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<META name="generator" content="HTML Tidy, see www.w3.org">
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<META http-equiv="Content-Type" content="text/html; charset=US-ASCII">
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<TITLE>PostgreSQL FAQ</TITLE>
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<BODY bgcolor="#ffffff" text="#000000" link="#ff0000" vlink="#a00000"
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alink="#0000ff">
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<H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1>
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<P>Last updated: Wed Jun 7 17:22:48 EDT 2006</P>
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<P>Current maintainer: Bruce Momjian (<A href=
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"mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)
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</P>
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<P>The most recent version of this document can be viewed at <A href=
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"http://www.postgresql.org/files/documentation/faqs/FAQ.html">
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http://www.postgresql.org/files/documentation/faqs/FAQ.html</A>.</P>
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<P>Platform-specific questions are answered at <A href=
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"http://www.postgresql.org/docs/faq/">
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http://www.postgresql.org/docs/faq/</A>.</P>
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<HR>
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<H2 align="center">General Questions</H2>
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<A href="#item1.1">1.1</A>) What is PostgreSQL? How is it pronounced?<BR>
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<A href="#item1.2">1.2</A>) Who controls PostgreSQL?<BR>
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<A href="#item1.3">1.3</A>) What is the copyright of PostgreSQL?<BR>
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<A href="#item1.4">1.4</A>) What platforms does PostgreSQL support?<BR>
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<A href="#item1.5">1.5</A>) Where can I get PostgreSQL?<BR>
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<A href="#item1.6">1.6</A>) What is the most recent release?<BR>
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<A href="#item1.7">1.7</A>) Where can I get support?<BR>
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<A href="#item1.8">1.8</A>) How do I submit a bug report?<BR>
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<A href="#item1.9">1.9</A>) How do I find out about known bugs or
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missing features?<BR>
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<A href="#item1.10">1.10</A>) What documentation is available?<BR>
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<A href="#item1.11">1.11</A>) How can I learn <SMALL>SQL</SMALL>?<BR>
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<A href="#item1.12">1.12</A>) How do I submit a patch or join the
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development team?<BR>
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<A href="#item1.13">1.13</A>) How does PostgreSQL compare to other
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<SMALL>DBMS</SMALL>s?<BR>
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<H2 align="center">User Client Questions</H2>
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<A href="#item2.1">2.1</A>) What interfaces are available for
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PostgreSQL?<BR>
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<A href="#item2.2">2.2</A>) What tools are available for using
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PostgreSQL with Web pages?<BR>
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<A href="#item2.3">2.3</A>) Does PostgreSQL have a graphical user
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interface?<BR>
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<H2 align="center">Administrative Questions</H2>
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<A href="#item3.1">3.1</A>) How do I install PostgreSQL somewhere other
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than <I>/usr/local/pgsql</I>?<BR>
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<A href="#item3.2">3.2</A>) How do I control connections from other
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hosts?<BR>
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<A href="#item3.3">3.3</A>) How do I tune the database engine for
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better performance?<BR>
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<A href="#item3.4">3.4</A>) What debugging features are available?<BR>
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<A href="#item3.5">3.5</A>) Why do I get <I>"Sorry, too many
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clients"</I> when trying to connect?<BR>
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<A href="#item3.6">3.6</A> What is the upgrade process for PostgreSQL?<BR>
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<A href="#item3.7">3.7</A>) What computer hardware should I use?<BR>
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<H2 align="center">Operational Questions</H2>
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<A href="#item4.1">4.1</A>) How do I <SMALL>SELECT</SMALL> only the
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first few rows of a query? A random row?<BR>
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<A href="#item4.2">4.2</A>) How do I find out what tables, indexes,
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databases, and users are defined? How do I see the queries used
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by <I>psql</I> to display them?<BR>
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<A href="#item4.3">4.3</A>) How do you change a column's data type?<BR>
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<A href="#item4.4">4.4</A>) What is the maximum size for a row, a
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table, and a database?<BR>
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<A href="#item4.5">4.5</A>) How much database disk space is required
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to store data from a typical text file?<BR>
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<A href="#item4.6">4.6</A>) Why are my queries slow? Why don't they
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use my indexes?<BR>
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<A href="#item4.7">4.7</A>) How do I see how the query optimizer is
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evaluating my query?<BR>
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<A href="#item4.8">4.8</A>) How do I perform regular expression
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searches and case-insensitive regular expression searches? How do I
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use an index for case-insensitive searches?<BR>
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<A href="#item4.9">4.9</A>) In a query, how do I detect if a field
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is <SMALL>NULL</SMALL>? How can I sort on whether a field is <SMALL>
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NULL</SMALL> or not?<BR>
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<A href="#item4.10">4.10</A>) What is the difference between the
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various character types?<BR>
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<A href="#item4.11.1">4.11.1</A>) How do I create a
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serial/auto-incrementing field?<BR>
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<A href="#item4.11.2">4.11.2</A>) How do I get the value of a
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<SMALL>SERIAL</SMALL> insert?<BR>
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<A href="#item4.11.3">4.11.3</A>) Doesn't <I>currval()</I>
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lead to a race condition with other users?<BR>
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<A href="#item4.11.4">4.11.4</A>) Why aren't my sequence numbers
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reused on transaction abort? Why are there gaps in the numbering of
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my sequence/SERIAL column?<BR>
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<A href="#item4.12">4.12</A>) What is an <SMALL>OID</SMALL>? What is a
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<SMALL>CTID</SMALL>?<BR>
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<A href="#item4.13">4.13</A>) Why do I get the error <I>"ERROR: Memory
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exhausted in AllocSetAlloc()"</I>?<BR>
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<A href="#item4.14">4.14</A>) How do I tell what PostgreSQL version I
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am running?<BR>
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<A href="#item4.15">4.15</A>) How do I create a column that will
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default to the current time?<BR>
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<A href="#item4.16">4.16</A>) How do I perform an outer join?<BR>
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<A href="#item4.17">4.17</A>) How do I perform queries using multiple
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databases?<BR>
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<A href="#item4.18">4.18</A>) How do I return multiple rows or columns
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from a function?<BR>
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<A href="#item4.19">4.19</A>) Why do I get "relation with OID #####
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does not exist" errors when accessing temporary tables in PL/PgSQL
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functions?<BR>
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<A href="#item4.20">4.20</A>) What replication solutions are available?<BR>
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<A href="#item4.21">4.21</A>) Why are my table and column names not
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recognized in my query?<BR>
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<HR>
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<H2 align="center">General Questions</H2>
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<H3 id="item1.1">1.1) What is PostgreSQL? How is it pronounced?</H3>
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<P>PostgreSQL is pronounced <I>Post-Gres-Q-L</I>, and is also sometimes
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referred to as just <I>Postgres</I>. An audio file is available in
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<a href="http://www.postgresql.org/files/postgresql.mp3">MP3 format</a> for
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those would like to hear the pronunciation.</P>
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<P>PostgreSQL is an object-relational database system that has the
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features of traditional commercial database systems with
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enhancements to be found in next-generation <SMALL>DBMS</SMALL>
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systems. PostgreSQL is free and the complete source code is
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available.</P>
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<P>PostgreSQL development is performed by a team of mostly volunteer
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developers spread throughout the world and communicating via the
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Internet. It is a community project and is not controlled by any
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company. To get involved, see the developer's FAQ at <A href=
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"http://www.postgresql.org/files/documentation/faqs/FAQ_DEV.html">
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http://www.postgresql.org/files/documentation/faqs/FAQ_DEV.html</A>
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</P>
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<H3 id="item1.2">1.2) Who controls PostgreSQL?<BR></H3>
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<P>If you are looking for a PostgreSQL gatekeeper, central committee,
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or controlling company, give up --- there isn't one. We do have a
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core committee and CVS committers, but these groups are more for
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administrative purposes than control. The project is directed by
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the community of developers and users, which anyone can join. All
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you need to do is subscribe to the mailing lists and participate in the
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discussions. (See the <a href="http://www.postgresql.org/docs/faqs.FAQ_DEV.html">
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Developer's FAQ</A> for information on how to get involved in PostgreSQL
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development.)</P>
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<H3 id="item1.3">1.3) What is the copyright of
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PostgreSQL?</H3>
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<P>PostgreSQL is distributed under the classic BSD license. Basically,
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it allows users to do anything they want with the code, including
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reselling binaries without the source code. The only restriction is
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that you not hold us legally liable for problems with the software.
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There is also the requirement that this copyright appear in all copies
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of the software. Here is the actual BSD license we use:</P>
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<P>PostgreSQL Data Base Management System</P>
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<P>Portions Copyright (c) 1996-2006, PostgreSQL Global Development Group
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Portions Copyright (c) 1994-1996 Regents of the University of California</P>
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<P>Permission to use, copy, modify, and distribute this software
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and its documentation for any purpose, without fee, and without a
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written agreement is hereby granted, provided that the above
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copyright notice and this paragraph and the following two
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paragraphs appear in all copies.</P>
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<P>IN NO EVENT SHALL THE UNIVERSITY OF CALIFORNIA BE LIABLE TO ANY
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PARTY FOR DIRECT, INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL
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DAMAGES, INCLUDING LOST PROFITS, ARISING OUT OF THE USE OF THIS
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SOFTWARE AND ITS DOCUMENTATION, EVEN IF THE UNIVERSITY OF
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CALIFORNIA HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.</P>
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<P>THE UNIVERSITY OF CALIFORNIA SPECIFICALLY DISCLAIMS ANY
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WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
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OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE
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SOFTWARE PROVIDED HEREUNDER IS ON AN "AS IS" BASIS, AND THE
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UNIVERSITY OF CALIFORNIA HAS NO OBLIGATIONS TO PROVIDE MAINTENANCE,
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SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.</P>
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<H3 id="item1.4">1.4) What platforms does PostgreSQL support?</H3>
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<P>In general, any modern Unix-compatible platform should be able to
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run PostgreSQL. The platforms that had received explicit testing at
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the time of release are listed in the installation
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instructions.</P>
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<P>PostgreSQL also runs natively on Microsoft Windows NT-based operating
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systems like Win2000, WinXP, and Win2003. A prepackaged installer is
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available at <a href= "http://pgfoundry.org/projects/pginstaller">
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http://pgfoundry.org/projects/pginstaller</a>. MSDOS-based versions
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of Windows (Win95, Win98, WinMe) can run PostgreSQL using Cygwin.</P>
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<p>There is also a Novell Netware 6 port at
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<a href="http://forge.novell.com">http://forge.novell.com</a>,
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and an OS/2 (eComStation) version at <a href=
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"http://hobbes.nmsu.edu/cgi-bin/h-search?sh=1&button=Search&key=postgreSQL&stype=all&sort=type&dir=%2F">
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http://hobbes.nmsu.edu/cgi-bin/h-search?sh=1&button=Search&key=postgreSQL&stype=all&sort=type&dir=%2F</a>.</p>
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<H3 id="item1.5">1.5) Where can I get PostgreSQL?</H3>
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<P>Via web browser, use <a href="http://www.postgresql.org/ftp/">
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http://www.postgresql.org/ftp/</a>, and via ftp, use
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<A href="ftp://ftp.PostgreSQL.org/pub/">
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ftp://ftp.PostgreSQL.org/pub/</A>.</P>
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<H3 id="item1.6">1.6) What is the most recent release?</H3>
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<P>The latest release of PostgreSQL is version 8.1.4.</P>
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<P>We plan to have a major release every year, with minor releases
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every few months.</P>
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<H3 id="item1.7">1.7) Where can I get support?</H3>
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<P>The PostgreSQL community provides assistance to many of its users
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via email. The main web site to subscribe to the email lists is
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<a href="http://www.postgresql.org/community/lists/">
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http://www.postgresql.org/community/lists/</a>. The <I>general</I>
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or <I>bugs</I> lists are a good place to start.</P>
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<P>The major IRC channel is <I>#postgresql</I> on Freenode
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(<I>irc.freenode.net</I>). To connect you can use the Unix
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program <CODE>irc -c '#postgresql' "$USER" irc.freenode.net</CODE>
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or use any other IRC clients. A Spanish one also exists
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on the same network, (<I>#postgresql-es</I>), and a French one,
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(<I>#postgresqlfr</I>). There is also a PostgreSQL channel on EFNet.</P>
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<P>A list of commercial support companies is available at <A href=
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"http://techdocs.postgresql.org/companies.php">
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http://techdocs.postgresql.org/companies.php</A>.</P>
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<H3 id="item1.8">1.8) How do I submit a bug report?</H3>
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<P>Visit the PostgreSQL bug form at <A href=
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"http://www.postgresql.org/support/submitbug">
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http://www.postgresql.org/support/submitbug</A>.
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Also check out our ftp site <A href=
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"ftp://ftp.PostgreSQL.org/pub/">ftp://ftp.PostgreSQL.org/pub/</A> to
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see if there is a more recent PostgreSQL version.</P>
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<P>Bugs submitted using the bug form or posted to any PostgreSQL mailing
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list typically generates one of the following replies:</P>
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<ul>
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<li>It is not a bug, and why</li>
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<li>It is a known bug and is already on the
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<A href="http://www.postgresql.org/docs/faqs.TODO.html">TODO</A> list</li>
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<li>The bug has been fixed in the current release</li>
|
|
|
|
|
<li>The bug has been fixed but is not packaged yet in an official
|
|
|
|
|
release</li>
|
|
|
|
|
<li>A request is made for more detailed information:
|
|
|
|
|
<ul>
|
|
|
|
|
<li>Operating system</li>
|
|
|
|
|
<li>PostgreSQL version</li>
|
|
|
|
|
<li>Reproducible test case</li>
|
|
|
|
|
<li>Debugging information</li>
|
|
|
|
|
<li>Debugger backtrace output</li>
|
|
|
|
|
</ul>
|
|
|
|
|
</li>
|
|
|
|
|
<li>The bug is new. The following might happen:
|
|
|
|
|
<ul>
|
|
|
|
|
<li>A patch is created and will be included in the next major
|
|
|
|
|
or minor release</li>
|
|
|
|
|
<li>The bug cannot be fixed immediately and is added
|
|
|
|
|
to the <A href="http://www.postgresql.org/docs/faqs.TODO.html">TODO</A>
|
|
|
|
|
list</li>
|
|
|
|
|
</ul>
|
|
|
|
|
</li>
|
|
|
|
|
</ul>
|
|
|
|
|
|
|
|
|
|
<H3 id="item1.9">1.9) How do I find out about known bugs or
|
|
|
|
|
missing features?</H3>
|
|
|
|
|
|
|
|
|
|
<P>PostgreSQL supports an extended subset of <SMALL>SQL:2003</SMALL>.
|
|
|
|
|
See our <A href="http://www.postgresql.org/docs/faqs.TODO.html">TODO</A>
|
|
|
|
|
list for known bugs, missing features, and future plans.</P>
|
|
|
|
|
|
|
|
|
|
<P>A feature request usually results in one of the following
|
|
|
|
|
replies:</P>
|
|
|
|
|
<ul>
|
|
|
|
|
<li>The feature is already on the
|
|
|
|
|
<A href="http://www.postgresql.org/docs/faqs.TODO.html">TODO</A>
|
|
|
|
|
list</li>
|
|
|
|
|
<li>The feature is not desired because:
|
|
|
|
|
<ul>
|
|
|
|
|
<li>It duplicates existing functionality that already
|
|
|
|
|
follows the SQL standard</li>
|
|
|
|
|
<li>The feature would increase code complexity but add little
|
|
|
|
|
benefit</li>
|
|
|
|
|
<li>The feature would be insecure or unreliable</li>
|
|
|
|
|
</ul>
|
|
|
|
|
</li>
|
|
|
|
|
<li>The new feature is added to the
|
|
|
|
|
<A href="http://www.postgresql.org/docs/faqs.TODO.html">TODO</A> list</li>
|
|
|
|
|
</ul>
|
|
|
|
|
|
|
|
|
|
<P>PostgreSQL does not use a bug tracking system because we find
|
|
|
|
|
it more efficient to respond directly to email and keep the
|
|
|
|
|
<A href="http://www.postgresql.org/docs/faqs.TODO.html">TODO</A>
|
|
|
|
|
list up-to-date. In practice, bugs don't last very long in the
|
|
|
|
|
software, and bugs that affect a large number of users are fixed
|
|
|
|
|
rapidly. The only place to find all changes, improvements, and
|
|
|
|
|
fixes in a PostgreSQL release is to read the
|
|
|
|
|
<a href="http://www.postgresql.org/developer/sourcecode/">CVS</a>
|
|
|
|
|
log messages. Even the release notes do not list every change
|
|
|
|
|
made to the software.</P>
|
|
|
|
|
|
|
|
|
|
<H3 id="item1.10">1.10) What documentation is available?</H3>
|
|
|
|
|
|
|
|
|
|
<P>PostgreSQL includes extensive documentation, including a large
|
|
|
|
|
manual, manual pages, and some test examples. See the <I>/doc</I>
|
|
|
|
|
directory. You can also browse the manuals online at <A href=
|
|
|
|
|
"http://www.PostgreSQL.org/docs">http://www.PostgreSQL.org/docs</A>.
|
|
|
|
|
</P>
|
|
|
|
|
|
|
|
|
|
<P>There are two PostgreSQL books available online at <A href=
|
|
|
|
|
"http://www.postgresql.org/docs/books/awbook.html">http://www.postgresql.org/docs/books/awbook.html</A>
|
|
|
|
|
and <A href=
|
|
|
|
|
"http://www.commandprompt.com/ppbook/">http://www.commandprompt.com/ppbook/</A>.
|
|
|
|
|
There are a number of PostgreSQL books available for purchase. One
|
|
|
|
|
of the most popular ones is by Korry Douglas. A list of book reviews
|
|
|
|
|
can be found at <A href=
|
|
|
|
|
"http://techdocs.postgresql.org/techdocs/bookreviews.php">http://techdocs.PostgreSQL.org/techdocs/bookreviews.php</A>.
|
|
|
|
|
There is also a collection of PostgreSQL technical articles at <A
|
|
|
|
|
href=
|
|
|
|
|
"http://techdocs.PostgreSQL.org/">http://techdocs.PostgreSQL.org/</A>.</P>
|
|
|
|
|
|
|
|
|
|
<P>The command line client program <I>psql</I> has some \d commands to show
|
|
|
|
|
information about types, operators, functions, aggregates, etc. - use \? to
|
|
|
|
|
display the available commands.</P>
|
|
|
|
|
|
|
|
|
|
<P>Our web site contains even more documentation.</P>
|
|
|
|
|
|
|
|
|
|
<H3 id="item1.11">1.11) How can I learn
|
|
|
|
|
<SMALL>SQL</SMALL>?</H3>
|
|
|
|
|
|
|
|
|
|
<P>First, consider the PostgreSQL-specific books mentioned above.
|
|
|
|
|
Another one is "Teach Yourself SQL in 21 Days, Second Edition"
|
|
|
|
|
at <A href=
|
|
|
|
|
"http://members.tripod.com/er4ebus/sql/index.htm">http://members.tripod.com/er4ebus/sql/index.htm</A>.
|
|
|
|
|
Many of our users like <I>The Practical SQL Handbook</I>,
|
|
|
|
|
Bowman, Judith S., et al., Addison-Wesley. Others like <I>The
|
|
|
|
|
Complete Reference SQL</I>, Groff et al., McGraw-Hill.</P>
|
|
|
|
|
|
|
|
|
|
<P>There are also many nice tutorials available online:
|
|
|
|
|
<UL>
|
|
|
|
|
<LI> <A href="http://www.intermedia.net/support/sql/sqltut.shtm">http://www.intermedia.net/support/sql/sqltut.shtm</A>
|
|
|
|
|
</LI>
|
|
|
|
|
<LI>
|
|
|
|
|
<A href="http://sqlcourse.com/">http://sqlcourse.com</A>
|
|
|
|
|
</LI>
|
|
|
|
|
<LI>
|
|
|
|
|
<A href="http://www.w3schools.com/sql/default.asp">http://www.w3schools.com/sql/default.asp</A>
|
|
|
|
|
</LI>
|
|
|
|
|
<LI><A href=
|
|
|
|
|
"http://mysite.verizon.net/Graeme_Birchall/id1.html">http://mysite.verizon.net/Graeme_Birchall/id1.html</A>
|
|
|
|
|
</LI>
|
|
|
|
|
</UL>
|
|
|
|
|
|
|
|
|
|
<H3 id="item1.12">1.12) How do I submit a patch or join the development
|
|
|
|
|
team?</H3>
|
|
|
|
|
|
|
|
|
|
<P>See the <a href="http://www.postgresql.org/docs/faqs.FAQ_DEV.html">
|
|
|
|
|
Developer's FAQ</A>.</P>
|
|
|
|
|
|
|
|
|
|
<H3 id="item1.13">1.13) How does PostgreSQL compare to other
|
|
|
|
|
<SMALL>DBMS</SMALL>s?</H3>
|
|
|
|
|
|
|
|
|
|
<P>There are several ways of measuring software: features,
|
|
|
|
|
performance, reliability, support, and price.</P>
|
|
|
|
|
|
|
|
|
|
<DL>
|
|
|
|
|
<DT><B>Features</B></DT>
|
|
|
|
|
|
|
|
|
|
<DD>PostgreSQL has most features present in large commercial
|
|
|
|
|
<SMALL>DBMS</SMALL>s, like transactions, subselects, triggers,
|
|
|
|
|
views, foreign key referential integrity, and sophisticated
|
|
|
|
|
locking. We have some features they do not have, like
|
|
|
|
|
user-defined types, inheritance, rules, and multi-version
|
|
|
|
|
concurrency control to reduce lock contention.<BR>
|
|
|
|
|
<BR>
|
|
|
|
|
</DD>
|
|
|
|
|
|
|
|
|
|
<DT><B>Performance</B></DT>
|
|
|
|
|
|
|
|
|
|
<DD>PostgreSQL's performance is comparable to other commercial and
|
|
|
|
|
open source databases. It is faster for some things, slower for
|
|
|
|
|
others. Our performance is usually +/-10% compared to other databases.
|
|
|
|
|
<BR>
|
|
|
|
|
</DD>
|
|
|
|
|
|
|
|
|
|
<DT><B>Reliability</B></DT>
|
|
|
|
|
|
|
|
|
|
<DD>We realize that a <SMALL>DBMS</SMALL> must be reliable, or it
|
|
|
|
|
is worthless. We strive to release well-tested, stable code that
|
|
|
|
|
has a minimum of bugs. Each release has at least one month of
|
|
|
|
|
beta testing, and our release history shows that we can provide
|
|
|
|
|
stable, solid releases that are ready for production use. We
|
|
|
|
|
believe we compare favorably to other database software in this
|
|
|
|
|
area.<BR>
|
|
|
|
|
<BR>
|
|
|
|
|
</DD>
|
|
|
|
|
|
|
|
|
|
<DT><B>Support</B></DT>
|
|
|
|
|
|
|
|
|
|
<DD>Our mailing lists provide contact with a large group of developers
|
|
|
|
|
and users to help resolve any problems encountered. While we cannot
|
|
|
|
|
guarantee a fix, commercial <SMALL>DBMS</SMALL>s do not always
|
|
|
|
|
supply a fix either. Direct access to developers, the user
|
|
|
|
|
community, manuals, and the source code often make PostgreSQL
|
|
|
|
|
support superior to other <SMALL>DBMS</SMALL>s. There is
|
|
|
|
|
commercial per-incident support available for those who need it.
|
|
|
|
|
(See <A href="#item1.7">FAQ section 1.7</A>.)<BR>
|
|
|
|
|
<BR>
|
|
|
|
|
</DD>
|
|
|
|
|
|
|
|
|
|
<DT><B>Price</B></DT>
|
|
|
|
|
|
|
|
|
|
<DD>We are free for all use, both commercial and non-commercial.
|
|
|
|
|
You can add our code to your product with no limitations, except
|
|
|
|
|
those outlined in our BSD-style license stated above.<BR>
|
|
|
|
|
<BR>
|
|
|
|
|
</DD>
|
|
|
|
|
</DL>
|
|
|
|
|
|
|
|
|
|
<HR>
|
|
|
|
|
|
|
|
|
|
<H2 align="center">User Client Questions</H2>
|
|
|
|
|
|
|
|
|
|
<H3 id="item2.1">2.1) What interfaces are available for
|
|
|
|
|
PostgreSQL?</H3>
|
|
|
|
|
|
|
|
|
|
<P>The PostgreSQL install includes only the <SMALL>C</SMALL> and embedded
|
|
|
|
|
<SMALL>C</SMALL> interfaces. All other interfaces are independent projects
|
|
|
|
|
that are downloaded separately; being separate allows them to have their
|
|
|
|
|
own release schedule and development teams.</P>
|
|
|
|
|
|
|
|
|
|
<P>Some programming languages like <SMALL>PHP</SMALL> include an
|
|
|
|
|
interface to PostgreSQL. Interfaces for languages like Perl,
|
|
|
|
|
<SMALL>TCL</SMALL>, Python, and many others are available at
|
|
|
|
|
<a href="http://gborg.postgresql.org">http://gborg.postgresql.org</A>
|
|
|
|
|
in the <I>Drivers/Interfaces</I> section and via Internet search.
|
|
|
|
|
</P>
|
|
|
|
|
|
|
|
|
|
<H3 id="item2.2">2.2) What tools are available for using
|
|
|
|
|
PostgreSQL with Web pages?</H3>
|
|
|
|
|
|
|
|
|
|
<P>A nice introduction to Database-backed Web pages can be seen at:
|
|
|
|
|
<A href="http://www.webreview.com">http://www.webreview.com</A></P>
|
|
|
|
|
|
|
|
|
|
<P>For Web integration, PHP (<A
|
|
|
|
|
href="http://www.php.net">http://www.php.net</A>) is an excellent
|
|
|
|
|
interface.</P>
|
|
|
|
|
|
|
|
|
|
<P>For complex cases, many use the Perl and DBD::Pg with CGI.pm or
|
|
|
|
|
mod_perl.</P>
|
|
|
|
|
|
|
|
|
|
<H3 id="item2.3">2.3) Does PostgreSQL have a graphical user
|
|
|
|
|
interface?</H3>
|
|
|
|
|
|
|
|
|
|
<P>There are a large number of GUI Tools that are available for PostgreSQL
|
|
|
|
|
from both commercial and open source developers. A detailed list can be
|
|
|
|
|
found in the <A href="http://www.postgresql.org/docs/techdocs.54">
|
|
|
|
|
PostgreSQL Community Documentation</A></P>
|
|
|
|
|
|
|
|
|
|
<HR>
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
<H2 align="center">Administrative Questions</H2>
|
|
|
|
|
|
|
|
|
|
<H3 id="item3.1">3.1) How do I install PostgreSQL somewhere
|
|
|
|
|
other than <I>/usr/local/pgsql</I>?</H3>
|
|
|
|
|
|
|
|
|
|
<P>Specify the <I>--prefix</I> option when running
|
|
|
|
|
<I>configure</I>.</P>
|
|
|
|
|
|
|
|
|
|
<H3 id="item3.2">3.2) How do I control connections from other
|
|
|
|
|
hosts?</H3>
|
|
|
|
|
|
|
|
|
|
<P>By default, PostgreSQL only allows connections from the local
|
|
|
|
|
machine using Unix domain sockets or TCP/IP connections. Other
|
|
|
|
|
machines will not be able to connect unless you modify
|
|
|
|
|
<I>listen_addresses</I> in the <I>postgresql.conf</I> file, enable
|
|
|
|
|
host-based authentication by modifying the
|
|
|
|
|
<I>$PGDATA/pg_hba.conf</I> file, and restart the server.</P>
|
|
|
|
|
|
|
|
|
|
<H3 id="item3.3">3.3) How do I tune the database engine for
|
|
|
|
|
better performance?</H3>
|
|
|
|
|
|
|
|
|
|
<P>There are three major areas for potential performance
|
|
|
|
|
improvement:</P>
|
|
|
|
|
|
|
|
|
|
<DL>
|
|
|
|
|
<DT><B>Query Changes</B></DT>
|
|
|
|
|
|
|
|
|
|
<DD>This involves modifying queries to obtain better
|
|
|
|
|
performance:
|
|
|
|
|
<ul>
|
|
|
|
|
<li>Creation of indexes, including expression and partial
|
|
|
|
|
indexes</li>
|
|
|
|
|
<li>Use of COPY instead of multiple <SMALL>INSERT</SMALL>s</li>
|
|
|
|
|
<li>Grouping of multiple statements into a single transaction to
|
|
|
|
|
reduce commit overhead</li>
|
|
|
|
|
<li>Use of <SMALL>CLUSTER</SMALL> when retrieving many rows from an
|
|
|
|
|
index</li>
|
|
|
|
|
<li>Use of <SMALL>LIMIT</SMALL> for returning a subset of a query's
|
|
|
|
|
output</li>
|
|
|
|
|
<li>Use of Prepared queries</li>
|
|
|
|
|
<li>Use of <SMALL>ANALYZE</SMALL> to maintain accurate optimizer
|
|
|
|
|
statistics</li>
|
|
|
|
|
<li>Regular use of <SMALL>VACUUM</SMALL> or <I>pg_autovacuum</I></li>
|
|
|
|
|
<li>Dropping of indexes during large data changes</li>
|
|
|
|
|
</ul><BR>
|
|
|
|
|
<BR>
|
|
|
|
|
</DD>
|
|
|
|
|
|
|
|
|
|
<DT><B>Server Configuration</B></DT>
|
|
|
|
|
|
|
|
|
|
<DD>A number of <I>postgresql.conf</I> settings affect performance.
|
|
|
|
|
For more details, see <a href=
|
|
|
|
|
"http://www.postgresql.org/docs/current/static/runtime.html">
|
|
|
|
|
Administration Guide/Server Run-time Environment/Run-time
|
|
|
|
|
Configuration</a> for a full listing, and for commentary see <a
|
|
|
|
|
href="http://www.varlena.com/varlena/GeneralBits/Tidbits/annotated_conf_e.html">
|
|
|
|
|
http://www.varlena.com/varlena/GeneralBits/Tidbits/annotated_conf_e.html</a>
|
|
|
|
|
and <a href="http://www.varlena.com/varlena/GeneralBits/Tidbits/perf.html">
|
|
|
|
|
http://www.varlena.com/varlena/GeneralBits/Tidbits/perf.html</a>.
|
|
|
|
|
<BR>
|
|
|
|
|
<BR>
|
|
|
|
|
</DD>
|
|
|
|
|
|
|
|
|
|
<DT><B>Hardware Selection</B></DT>
|
|
|
|
|
|
|
|
|
|
<DD>The effect of hardware on performance is detailed in <a
|
|
|
|
|
href="http://candle.pha.pa.us/main/writings/pgsql/hw_performance/index.html">
|
|
|
|
|
http://candle.pha.pa.us/main/writings/pgsql/hw_performance/index.html</a> and
|
|
|
|
|
<a href="http://www.powerpostgresql.com/PerfList/">
|
|
|
|
|
http://www.powerpostgresql.com/PerfList/</a>.
|
|
|
|
|
<BR>
|
|
|
|
|
<BR>
|
|
|
|
|
</DD>
|
|
|
|
|
</DL>
|
|
|
|
|
|
|
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|
|
<H3 id="item3.4">3.4) What debugging features are
|
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|
|
|
available?</H3>
|
|
|
|
|
|
|
|
|
|
<P>There are many <CODE>log_*</CODE> server configuration variables
|
|
|
|
|
that enable printing of query and process statistics which can be
|
|
|
|
|
very useful for debugging and performance measurements.</P>
|
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|
|
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|
|
|
|
<H3 id="item3.5">3.5) Why do I get <I>"Sorry, too many
|
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|
|
|
clients"</I> when trying to connect?</H3>
|
|
|
|
|
|
|
|
|
|
<P>You have reached the default limit is 100 database sessions. You
|
|
|
|
|
need to increase the <I>postmaster</I>'s limit on how many
|
|
|
|
|
concurrent backend processes it can start by changing the
|
|
|
|
|
<I>max_connections</I> value in <I>postgresql.conf</I> and
|
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|
|
|
restarting the <I>postmaster</I>.</P>
|
|
|
|
|
|
|
|
|
|
<H3 id="item3.6">3.6) What is the upgrade process for PostgreSQL?</H3>
|
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|
|
|
|
|
|
|
|
<P>The PostgreSQL team makes only bug fixes in minor releases,
|
|
|
|
|
so, for example, upgrading from 7.4.8 to 7.4.9 does not require
|
|
|
|
|
a dump and restore; merely stop the database server, install
|
|
|
|
|
the updated binaries, and restart the server.</P>
|
|
|
|
|
|
|
|
|
|
<P>All users should upgrade to the most recent minor release as
|
|
|
|
|
soon as it is available. While every upgrade has some risk,
|
|
|
|
|
PostgreSQL minor releases are designed to fix only common bugs
|
|
|
|
|
with the least risk. The community considers <i>not</i> upgrading
|
|
|
|
|
more risky that upgrading.</P>
|
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|
|
|
|
|
|
|
|
<P>Major releases (e.g. from 7.3 to 7.4) often change the internal
|
|
|
|
|
format of system tables and data files. These changes are often complex,
|
|
|
|
|
so we don't maintain backward compatibility for data files. A dump/reload
|
|
|
|
|
of the database is required for major upgrades.</P>
|
|
|
|
|
|
|
|
|
|
<H3 id="item3.7">3.7) What computer hardware should I use?</H3>
|
|
|
|
|
|
|
|
|
|
<P>Because PC hardware is mostly compatible, people tend to believe that
|
|
|
|
|
all PC hardware is of equal quality. It is not. ECC RAM, SCSI, and
|
|
|
|
|
quality motherboards are more reliable and have better performance than
|
|
|
|
|
less expensive hardware. PostgreSQL will run on almost any hardware,
|
|
|
|
|
but if reliability and performance are important it is wise to
|
|
|
|
|
research your hardware options thoroughly. Our email lists can be used
|
|
|
|
|
to discuss hardware options and tradeoffs.</P>
|
|
|
|
|
|
|
|
|
|
<HR>
|
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|
|
|
|
|
|
|
|
<H2 align="center">Operational Questions</H2>
|
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|
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|
|
<H3 id="item4.1">4.1) How do I <SMALL>SELECT</SMALL> only the
|
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|
|
|
first few rows of a query? A random row?</H3>
|
|
|
|
|
|
|
|
|
|
<P>To retrieve only a few rows, if you know at the number of rows
|
|
|
|
|
needed at the time of the <SMALL>SELECT</SMALL> use
|
|
|
|
|
<SMALL>LIMIT</SMALL> . If an index matches the <SMALL>ORDER
|
|
|
|
|
BY</SMALL> it is possible the entire query does not have to be
|
|
|
|
|
executed. If you don't know the number of rows at
|
|
|
|
|
<SMALL>SELECT</SMALL> time, use a cursor and
|
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|
|
|
<SMALL>FETCH</SMALL>.</P>
|
|
|
|
|
|
|
|
|
|
<P>To <SMALL>SELECT</SMALL> a random row, use:</P>
|
|
|
|
|
<PRE>
|
|
|
|
|
SELECT col
|
|
|
|
|
FROM tab
|
|
|
|
|
ORDER BY random()
|
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|
|
LIMIT 1;
|
|
|
|
|
</PRE>
|
|
|
|
|
|
|
|
|
|
<H3 id="item4.2">4.2) How do I find out what tables, indexes,
|
|
|
|
|
databases, and users are defined? How do I see the queries used
|
|
|
|
|
by <I>psql</I> to display them?</H3>
|
|
|
|
|
|
|
|
|
|
<P>Use the \dt command to see tables in <I>psql</I>. For a complete list of
|
|
|
|
|
commands inside psql you can use \?. Alternatively you can read the source
|
|
|
|
|
code for <I>psql</I> in file <I>pgsql/src/bin/psql/describe.c</I>, it
|
|
|
|
|
contains <SMALL>SQL</SMALL> commands that generate the output for
|
|
|
|
|
<I>psql</I>'s backslash commands. You can also start <I>psql</I> with the
|
|
|
|
|
<I>-E</I> option so it will print out the queries it uses to execute the
|
|
|
|
|
commands you give. PostgreSQL also provides an <SMALL>SQL</SMALL> compliant
|
|
|
|
|
INFORMATION SCHEMA interface you can query to get information about the
|
|
|
|
|
database.</P>
|
|
|
|
|
|
|
|
|
|
<P>There are also system tables beginning with <I>pg_</I> that describe
|
|
|
|
|
these too.</P>
|
|
|
|
|
|
|
|
|
|
<P>Use <I>psql -l</I> will list all databases.</P>
|
|
|
|
|
|
|
|
|
|
<P>Also try the file <I>pgsql/src/tutorial/syscat.source</I>. It
|
|
|
|
|
illustrates many of the <SMALL>SELECT</SMALL>s needed to get
|
|
|
|
|
information from the database system tables.</P>
|
|
|
|
|
|
|
|
|
|
<H3 id="item4.3">4.3) How do you change a column's data type?</H3>
|
|
|
|
|
|
|
|
|
|
<P>Changing the data type of a column can be done easily in 8.0
|
|
|
|
|
and later with <SMALL>ALTER TABLE ALTER COLUMN TYPE</SMALL>.</P>
|
|
|
|
|
|
|
|
|
|
<P>In earlier releases, do this:</P>
|
|
|
|
|
<PRE>
|
|
|
|
|
BEGIN;
|
|
|
|
|
ALTER TABLE tab ADD COLUMN new_col <i>new_data_type</i>;
|
|
|
|
|
UPDATE tab SET new_col = CAST(old_col AS <i>new_data_type</i>);
|
|
|
|
|
ALTER TABLE tab DROP COLUMN old_col;
|
|
|
|
|
COMMIT;
|
|
|
|
|
</PRE>
|
|
|
|
|
<P>You might then want to do <I>VACUUM FULL tab</I> to reclaim the
|
|
|
|
|
disk space used by the expired rows.</P>
|
|
|
|
|
|
|
|
|
|
<H3 id="item4.4">4.4) What is the maximum size for a row, a
|
|
|
|
|
table, and a database?</H3>
|
|
|
|
|
|
|
|
|
|
<P>These are the limits:</P>
|
|
|
|
|
<BLOCKQUOTE>
|
|
|
|
|
<TABLE>
|
|
|
|
|
<TR><TD>Maximum size for a database?</TD><TD>unlimited (32 TB databases
|
|
|
|
|
exist)</TD></TR>
|
|
|
|
|
<TR><TD>Maximum size for a table?</TD><TD>32 TB</TD></TR>
|
|
|
|
|
<TR><TD>Maximum size for a row?</TD><TD>400 GB</TD></TR>
|
|
|
|
|
<TR><TD>Maximum size for a field?</TD><TD>1 GB</TD></TR>
|
|
|
|
|
<TR><TD>Maximum number of rows in a table?</TD><TD>unlimited</TD></TR>
|
|
|
|
|
<TR><TD>Maximum number of columns in a table?</TD><TD>250-1600 depending
|
|
|
|
|
on column types</TD></TR>
|
|
|
|
|
<TR><TD>Maximum number of indexes on a
|
|
|
|
|
table?</TD><TD>unlimited</TD></TR>
|
|
|
|
|
</TABLE>
|
|
|
|
|
</BLOCKQUOTE>
|
|
|
|
|
|
|
|
|
|
<P>Of course, these are not actually unlimited, but limited to
|
|
|
|
|
available disk space and memory/swap space. Performance may suffer
|
|
|
|
|
when these values get unusually large.</P>
|
|
|
|
|
|
|
|
|
|
<P>The maximum table size of 32 TB does not require large file
|
|
|
|
|
support from the operating system. Large tables are stored as
|
|
|
|
|
multiple 1 GB files so file system size limits are not
|
|
|
|
|
important.</P>
|
|
|
|
|
|
|
|
|
|
<P>The maximum table size, row size, and maximum number of columns
|
|
|
|
|
can be quadrupled by increasing the default block size to 32k. The
|
|
|
|
|
maximum table size can also be increased using table partitioning.</P>
|
|
|
|
|
|
|
|
|
|
<P>One limitation is that indexes can not be created on columns
|
|
|
|
|
longer than about 2,000 characters. Fortunately, such indexes are
|
|
|
|
|
rarely needed. Uniqueness is best guaranteed by a function index
|
|
|
|
|
of an MD5 hash of the long column, and full text indexing
|
|
|
|
|
allows for searching of words within the column.</P>
|
|
|
|
|
|
|
|
|
|
<H3 id="item4.5">4.5) How much database disk space is required
|
|
|
|
|
to store data from a typical text file?</H3>
|
|
|
|
|
|
|
|
|
|
<P>A PostgreSQL database may require up to five times the disk
|
|
|
|
|
space to store data from a text file.</P>
|
|
|
|
|
|
|
|
|
|
<P>As an example, consider a file of 100,000 lines with an integer
|
|
|
|
|
and text description on each line. Suppose the text string
|
|
|
|
|
avergages twenty bytes in length. The flat file would be 2.8 MB.
|
|
|
|
|
The size of the PostgreSQL database file containing this data can
|
|
|
|
|
be estimated as 5.6 MB:</P>
|
|
|
|
|
<PRE>
|
|
|
|
|
28 bytes: each row header (approximate)
|
|
|
|
|
24 bytes: one int field and one text field
|
|
|
|
|
+ 4 bytes: pointer on page to tuple
|
|
|
|
|
----------------------------------------
|
|
|
|
|
56 bytes per row
|
|
|
|
|
|
|
|
|
|
The data page size in PostgreSQL is 8192 bytes (8 KB), so:
|
|
|
|
|
|
|
|
|
|
8192 bytes per page
|
|
|
|
|
------------------- = 146 rows per database page (rounded down)
|
|
|
|
|
56 bytes per row
|
|
|
|
|
|
|
|
|
|
100000 data rows
|
|
|
|
|
-------------------- = 685 database pages (rounded up)
|
|
|
|
|
146 rows per page
|
|
|
|
|
|
|
|
|
|
685 database pages * 8192 bytes per page = 5,611,520 bytes (5.6 MB)
|
|
|
|
|
</PRE>
|
|
|
|
|
|
|
|
|
|
<P>Indexes do not require as much overhead, but do contain the data
|
|
|
|
|
that is being indexed, so they can be large also.</P>
|
|
|
|
|
|
|
|
|
|
<P><SMALL>NULL</SMALL>s are stored as bitmaps, so they
|
|
|
|
|
use very little space.</P>
|
|
|
|
|
|
|
|
|
|
<H3 id="item4.6">4.6) Why are my queries slow? Why don't they
|
|
|
|
|
use my indexes?</H3>
|
|
|
|
|
|
|
|
|
|
<P>Indexes are not used by every query. Indexes are used only if the
|
|
|
|
|
table is larger than a minimum size, and the query selects only a
|
|
|
|
|
small percentage of the rows in the table. This is because the random
|
|
|
|
|
disk access caused by an index scan can be slower than a straight read
|
|
|
|
|
through the table, or sequential scan. </P>
|
|
|
|
|
|
|
|
|
|
<P>To determine if an index should be used, PostgreSQL must have
|
|
|
|
|
statistics about the table. These statistics are collected using
|
|
|
|
|
<SMALL>VACUUM ANALYZE</SMALL>, or simply <SMALL>ANALYZE</SMALL>.
|
|
|
|
|
Using statistics, the optimizer knows how many rows are in the
|
|
|
|
|
table, and can better determine if indexes should be used.
|
|
|
|
|
Statistics are also valuable in determining optimal join order and
|
|
|
|
|
join methods. Statistics collection should be performed
|
|
|
|
|
periodically as the contents of the table change.</P>
|
|
|
|
|
|
|
|
|
|
<P>Indexes are normally not used for <SMALL>ORDER BY</SMALL> or to
|
|
|
|
|
perform joins. A sequential scan followed by an explicit sort is
|
|
|
|
|
usually faster than an index scan of a large table.
|
|
|
|
|
However, <SMALL>LIMIT</SMALL> combined with <SMALL>ORDER BY</SMALL>
|
|
|
|
|
often will use an index because only a small portion of the table
|
|
|
|
|
is returned.</P>
|
|
|
|
|
|
|
|
|
|
<P>If you believe the optimizer is incorrect in choosing a
|
|
|
|
|
sequential scan, use <CODE>SET enable_seqscan TO 'off'</CODE> and
|
|
|
|
|
run query again to see if an index scan is indeed faster.</P>
|
|
|
|
|
|
|
|
|
|
<P>When using wild-card operators such as <SMALL>LIKE</SMALL> or
|
|
|
|
|
<I>~</I>, indexes can only be used in certain circumstances:</P>
|
|
|
|
|
<UL>
|
|
|
|
|
<LI>The beginning of the search string must be anchored to the start
|
|
|
|
|
of the string, i.e.
|
|
|
|
|
<UL>
|
|
|
|
|
<LI><SMALL>LIKE</SMALL> patterns must not start with <I>%</I>.</LI>
|
|
|
|
|
<LI><I>~</I> (regular expression) patterns must start with
|
|
|
|
|
<I>^</I>.</LI>
|
|
|
|
|
</UL></LI>
|
|
|
|
|
<LI>The search string can not start with a character class,
|
|
|
|
|
e.g. [a-e].</LI>
|
|
|
|
|
<LI>Case-insensitive searches such as <SMALL>ILIKE</SMALL> and
|
|
|
|
|
<I>~*</I> do not utilize indexes. Instead, use expression
|
|
|
|
|
indexes, which are described in section <a href="#item4.8">4.8</a>.</LI>
|
|
|
|
|
<LI>The default <I>C</I> locale must be used during
|
|
|
|
|
<i>initdb</i> because it is not possible to know the next-greatest
|
|
|
|
|
character in a non-C locale. You can create a special
|
|
|
|
|
<CODE>text_pattern_ops</CODE> index for such cases that work only
|
|
|
|
|
for <SMALL>LIKE</SMALL> indexing.
|
|
|
|
|
</LI>
|
|
|
|
|
</UL>
|
|
|
|
|
|
|
|
|
|
<P>In pre-8.0 releases, indexes often can not be used unless the data
|
|
|
|
|
types exactly match the index's column types. This was particularly
|
|
|
|
|
true of int2, int8, and numeric column indexes.</P>
|
|
|
|
|
|
|
|
|
|
<H3 id="item4.7">4.7) How do I see how the query optimizer is
|
|
|
|
|
evaluating my query?</H3>
|
|
|
|
|
|
|
|
|
|
<P>See the <SMALL>EXPLAIN</SMALL> manual page.</P>
|
|
|
|
|
|
|
|
|
|
<H3 id="item4.8">4.8) How do I perform regular expression
|
|
|
|
|
searches and case-insensitive regular expression searches? How do I
|
|
|
|
|
use an index for case-insensitive searches?</H3>
|
|
|
|
|
|
|
|
|
|
<P>The <I>~</I> operator does regular expression matching, and
|
|
|
|
|
<I>~*</I> does case-insensitive regular expression matching. The
|
|
|
|
|
case-insensitive variant of <SMALL>LIKE</SMALL> is called
|
|
|
|
|
<SMALL>ILIKE</SMALL>.</P>
|
|
|
|
|
|
|
|
|
|
<P>Case-insensitive equality comparisons are normally expressed
|
|
|
|
|
as:</P>
|
|
|
|
|
<PRE>
|
|
|
|
|
SELECT *
|
|
|
|
|
FROM tab
|
|
|
|
|
WHERE lower(col) = 'abc';
|
|
|
|
|
</PRE>
|
|
|
|
|
This will not use an standard index. However, if you create an
|
|
|
|
|
expression index, it will be used:
|
|
|
|
|
<PRE>
|
|
|
|
|
CREATE INDEX tabindex ON tab (lower(col));
|
|
|
|
|
</PRE>
|
|
|
|
|
<P>If the above index is created as <SMALL>UNIQUE</SMALL>, though
|
|
|
|
|
the column can store upper and lowercase characters, it can not have
|
|
|
|
|
identical values that differ only in case. To force a particular
|
|
|
|
|
case to be stored in the column, use a <SMALL>CHECK</SMALL>
|
|
|
|
|
constraint or a trigger.</P>
|
|
|
|
|
|
|
|
|
|
<H3 id="item4.9">4.9) In a query, how do I detect if a field
|
|
|
|
|
is <SMALL>NULL</SMALL>? How can I sort on whether a field is <SMALL>
|
|
|
|
|
NULL</SMALL> or not?</H3>
|
|
|
|
|
|
|
|
|
|
<P>You test the column with <SMALL>IS NULL</SMALL> and <SMALL>IS
|
|
|
|
|
NOT NULL</SMALL>, like this:</P>
|
|
|
|
|
|
|
|
|
|
<PRE>
|
|
|
|
|
SELECT *
|
|
|
|
|
FROM tab
|
|
|
|
|
WHERE col IS NULL;
|
|
|
|
|
</PRE>
|
|
|
|
|
|
|
|
|
|
<P>To sort by the <SMALL>NULL</SMALL> status, use the <SMALL>IS NULL</SMALL>
|
|
|
|
|
and <SMALL>IS NOT NULL</SMALL> modifiers in your <SMALL>ORDER BY</SMALL> clause.
|
|
|
|
|
Things that are <I>true</I> will sort higher than things that are <I>false</I>,
|
|
|
|
|
so the following will put NULL entries at the top of the resulting list:</P>
|
|
|
|
|
|
|
|
|
|
<PRE>
|
|
|
|
|
SELECT *
|
|
|
|
|
FROM tab
|
|
|
|
|
ORDER BY (col IS NOT NULL)
|
|
|
|
|
</PRE>
|
|
|
|
|
|
|
|
|
|
<H3 id="item4.10">4.10) What is the difference between the
|
|
|
|
|
various character types?</H3>
|
|
|
|
|
<BLOCKQUOTE>
|
|
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<TABLE>
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<TR><TH>Type</TH><TH>Internal Name</TH><TH>Notes</TH></TR>
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<TR><TD>VARCHAR(n)</TD><TD>varchar</TD><TD>size specifies maximum
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length, no padding</TD></TR>
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<TR><TD>CHAR(n)</TD><TD>bpchar</TD><TD>blank padded to the specified
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fixed length</TD></TR>
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<TR><TD>TEXT</TD><TD>text</TD><TD>no specific upper limit on
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length</TD></TR>
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<TR><TD>BYTEA</TD><TD>bytea</TD><TD>variable-length byte array
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(null-byte safe)</TD></TR>
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<TR><TD>"char"</TD><TD>char</TD><TD>one character</TD></TR>
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</TABLE>
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</BLOCKQUOTE>
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<P>You will see the internal name when examining system catalogs
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and in some error messages.</P>
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<P>The first four types above are "varlena" types (i.e., the first
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four bytes on disk are the length, followed by the data). Thus the
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actual space used is slightly greater than the declared size.
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However, long values are also subject to compression, so the space
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on disk might also be less than expected.</P>
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<SMALL>VARCHAR(n)</SMALL> is best when storing variable-length
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strings and it limits how long a string can be. <SMALL>TEXT</SMALL>
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is for strings of unlimited length, with a maximum of one gigabyte.
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<P><SMALL>CHAR(n)</SMALL> is for storing strings that are all the
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same length. <SMALL>CHAR(n)</SMALL> pads with blanks to the specified
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length, while <SMALL>VARCHAR(n)</SMALL> only stores the characters
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supplied. <SMALL>BYTEA</SMALL> is for storing binary data,
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particularly values that include <SMALL>NULL</SMALL> bytes. All the
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types described here have similar performance characteristics.</P>
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<H3 id="item4.11.1">4.11.1) How do I create a
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serial/auto-incrementing field?</H3>
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<P>PostgreSQL supports a <SMALL>SERIAL</SMALL> data type. It
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auto-creates a sequence. For example, this:</P>
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<PRE>
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CREATE TABLE person (
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id SERIAL,
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name TEXT
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);
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</PRE>
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is automatically translated into this:
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<PRE>
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CREATE SEQUENCE person_id_seq;
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CREATE TABLE person (
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id INT4 NOT NULL DEFAULT nextval('person_id_seq'),
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name TEXT
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);
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</PRE>
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See the <I>create_sequence</I> manual page for more information
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about sequences.
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<H3 id="item4.11.2">4.11.2) How do I get the value of a
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<SMALL>SERIAL</SMALL> insert?</H3>
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<P>One approach is to retrieve the next <SMALL>SERIAL</SMALL> value
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from the sequence object with the <I>nextval()</I> function
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<I>before</I> inserting and then insert it explicitly. Using the
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example table in <A href="#item4.11.1">4.11.1</A>, an example in a
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pseudo-language would look like this:</P>
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<PRE>
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new_id = execute("SELECT nextval('person_id_seq')");
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execute("INSERT INTO person (id, name) VALUES (new_id, 'Blaise Pascal')");
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</PRE>
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You would then also have the new value stored in <CODE>new_id</CODE>
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for use in other queries (e.g., as a foreign key to the <CODE>person
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</CODE> table). Note that the name of the automatically created
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<SMALL>SEQUENCE</SMALL> object will be named <<I>table</I>>_<<I>
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serialcolumn</I>>_<I>seq</I>, where <I>table</I> and <I>serialcolumn</I>
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are the names of your table and your <SMALL>SERIAL</SMALL> column,
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respectively.
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<P>Alternatively, you could retrieve the assigned <SMALL>SERIAL</SMALL>
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value with the <I>currval()</I> function <I>after</I> it was inserted by
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default, e.g.,</P>
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<PRE>
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execute("INSERT INTO person (name) VALUES ('Blaise Pascal')");
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new_id = execute("SELECT currval('person_id_seq')");
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|
</PRE>
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<H3 id="item4.11.3">4.11.3) Doesn't <I>currval()</I>
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lead to a race condition with other users?</H3>
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<P>No. <I>currval()</I> returns the current value assigned by your
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session, not by all sessions.</P>
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<H3 id="item4.11.4">4.11.4) Why aren't my sequence numbers
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reused on transaction abort? Why are there gaps in the numbering of
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my sequence/SERIAL column?</H3>
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<P>To improve concurrency, sequence values are given out to running
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transactions as needed and are not locked until the transaction
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completes. This causes gaps in numbering from aborted
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transactions.</P>
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|
<H3 id="item4.12">4.12) What is an <SMALL>OID</SMALL>? What is
|
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|
a <SMALL>CTID</SMALL>?</H3>
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|
<P>Every row that is created in PostgreSQL gets a unique
|
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|
<SMALL>OID</SMALL> unless created <SMALL>WITHOUT OIDS</SMALL>.
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|
O<SMALL>ID</SMALL>s are automatically assigned unique 4-byte
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|
integers that are unique across the entire installation. However,
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|
they overflow at 4 billion, and then the O<SMALL>ID</SMALL>s start
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|
being duplicated. PostgreSQL uses <SMALL>OID</SMALL>s to link its
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|
|
internal system tables together.</P>
|
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|
<P>To uniquely number columns in user tables, it is best to use
|
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|
<SMALL>SERIAL</SMALL> rather than O<SMALL>ID</SMALL>s because
|
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|
<SMALL>SERIAL</SMALL> sequences are unique only within a single
|
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|
|
|
table. and are therefore less likely to overflow.
|
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|
|
<SMALL>SERIAL8</SMALL> is available for storing eight-byte sequence
|
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|
|
values.</P>
|
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|
<P>C<SMALL>TID</SMALL>s are used to identify specific physical rows
|
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|
|
|
with block and offset values. C<SMALL>TID</SMALL>s change after rows
|
|
|
|
|
are modified or reloaded. They are used by index entries to point
|
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|
|
to physical rows.</P>
|
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|
|
<H3 id="item4.13">4.13) Why do I get the error <I>"ERROR:
|
|
|
|
|
Memory exhausted in AllocSetAlloc()"</I>?</H3>
|
|
|
|
|
|
|
|
|
|
<P>You probably have run out of virtual memory on your system,
|
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|
|
|
or your kernel has a low limit for certain resources. Try this
|
|
|
|
|
before starting <I>postmaster</I>:</P>
|
|
|
|
|
<PRE>
|
|
|
|
|
ulimit -d 262144
|
|
|
|
|
limit datasize 256m
|
|
|
|
|
</PRE>
|
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|
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|
|
|
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|
|
Depending on your shell, only one of these may succeed, but it will
|
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|
|
|
set your process data segment limit much higher and perhaps allow
|
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|
|
|
the query to complete. This command applies to the current process,
|
|
|
|
|
and all subprocesses created after the command is run. If you are
|
|
|
|
|
having a problem with the <SMALL>SQL</SMALL> client because the
|
|
|
|
|
backend is returning too much data, try it before starting the
|
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|
|
|
client.
|
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|
|
|
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|
|
<H3 id="item4.14">4.14) How do I tell what PostgreSQL version
|
|
|
|
|
I am running?</H3>
|
|
|
|
|
|
|
|
|
|
<P>From <I>psql</I>, type <CODE>SELECT version();</CODE></P>
|
|
|
|
|
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|
|
<H3 id="item4.15">4.15) How do I create a column that will
|
|
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|
|
default to the current time?</H3>
|
|
|
|
|
|
|
|
|
|
<P>Use <I>CURRENT_TIMESTAMP</I>:</P>
|
|
|
|
|
<PRE>
|
|
|
|
|
CREATE TABLE test (x int, modtime TIMESTAMP DEFAULT CURRENT_TIMESTAMP );
|
|
|
|
|
</PRE>
|
|
|
|
|
|
|
|
|
|
<H3 id="item4.16">4.16) How do I perform an outer join?</H3>
|
|
|
|
|
|
|
|
|
|
<P>PostgreSQL supports outer joins using the SQL standard syntax.
|
|
|
|
|
Here are two examples:</P>
|
|
|
|
|
<PRE>
|
|
|
|
|
SELECT *
|
|
|
|
|
FROM t1 LEFT OUTER JOIN t2 ON (t1.col = t2.col);
|
|
|
|
|
</PRE>
|
|
|
|
|
or
|
|
|
|
|
<PRE>
|
|
|
|
|
SELECT *
|
|
|
|
|
FROM t1 LEFT OUTER JOIN t2 USING (col);
|
|
|
|
|
</PRE>
|
|
|
|
|
|
|
|
|
|
<P>These identical queries join t1.col to t2.col, and also return
|
|
|
|
|
any unjoined rows in t1 (those with no match in t2). A
|
|
|
|
|
<SMALL>RIGHT</SMALL> join would add unjoined rows of t2. A
|
|
|
|
|
<SMALL>FULL</SMALL> join would return the matched rows plus all
|
|
|
|
|
unjoined rows from t1 and t2. The word <SMALL>OUTER</SMALL> is
|
|
|
|
|
optional and is assumed in <SMALL>LEFT</SMALL>,
|
|
|
|
|
<SMALL>RIGHT</SMALL>, and <SMALL>FULL</SMALL> joins. Ordinary joins
|
|
|
|
|
are called <SMALL>INNER</SMALL> joins.</P>
|
|
|
|
|
|
|
|
|
|
<H3 id="item4.17">4.17) How do I perform queries using
|
|
|
|
|
multiple databases?</H3>
|
|
|
|
|
|
|
|
|
|
<P>There is no way to query a database other than the current one.
|
|
|
|
|
Because PostgreSQL loads database-specific system catalogs, it is
|
|
|
|
|
uncertain how a cross-database query should even behave.</P>
|
|
|
|
|
|
|
|
|
|
<P><I>contrib/dblink</I> allows cross-database queries using
|
|
|
|
|
function calls. Of course, a client can also make simultaneous
|
|
|
|
|
connections to different databases and merge the results on the
|
|
|
|
|
client side.</P>
|
|
|
|
|
|
|
|
|
|
<H3 id="item4.18">4.18) How do I return multiple rows or
|
|
|
|
|
columns from a function?</H3>
|
|
|
|
|
|
|
|
|
|
<P>It is easy using set-returning functions,
|
|
|
|
|
<a href="http://techdocs.postgresql.org/guides/SetReturningFunctions">
|
|
|
|
|
http://techdocs.postgresql.org/guides/SetReturningFunctions</a></P>.
|
|
|
|
|
|
|
|
|
|
<H3 id="item4.19">4.19) Why do I get "relation with OID #####
|
|
|
|
|
does not exist" errors when accessing temporary tables in PL/PgSQL
|
|
|
|
|
functions?</H3>
|
|
|
|
|
|
|
|
|
|
<P>PL/PgSQL caches function scripts, and an unfortunate side effect
|
|
|
|
|
is that if a PL/PgSQL function accesses a temporary table, and that
|
|
|
|
|
table is later dropped and recreated, and the function called again,
|
|
|
|
|
the function will fail because the cached function contents still
|
|
|
|
|
point to the old temporary table. The solution is to use
|
|
|
|
|
<SMALL>EXECUTE</SMALL> for temporary table access in PL/PgSQL. This
|
|
|
|
|
will cause the query to be reparsed every time.</P>
|
|
|
|
|
|
|
|
|
|
<H3 id="item4.20">4.20) What replication solutions are available?
|
|
|
|
|
</H3>
|
|
|
|
|
|
|
|
|
|
<P>Though "replication" is a single term, there are several technologies
|
|
|
|
|
for doing replication, with advantages and disadvantages for each.</P>
|
|
|
|
|
|
|
|
|
|
<P>Master/slave replication allows a single master to receive read/write
|
|
|
|
|
queries, while slaves can only accept read/<SMALL>SELECT</SMALL>
|
|
|
|
|
queries. The most popular freely available master-slave PostgreSQL
|
|
|
|
|
replication solution is <A
|
|
|
|
|
href="http://gborg.postgresql.org/project/slony1/projdisplay.php">
|
|
|
|
|
Slony-I</A>.</P>
|
|
|
|
|
|
|
|
|
|
<P>Multi-master replication allows read/write queries to be sent to
|
|
|
|
|
multiple replicated computers. This capability also has a severe impact
|
|
|
|
|
on performance due to the need to synchronize changes between servers.
|
|
|
|
|
<A href="http://pgfoundry.org/projects/pgcluster/">Pgcluster</a> is the
|
|
|
|
|
most popular such solution freely available for PostgreSQL.</P>
|
|
|
|
|
|
|
|
|
|
<P>There are also commercial and hardware-based replication solutions
|
|
|
|
|
available supporting a variety of replication models.</P>
|
|
|
|
|
|
|
|
|
|
<H3 id="item4.21">4.21) Why are my table and column names not
|
|
|
|
|
recognized in my query?</H3>
|
|
|
|
|
|
|
|
|
|
<P>The most common cause is the use of double-quotes around table or
|
|
|
|
|
column names during table creation. When double-quotes are used,
|
|
|
|
|
table and column names (called identifiers) are stored <a
|
|
|
|
|
href="http://www.postgresql.org/docs/8.0/static/sql-syntax.html#SQL-SYNTAX-IDENTIFIERS">
|
|
|
|
|
case-sensitive</a>, meaning you must use double-quotes when
|
|
|
|
|
referencing the names in a query. Some interfaces, like pgAdmin,
|
|
|
|
|
automatically double-quote identifiers during table creation. So,
|
|
|
|
|
for identifiers to be recognized, you must either:
|
|
|
|
|
<UL>
|
|
|
|
|
<LI>Avoid double-quoting identifiers when creating tables</LI>
|
|
|
|
|
<LI>Use only lowercase characters in identifiers</LI>
|
|
|
|
|
<LI>Double-quote identifiers when referencing them in queries</LI>
|
|
|
|
|
</UL>
|
|
|
|
|
</BODY>
|
|
|
|
|
</HTML>
|
