell you to update to a newer version to see if the bug happens there Or we might decide that the bug cannot be fixed before some major rewrite we might be planning is done Or perhaps it is simply too hard and there are more important things on the agenda If you need help immediately consider obtaining a commercial support contract 5 1 Identifying Bugs Before you report a bug please read and re read the documentation to verify that you can really do whatever it is you are trying If it is not clear from the documentation whether you can do something or not please report that too it is a bug in the documentation If it turns out that the program does something different from what the documentation says that is a bug That might include but is not limited to the following circumstances A program terminates with a fatal signal or an operating system error message that would point to a problem in the program A counterexample might be a disk full message since you have to fix that yourself A program produces the wrong output for any given input xi A program refuses
69. uld keep this in mind because the files that can be accessed on a client machine may not be accessible or may only be accessed using a different filename on the database server machine You should also be aware that the postmaster and postgres servers run with the user id of the Postgres superuser Note that the Postgres superuser does not have to be a special user e g a user named postgres Furthermore the Postgres superuser should definitely not be the Unix superuser root In any case all files relating to a database should belong to this Postgres superuser 22 Chapter 3 Getting Started How to begin work with Postgres for a new user Some of the steps required to use Postgres can be performed by any Postgres user and some must be done by the site database administrator This site administrator is the person who installed the software created the database directories and started the postmaster process This person does not have to be the Unix superuser root or the computer system administrator a person can install and use Postgres without any special accounts or privileges If you are installing Postgres yourself then refer to the Administrator s Guide for instructions on installation and return to this guide when the installation is complete Throughout this manual any examples that begin with the character are commands that should be typed at the Unix shell prompt Examples that begin with the character ar
70. word AS is the new title of the second column This technique can be used for every element of the target list to assign a new title to the resulting column This new title is often referred to as alias The alias cannot be used throughout the rest of the query 1 4 1 2 Joins The following example shows how joins are realized in SQL To join the three tables SUPPLIER PART and SELLS over their common attributes we formulate the following statement SELECT S SNAME P PNAME FROM SUPPLIER S PART P SELLS SE WHERE S SNO SE SNO AND P PNO SE PNO and get the following table as a result SNAME PNAME Ee 4 Smith Screw Smith Nut Jones Cam Adams Screw Adams Bolt Blake Nut Blake Bolt Blake Cam In the FROM clause we introduced an alias name for every relation because there are common named attributes SNO and PNO among the relations Now we can distinguish between the common named attributes by simply prefixing the attribute name with the alias name followed by a dot The join is calculated in the same way as shown in An Inner Join First the Cartesian product SUPPLIER x PART x SELLS is derived Now only those tuples satisfying the conditions given in the WHERE clause are selected i e the common named attributes have to be equal Finally we project out all columns but S SNAME and P PNAME Another way to perform joins is to use the SQL JOIN syntax as follows select sname pname from supplier JOIN sells
71. xt 22 The above query will create a table named SAL EMP with a text string name a one dimensional array of integer pay by quarter which represents the employee s salary by quarter and a two dimensional array of text schedule which represents the employee s weekly schedule Now we do some JNSERTs note that when appending to an array we enclose the values within braces and separate them by commas If you know C this is not unlike the syntax for initializing structures INSERT INTO SAL EMP VALUES Bill 10000 10000 10000 10000 meeting lunch INSERT INTO SAL_EMP VALUES Carol 20000 25000 25000 25000 talk consult meeting By default Postgres uses the one based numbering convention for arrays that is an array of n elements starts with array 1 and ends with array n Now we can run some queries on SAL_EMP First we show how to access a single element of an array at a time This query retrieves the names of the employees whose pay changed in the second quarter SE H ECT name FROM SAL_EMP WHERE SAL EMP pay by quarter 1 lt gt SAL EMP pay by quarter 2 This query retrieves the third quarter pay of all employees SE n ECT SAL EMP pay by quarter 3 FROM SAL EMP pay by quarter 10000 25000 We can also access arbitrary slices o

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