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<META name="generator" content="HTML Tidy, see www.w3.org">
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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: Tue Oct 17 00:21:20 EDT 2000</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>)<BR>
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</P>
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<P>The most recent version of this document can be viewed at <A
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href=
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"http://www.Postgresql.org/docs/faq-english.html">http://www.PostgreSQL.org/docs/faq-english.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/">http://www.PostgreSQL.org/docs/</A>.</P>
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<HR>
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<CENTER>
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<H2>General Questions</H2>
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</CENTER>
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<A href="#1.1">1.1</A>) What is PostgreSQL?<BR>
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<A href="#1.2">1.2</A>) What's the copyright on PostgreSQL?<BR>
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<A href="#1.3">1.3</A>) What Unix platforms does PostgreSQL run
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on?<BR>
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<A href="#1.4">1.4</A>) What non-unix ports are available?<BR>
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<A href="#1.5">1.5</A>) Where can I get PostgreSQL?<BR>
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<A href="#1.6">1.6</A>) Where can I get support?<BR>
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<A href="#1.7">1.7</A>) What is the latest release?<BR>
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<A href="#1.8">1.8</A>) What documentation is available?<BR>
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<A href="#1.9">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="#1.10">1.10</A>) How can I learn <SMALL>SQL</SMALL>?<BR>
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<A href="#1.11">1.11</A>) Is PostgreSQL Y2K compliant?<BR>
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<A href="#1.12">1.12</A>) How do I join the development team?<BR>
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<A href="#1.13">1.13</A>) How do I submit a bug report?<BR>
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<A href="#1.14">1.14</A>) How does PostgreSQL compare to other
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<SMALL>DBMS</SMALL>'s?<BR>
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<CENTER>
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<H2>User Client Questions</H2>
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</CENTER>
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<A href="#2.1">2.1</A>) Are there <SMALL>ODBC</SMALL> drivers for
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PostgreSQL?<BR>
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<A href="#2.2">2.2</A>) What tools are available for use PostgreSQL
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with Web pages?<BR>
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<A href="#2.3">2.3</A>) Does PostgreSQL have a graphical user
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interface? A report generator? An embedded query language
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interface?<BR>
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<A href="#2.4">2.4</A>) What languages are available to
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communicate with PostgreSQL?<BR>
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<CENTER>
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<H2>Administrative Questions</H2>
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</CENTER>
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<A href="#3.1">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="#3.2">3.2</A>) When I start the <I>postmaster</I>, I get
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a <I>Bad System Call</I> or core dumped message. Why?<BR>
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<A href="#3.3">3.3</A>) When I try to start the <I>postmaster</I>,
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I get <I>IpcMemoryCreate</I> errors. Why?<BR>
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<A href="#3.4">3.4</A>) When I try to start the <I>postmaster</I>,
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I get <I>IpcSemaphoreCreate</I> errors. Why?<BR>
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<A href="#3.5">3.5</A>) How do I prevent other hosts from
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accessing my PostgreSQL database?<BR>
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<A href="#3.6">3.6</A>) Why can't I connect to my database from
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another machine?<BR>
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<A href="#3.7">3.7</A>) How do I tune the database engine for
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better performance?<BR>
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<A href="#3.8">3.8</A>) What debugging features are available?<BR>
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<A href="#3.9">3.9</A>) I get <I>"Sorry, too many clients"</I>
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when trying to connect. Why?<BR>
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<A href="#3.10">3.10</A>) What are the <I>pg_sorttempNNN.NN</I>
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files in my database directory?<BR>
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<CENTER>
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<H2>Operational Questions</H2>
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</CENTER>
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<A href="#4.1">4.1</A>) Why is the system confused about commas,
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decimal points, and date formats.<BR>
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<A href="#4.2">4.2</A>) What is the exact difference between
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binary cursors and normal cursors?<BR>
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<A href="#4.3">4.3</A>) How do I <SMALL>SELECT</SMALL> only the
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first few rows of a query?<BR>
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<A href="#4.4">4.4</A>) How do I get a list of tables or other
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things I can see in <I>psql?</I><BR>
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<A href="#4.5">4.5</A>) How do you remove a column from a
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table?<BR>
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<A href="#4.6">4.6</A>) What is the maximum size for a row, table,
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database?<BR>
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<A href="#4.7">4.7</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="#4.8">4.8</A>) How do I find out what tables or
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indexes are defined in the database?<BR>
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<A href="#4.9">4.9</A>) My queries are slow or don't make use of
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the indexes. Why?<BR>
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<A href="#4.10">4.10</A>) How do I see how the query optimizer is
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evaluating my query?<BR>
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<A href="#4.11">4.11</A>) What is an R-tree index?<BR>
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<A href="#4.12">4.12</A>) What is the Genetic Query Optimizer?<BR>
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<A href="#4.13">4.13</A>) How do I perform regular expression searches
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and case-insensitive regular expression searches?<BR>
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<A href="#4.14">4.14</A>) In a query, how do I detect if a field
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is <SMALL>NULL</SMALL>?<BR>
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<A href="#4.15">4.15</A>) What is the difference between the
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various character types?<BR>
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<A href="#4.16.1">4.16.1</A>) How do I create a
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serial/auto-incrementing field?<BR>
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<A href="#4.16.2">4.16.2</A>) How do I get the value of a
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<SMALL>SERIAL</SMALL> insert?<BR>
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<A href="#4.16.3">4.16.3</A>) Don't <I>currval()</I> and
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<I>nextval()</I> lead to a race condition with other users?<BR>
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<A href="#4.17">4.17</A>) What is an <SMALL>OID</SMALL>? What is a
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<SMALL>TID</SMALL>?<BR>
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<A href="#4.18">4.18</A>) What is the meaning of some of the terms
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used in PostgreSQL?<BR>
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<A href="#4.19">4.19</A>) Why do I get the error <I>"FATAL: palloc
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failure: memory exhausted?"</I><BR>
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<A href="#4.20">4.20</A>) How do I tell what PostgreSQL version I
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am running? <BR>
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<A href="#4.21">4.21</A>) My large-object operations get
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<I>invalid large obj descriptor.</I> Why?<BR>
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<A href="#4.22">4.22</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="#4.23">4.23</A>) Why are my subqueries using
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<CODE><SMALL>IN</SMALL></CODE> so slow?<BR>
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<A href="#4.24">4.24</A>) How do I perform an outer join?<BR>
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<CENTER>
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<H2>Extending PostgreSQL</H2>
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</CENTER>
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<A href="#5.1">5.1</A>) I wrote a user-defined function. When I run
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it in <I>psql,</I> why does it dump core?<BR>
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<A href="#5.2">5.2</A>) What does the message
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<I>"NOTICE:PortalHeapMemoryFree: 0x402251d0 not in alloc set!"</I>
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mean?<BR>
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<A href="#5.3">5.3</A>) How can I contribute some nifty new types
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and functions to PostgreSQL?<BR>
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<A href="#5.4">5.4</A>) How do I write a C function to return a
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tuple?<BR>
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<A href="#5.5">5.5</A>) I have changed a source file. Why does the
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recompile not see the change?<BR>
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<HR>
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<CENTER>
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<H2>General Questions</H2>
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</CENTER>
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<H4><A name="1.1">1.1</A>) What is PostgreSQL?</H4>
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<P>PostgreSQL is an enhancement of the POSTGRES database management
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system, a next-generation <SMALL>DBMS</SMALL> research prototype.
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While PostgreSQL retains the powerful data model and rich data
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types of POSTGRES, it replaces the PostQuel query language with an
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extended subset of <SMALL>SQL</SMALL>. PostgreSQL is free and the
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complete source is available.</P>
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<P>PostgreSQL development is performed by a team of Internet
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developers who all subscribe to the PostgreSQL development mailing
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list. The current coordinator is Marc G. Fournier (<A href=
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"mailto:scrappy@PostgreSQL.org">scrappy@PostgreSQL.org</A>). (See
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below on how to join). This team is now responsible for all
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development of PostgreSQL.</P>
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<P>The authors of PostgreSQL 1.01 were Andrew Yu and Jolly Chen.
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Many others have contributed to the porting, testing, debugging,
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and enhancement of the code. The original Postgres code, from which
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PostgreSQL is derived, was the effort of many graduate students,
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undergraduate students, and staff programmers working under the
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direction of Professor Michael Stonebraker at the University of
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California, Berkeley.</P>
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<P>The original name of the software at Berkeley was Postgres. When
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<SMALL>SQL</SMALL> functionality was added in 1995, its name was
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changed to Postgres95. The name was changed at the end of 1996 to
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PostgreSQL.</P>
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<P>It is pronounced <I>Post-Gres-Q-L.</I></P>
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<H4><A name="1.2">1.2</A>) What's the copyright on PostgreSQL?</H4>
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<P>PostgreSQL is subject to the following COPYRIGHT:</P>
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<P>PostgreSQL Data Base Management System</P>
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<P>Portions copyright (c) 1996-2001, PostgreSQL Global Development Group Portions
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Copyright (c) 1994-6 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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<H4><A name="1.3">1.3</A>) What Unix platforms does PostgreSQL run
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on?</H4>
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<P>In general, a 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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<H4><A name="1.4">1.4</A>) What non-unix ports are available?</H4>
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<H6>Client</H6>
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<P>It is possible to compile the <I>libpq</I> C library, psql, and
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other interfaces and binaries to run on MS Windows platforms. In
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this case, the client is running on MS Windows, and communicates
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via TCP/IP to a server running on one of our supported Unix
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platforms. A file <I>win31.mak</I> is included in the distribution
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for making a Win32 <I>libpq</I> library and psql. PostgreSQL also
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communicates with <SMALL>ODBC</SMALL> clients.</P>
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<H6>Server</H6>
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<P>The database server can run on Windows NT and later using
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Cygwin, the Cygnus Unix/NT porting library. See
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<I>pgsql/doc/FAQ_MSWIN</I> in the distribution or the MS Windows
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FAQ on our web site. The database server does not run on MS Windows
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9X because Cygwin does not support the required features on those
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platforms. We have no plan to do a native port to any Microsoft
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platform.</P>
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<H4><A name="1.5">1.5</A>) Where can I get PostgreSQL?</H4>
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<P>The primary anonymous ftp site for PostgreSQL is <A href=
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"ftp://ftp.PostgreSQL.org/pub">ftp://ftp.PostgreSQL.org/pub</A>.
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For mirror sites, see our main Web site.</P>
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<H4><A name="1.6">1.6</A>) Where can I get support?</H4>
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<P>The main mailing list is: <A href=
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"mailto:pgsql-general@PostgreSQL.org">pgsql-general@PostgreSQL.org</A>.
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It is available for discussion of matters pertaining to PostgreSQL.
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To subscribe, send mail with the following lines in the body (not
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the subject line)</P>
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<PRE>
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subscribe
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end
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</PRE>
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<P>to <A href=
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"mailto:pgsql-general-request@PostgreSQL.org">pgsql-general-request@PostgreSQL.org</A>.</P>
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<P>There is also a digest list available. To subscribe to this
|
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|
list, send email to: <A href=
|
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|
"mailto:pgsql-general-digest-request@PostgreSQL.org">pgsql-general-digest-request@PostgreSQL.org</A>
|
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with a body of:</P>
|
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<PRE>
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subscribe
|
|
|
|
|
end
|
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</PRE>
|
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Digests are sent out to members of this list whenever the main list
|
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|
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|
has received around 30k of messages.
|
|
|
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<P>The bugs mailing list is available. To subscribe to this list,
|
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|
|
send email to <A href=
|
|
|
|
|
"mailto:pgsql-bugs-request@PostgreSQL.org">pgsql-bugs-request@PostgreSQL.org</A>
|
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with a body of:</P>
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<PRE>
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subscribe
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|
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end
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</PRE>
|
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There is also a developers discussion mailing list available. To
|
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|
subscribe to this list, send email to <A href=
|
|
|
|
|
"mailto:pgsql-hackers-request@PostgreSQL.org">pgsql-hackers-request@PostgreSQL.org</A>
|
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with a body of:
|
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|
|
|
<PRE>
|
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|
subscribe
|
|
|
|
|
end
|
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|
|
</PRE>
|
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|
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|
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<P>Additional mailing lists and information about PostgreSQL can be
|
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found via the PostgreSQL WWW home page at:</P>
|
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|
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|
<BLOCKQUOTE>
|
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<A href="http://www.PostgreSQL.org">http://www.PostgreSQL.org</A>
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</BLOCKQUOTE>
|
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<P>There is also an IRC channel on EFNet, channel
|
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<I>#PostgreSQL.</I> I use the unix command <CODE>irc -c
|
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|
'#PostgreSQL' "$USER" irc.phoenix.net.</CODE></P>
|
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<P>A list of commercial support companies is available at <A href=
|
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|
"http://www.postgresql.org/users-lounge/commercial-support.html">http://www.postgresql.org/users-lounge/commercial-support.html</A>.</P>
|
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<H4><A name="1.7">1.7</A>) What is the latest release?</H4>
|
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<P>The latest release of PostgreSQL is version 7.0.3.</P>
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<P>We plan to have major releases every four months.</P>
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<H4><A name="1.8">1.8</A>) What documentation is available?</H4>
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<P>Several manuals, manual pages, and some small test examples are
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included in the distribution. See the <I>/doc</I> directory. You
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can also browse the manual online at <A href=
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"http://www.PostgreSQL.org/docs/postgres">http://www.PostgreSQL.org/docs/postgres</A>.</P>
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<P>There is a PostgreSQL book available at <A href=
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|
"http://www.PostgreSQL.org/docs/awbook.html">http://www.PostgreSQL.org/docs/awbook.html</A>.</P>
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<P><I>psql</I> has some nice \d commands to show information about
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types, operators, functions, aggregates, etc.</P>
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<P>Our Web site contains even more documentation.</P>
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<H4><A name="1.9">1.9</A>) How do I find out about known bugs or
|
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|
missing features?</H4>
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<P>PostgreSQL supports an extended subset of <SMALL>SQL</SMALL>-92.
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See our <A href="http://www.PostgreSQL.org/docs/todo.html">TODO</A>
|
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|
list for known bugs, missing features, and future plans.</P>
|
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<H4><A name="1.10">1.10</A>) How can I learn
|
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<SMALL>SQL</SMALL>?</H4>
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<P>The PostgreSQL book at <A href=
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|
"http://www.PostgreSQL.org/docs/awbook.html">http://www.PostgreSQL.org/docs/awbook.html</A>
|
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teaches <SMALL>SQL</SMALL>. There is a nice tutorial at <A href=
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|
"http://w3.one.net/~jhoffman/sqltut.htm">http://w3.one.net/~jhoffman/sqltut.htm</A>
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and at <A href=
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|
"http://ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM">
|
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|
http://ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM.</A></P>
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<P>Another one is "Teach Yourself SQL in 21 Days, Second Edition"
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at <A href=
|
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|
"http://members.tripod.com/er4ebus/sql/index.htm">http://members.tripod.com/er4ebus/sql/index.htm</A></P>
|
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<P>Many of our users like <I>The Practical SQL Handbook</I>,
|
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|
Bowman, Judith S., et al., Addison-Wesley. Others like <I>The
|
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|
|
Complete Reference SQL</I>, Groff et al., McGraw-Hill.</P>
|
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|
<H4><A name="1.11">1.11</A>) Is PostgreSQL Y2K compliant?</H4>
|
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|
<P>Yes, we easily handle dates past the year 2000AD, and before
|
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|
2000BC.</P>
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|
<H4><A name="1.12">1.12</A>) How do I join the development
|
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|
|
team?</H4>
|
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|
|
<P>First, download the latest source and read the PostgreSQL
|
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|
|
|
Developers documentation on our Web site, or in the distribution.
|
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|
|
|
Second, subscribe to the <I>pgsql-hackers</I> and
|
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|
|
|
<I>pgsql-patches</I> mailing lists. Third, submit high-quality
|
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|
|
|
patches to pgsql-patches.</P>
|
|
|
|
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|
|
<P>There are about a dozen people who have commit privileges to the
|
|
|
|
|
PostgreSQL <SMALL>CVS</SMALL> archive. They each have submitted so
|
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|
|
many high-quality patches that it was impossible for the existing
|
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|
|
|
committers to keep up, and we had confidence that patches they
|
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|
|
committed were of high quality.</P>
|
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|
|
<H4><A name="1.13">1.13</A>) How do I submit a bug report?</H4>
|
|
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|
|
<P>Fill out the "bug-template" file and send it to: <A href=
|
|
|
|
|
"mailto:pgsql-bugs@PostgreSQL.org">pgsql-bugs@PostgreSQL.org</A></P>
|
|
|
|
|
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|
|
|
|
<P>Also check out our ftp site <A href=
|
|
|
|
|
"ftp://ftp.PostgreSQL.org/pub">ftp://ftp.PostgreSQL.org/pub</A> to
|
|
|
|
|
see if there is a more recent PostgreSQL version or patches.</P>
|
|
|
|
|
|
|
|
|
|
<H4><A name="1.14">1.14</A>) How does PostgreSQL compare to other
|
|
|
|
|
<SMALL>DBMS</SMALL>'s?</H4>
|
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|
|
<P>There are several ways of measuring software: features,
|
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|
|
performance, reliability, support, and price.</P>
|
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|
|
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|
|
|
<DL>
|
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|
|
|
<DT><B>Features</B></DT>
|
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|
|
<DD>PostgreSQL has most features present in large commercial
|
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|
|
<SMALL>DBMS</SMALL>'s, like transactions, subselects, triggers,
|
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|
|
views, foreign key referential integrity, and sophisticated
|
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|
|
|
locking. We have some features they don't have, like user-defined
|
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|
|
|
types, inheritance, rules, and multi-version concurrency control
|
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|
|
to reduce lock contention.<BR>
|
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|
|
<BR>
|
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|
|
</DD>
|
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|
|
<DT><B>Performance</B></DT>
|
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|
|
<DD>PostgreSQL runs in two modes. Normal <I>fsync</I> mode
|
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|
|
flushes every completed transaction to disk, guaranteeing that if
|
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|
|
the OS crashes or loses power in the next few seconds, all your
|
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|
|
|
data is safely stored on disk. In this mode, we are slower than
|
|
|
|
|
most commercial databases, partly because few of them do such
|
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|
|
conservative flushing to disk in their default modes. In
|
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|
|
<I>no-fsync</I> mode, we are usually faster than commercial
|
|
|
|
|
databases, though in this mode, an OS crash could cause data
|
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|
|
corruption. We are working to provide an intermediate mode that
|
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|
|
suffers less performance overhead than full fsync mode, and will
|
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|
|
allow data integrity within 30 seconds of an OS crash.<BR>
|
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|
|
<BR>
|
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|
|
In comparison to MySQL or leaner database systems, we are slower
|
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|
|
|
on inserts/updates because we have transaction overhead. Of
|
|
|
|
|
course, MySQL doesn't have any of the features mentioned in the
|
|
|
|
|
<I>Features</I> section above. We are built for flexibility and
|
|
|
|
|
features, though we continue to improve performance through
|
|
|
|
|
profiling and source code analysis. There is an interesting Web
|
|
|
|
|
page comparing PostgreSQL to MySQL at <A href=
|
|
|
|
|
"http://openacs.org/why-not-mysql.html">http://openacs.org/why-not-mysql.html</A><BR>
|
|
|
|
|
|
|
|
|
|
<BR>
|
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|
|
We handle each user connection by creating a Unix process.
|
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|
|
|
Backend processes share data buffers and locking information.
|
|
|
|
|
With multiple CPU's, multiple backends can easily run on
|
|
|
|
|
different CPU's.<BR>
|
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|
|
<BR>
|
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|
|
</DD>
|
|
|
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|
|
|
<DT><B>Reliability</B></DT>
|
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|
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|
|
<DD>We realize that a <SMALL>DBMS</SMALL> must be reliable, or it
|
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|
|
is worthless. We strive to release well-tested, stable code that
|
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|
|
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
|
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|
|
believe we compare favorably to other database software in this
|
|
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|
|
area.<BR>
|
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|
|
<BR>
|
|
|
|
|
</DD>
|
|
|
|
|
|
|
|
|
|
<DT><B>Support</B></DT>
|
|
|
|
|
|
|
|
|
|
<DD>Our mailing list provides a large group of developers and
|
|
|
|
|
users to help resolve any problems encountered. While we can not
|
|
|
|
|
guarantee a fix, commercial <SMALL>DBMS</SMALL>'s don't 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 support FAQ item.)<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>
|
|
|
|
|
|
|
|
|
|
<CENTER>
|
|
|
|
|
<H2>User Client Questions</H2>
|
|
|
|
|
</CENTER>
|
|
|
|
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|
|
<H4><A name="2.1">2.1</A>) Are there <SMALL>ODBC</SMALL> drivers
|
|
|
|
|
for PostgreSQL?</H4>
|
|
|
|
|
|
|
|
|
|
<P>There are two <SMALL>ODBC</SMALL> drivers available, PsqlODBC
|
|
|
|
|
and OpenLink <SMALL>ODBC</SMALL>.</P>
|
|
|
|
|
|
|
|
|
|
<P>PsqlODBC is included in the distribution. More information about
|
|
|
|
|
it can be gotten from <A href=
|
|
|
|
|
"ftp://ftp.PostgreSQL.org/pub/odbc/">ftp://ftp.PostgreSQL.org/pub/odbc/</A>.</P>
|
|
|
|
|
|
|
|
|
|
<P>OpenLink <SMALL>ODBC</SMALL> can be gotten from <A href=
|
|
|
|
|
"http://www.openlinksw.com/">http://www.openlinksw.com</A>. It
|
|
|
|
|
works with their standard <SMALL>ODBC</SMALL> client software so
|
|
|
|
|
you'll have PostgreSQL <SMALL>ODBC</SMALL> available on every
|
|
|
|
|
client platform they support (Win, Mac, Unix, VMS).</P>
|
|
|
|
|
|
|
|
|
|
<P>They will probably be selling this product to people who need
|
|
|
|
|
commercial-quality support, but a freeware version will always be
|
|
|
|
|
available. Questions to <A href=
|
|
|
|
|
"mailto:postgres95@openlink.co.uk">postgres95@openlink.co.uk</A>.</P>
|
|
|
|
|
|
|
|
|
|
<P>See also the <A href=
|
|
|
|
|
"http://www.PostgreSQL.org/docs/programmer/odbc.htm">ODBC chapter
|
|
|
|
|
of the Programmer's Guide</A>.</P>
|
|
|
|
|
|
|
|
|
|
<H4><A name="2.2">2.2</A>) What tools are available for using
|
|
|
|
|
PostgreSQL with Web pages?</H4>
|
|
|
|
|
|
|
|
|
|
<P>A nice introduction to Database-backed Web pages can be seen at:
|
|
|
|
|
<A href="http://www.webtools.com">http://www.webtools.com</A></P>
|
|
|
|
|
|
|
|
|
|
<P>There is also one at <A href=
|
|
|
|
|
"http://www.phone.net/home/mwm/hotlist/">http://www.phone.net/home/mwm/hotlist/.</A></P>
|
|
|
|
|
|
|
|
|
|
<P>For Web integration, PHP is an excellent interface. It is at <A
|
|
|
|
|
href="http://www.php.net">http://www.php.net</A></P>
|
|
|
|
|
|
|
|
|
|
<P>For complex cases, many use the Perl interface and CGI.pm.</P>
|
|
|
|
|
|
|
|
|
|
<P>A WWW gateway based on WDB using Perl can be downloaded from <A
|
|
|
|
|
href=
|
|
|
|
|
"http://www.eol.ists.ca/~dunlop/wdb-p95">http://www.eol.ists.ca/~dunlop/wdb-p95</A></P>
|
|
|
|
|
|
|
|
|
|
<H4><A name="2.3">2.3</A>) Does PostgreSQL have a graphical user
|
|
|
|
|
interface? A report generator? An embedded query language
|
|
|
|
|
interface?</H4>
|
|
|
|
|
|
|
|
|
|
<P>We have a nice graphical user interface called <I>pgaccess,</I>
|
|
|
|
|
which is shipped as part of the distribution. <I>Pgaccess</I> also
|
|
|
|
|
has a report generator. The Web page is <A href=
|
|
|
|
|
"http://www.flex.ro/pgaccess">http://www.flex.ro/pgaccess</A></P>
|
|
|
|
|
|
|
|
|
|
<P>We also include <I>ecpg,</I> which is an embedded SQL query
|
|
|
|
|
language interface for C.</P>
|
|
|
|
|
|
|
|
|
|
<H4><A name="2.4">2.4</A>) What languages are available to
|
|
|
|
|
communicate with PostgreSQL?</H4>
|
|
|
|
|
|
|
|
|
|
<P>We have:</P>
|
|
|
|
|
|
|
|
|
|
<UL>
|
|
|
|
|
<LI>C (libpq)</LI>
|
|
|
|
|
|
|
|
|
|
<LI>C++ (libpq++)</LI>
|
|
|
|
|
|
|
|
|
|
<LI>Embedded C (ecpg)</LI>
|
|
|
|
|
|
|
|
|
|
<LI>Java (jdbc)</LI>
|
|
|
|
|
|
|
|
|
|
<LI>Perl (perl5)</LI>
|
|
|
|
|
|
|
|
|
|
<LI>ODBC (odbc)</LI>
|
|
|
|
|
|
|
|
|
|
<LI>Python (PyGreSQL)</LI>
|
|
|
|
|
|
|
|
|
|
<LI>TCL (libpgtcl)</LI>
|
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|
|
|
|
|
|
|
|
<LI>C Easy API (libpgeasy)</LI>
|
|
|
|
|
|
|
|
|
|
<LI>Embedded <SMALL>HTML</SMALL> (<A href=
|
|
|
|
|
"http://www.php.net">PHP from http://www.php.net</A>)</LI>
|
|
|
|
|
</UL>
|
|
|
|
|
<HR>
|
|
|
|
|
|
|
|
|
|
<CENTER>
|
|
|
|
|
<H2>Administrative Questions</H2>
|
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|
|
</CENTER>
|
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|
<H4><A name="3.1">3.1</A>) How do I install PostgreSQL somewhere
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|
other than <I>/usr/local/pgsql?</I></H4>
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<P>Specify the <I>--prefix</I> option when running
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<I>configure.</I></P>
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<H4><A name="3.2">3.2</A>) When I start the <I>postmaster</I>, I
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get a <I>Bad System Call</I> or core dumped message. Why?</H4>
|
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|
<P>It could be a variety of problems, but first check to see that
|
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|
you have System V extensions installed in your kernel. PostgreSQL
|
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requires kernel support for shared memory and semaphores.</P>
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<H4><A name="3.3">3.3</A>) When I try to start the
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|
<I>postmaster,</I> I get <I>IpcMemoryCreate</I> errors. Why?</H4>
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|
<P>You either do not have shared memory configured properly in your
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|
kernel or you need to enlarge the shared memory available in the
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|
kernel. The exact amount you need depends on your architecture and
|
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|
how many buffers and backend processes you configure for the
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|
<I>postmaster.</I> For most systems, with default numbers of
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|
buffers and processes, you need a minimum of ~1MB.</P>
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|
<H4><A name="3.4">3.4</A>) When I try to start the
|
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|
<I>postmaster,</I> I get <I>IpcSemaphoreCreate</I> errors.
|
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|
Why?</H4>
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<P>If the error message is <I>IpcSemaphoreCreate: semget failed (No
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|
space left on device)</I> then your kernel is not configured with
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|
enough semaphores. Postgres needs one semaphore per potential
|
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|
|
backend process. A temporary solution is to start the
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|
|
<I>postmaster</I> with a smaller limit on the number of backend
|
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|
processes. Use <I>-N</I> with a parameter less than the default of
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|
32. A more permanent solution is to increase your kernel's
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|
<SMALL>SEMMNS</SMALL> and <SMALL>SEMMNI</SMALL> parameters.</P>
|
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|
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|
|
<P>If the error message is something else, you might not have
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|
semaphore support configured in your kernel at all.</P>
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|
<H4><A name="3.5">3.5</A>) How do I prevent other hosts from
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|
accessing my PostgreSQL database?</H4>
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|
<P>By default, PostgreSQL only allows connections from the local
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|
machine using Unix domain sockets. Other machines will not be able
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|
to connect unless you add the <I>-i</I> flag to the
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<I>postmaster,</I> <B>and</B> enable host-based authentication by
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modifying the file <I>$PGDATA/pg_hba.conf</I> accordingly. This
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will allow TCP/IP connections.</P>
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|
<P>Inoperative semaphores can also cause crashes during heavy
|
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|
database access.</P>
|
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|
<H4><A name="3.6">3.6</A>) Why can't I connect to my database from
|
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|
|
another machine?</H4>
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|
|
<P>The default configuration allows only unix domain socket
|
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|
|
connections from the local machine. To enable TCP/IP connections,
|
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|
|
make sure the <I>postmaster</I> has been started with the <I>-i</I>
|
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|
|
|
option, and add an appropriate host entry to the file
|
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|
|
<I>pgsql/data/pg_hba.conf</I>.</P>
|
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|
<H4><A name="3.7">3.7</A>) How do I tune the database engine for
|
|
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|
|
better performance?</H4>
|
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|
<P>Certainly, indices can speed up queries. The
|
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|
|
<SMALL>EXPLAIN</SMALL> command allows you to see how PostgreSQL is
|
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|
|
|
interpreting your query, and which indices are being used.</P>
|
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|
<P>If you are doing a lot of <SMALL>INSERTs</SMALL>, consider doing
|
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|
|
them in a large batch using the <SMALL>COPY</SMALL> command. This
|
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|
|
|
is much faster than individual <SMALL>INSERTS.</SMALL> Second,
|
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|
|
statements not in a <SMALL>BEGIN WORK/COMMIT</SMALL> transaction
|
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|
|
|
block are considered to be in their own transaction. Consider
|
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|
|
performing several statements in a single transaction block. This
|
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|
|
reduces the transaction overhead. Also consider dropping and
|
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|
|
|
recreating indices when making large data changes.</P>
|
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|
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|
|
<P>There are several tuning options. You can disable <I>fsync()</I>
|
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|
|
|
by starting the <I>postmaster</I> with a <I>-o -F</I> option. This
|
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|
|
will prevent <I>fsync()'s</I> from flushing to disk after every
|
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|
|
|
transaction.</P>
|
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|
<P>You can also use the <I>postmaster</I> <I>-B</I> option to
|
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|
|
|
increase the number of shared memory buffers used by the backend
|
|
|
|
|
processes. If you make this parameter too high, the
|
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|
|
<I>postmaster</I> may not start because you've exceeded your
|
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|
|
|
kernel's limit on shared memory space. Each buffer is 8K and the
|
|
|
|
|
default is 64 buffers.</P>
|
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|
|
<P>You can also use the backend <I>-S</I> option to increase the
|
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|
|
maximum amount of memory used by the backend process for temporary
|
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|
|
sorts. The <I>-S</I> value is measured in kilobytes, and the
|
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|
|
|
default is 512 (ie, 512K).</P>
|
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|
|
<P>You can also use the <SMALL>CLUSTER</SMALL> command to group
|
|
|
|
|
data in tables to match an index. See the <SMALL>CLUSTER</SMALL>
|
|
|
|
|
manual page for more details.</P>
|
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|
|
<H4><A name="3.8">3.8</A>) What debugging features are
|
|
|
|
|
available?</H4>
|
|
|
|
|
|
|
|
|
|
<P>PostgreSQL has several features that report status information
|
|
|
|
|
that can be valuable for debugging purposes.</P>
|
|
|
|
|
|
|
|
|
|
<P>First, by running <I>configure</I> with the --enable-cassert
|
|
|
|
|
option, many <I>assert()'s</I> monitor the progress of the backend
|
|
|
|
|
and halt the program when something unexpected occurs.</P>
|
|
|
|
|
|
|
|
|
|
<P>Both <I>postmaster</I> and <I>postgres</I> have several debug
|
|
|
|
|
options available. First, whenever you start the <I>postmaster,</I>
|
|
|
|
|
make sure you send the standard output and error to a log file,
|
|
|
|
|
like:</P>
|
|
|
|
|
<PRE>
|
|
|
|
|
cd /usr/local/pgsql
|
|
|
|
|
./bin/postmaster >server.log 2>&1 &
|
|
|
|
|
</PRE>
|
|
|
|
|
|
|
|
|
|
<P>This will put a server.log file in the top-level PostgreSQL
|
|
|
|
|
directory. This file contains useful information about problems or
|
|
|
|
|
errors encountered by the server. <I>Postmaster</I> has a <I>-d</I>
|
|
|
|
|
option that allows even more detailed information to be reported.
|
|
|
|
|
The <I>-d</I> option takes a number that specifies the debug level.
|
|
|
|
|
Be warned that high debug level values generate large log
|
|
|
|
|
files.</P>
|
|
|
|
|
|
|
|
|
|
<P>If the <I>postmaster</I> is not running, you can actually run
|
|
|
|
|
the <I>postgres</I> backend from the command line, and type your
|
|
|
|
|
<SMALL>SQL</SMALL> statement directly. This is recommended
|
|
|
|
|
<B>only</B> for debugging purposes. Note that a newline terminates
|
|
|
|
|
the query, not a semicolon. If you have compiled with debugging
|
|
|
|
|
symbols, you can use a debugger to see what is happening. Because
|
|
|
|
|
the backend was not started from the <I>postmaster,</I> it is not
|
|
|
|
|
running in an identical environment and locking/backend interaction
|
|
|
|
|
problems may not be duplicated.</P>
|
|
|
|
|
|
|
|
|
|
<P>If the <I>postmaster</I> is running, start <I>psql</I> in one
|
|
|
|
|
window, then find the <SMALL>PID</SMALL> of the <I>postgres</I>
|
|
|
|
|
process used by <I>psql.</I> Use a debugger to attach to the
|
|
|
|
|
<I>postgres</I> <SMALL>PID.</SMALL> You can set breakpoints in the
|
|
|
|
|
debugger and issue queries from <I>psql.</I> If you are debugging
|
|
|
|
|
<I>postgres</I> startup, you can set PGOPTIONS="-W n", then start
|
|
|
|
|
<I>psql.</I> This will cause startup to delay for <I>n</I> seconds
|
|
|
|
|
so you can attach with the debugger and trace through the startup
|
|
|
|
|
sequence.</P>
|
|
|
|
|
|
|
|
|
|
<P>The <I>postgres</I> program has <I>-s, -A,</I> and <I>-t</I>
|
|
|
|
|
options that can be very useful for debugging and performance
|
|
|
|
|
measurements.</P>
|
|
|
|
|
|
|
|
|
|
<P>You can also compile with profiling to see what functions are
|
|
|
|
|
taking execution time. The backend profile files will be deposited
|
|
|
|
|
in the <I>pgsql/data/base/dbname</I> directory. The client profile
|
|
|
|
|
file will be put in the client's current directory.</P>
|
|
|
|
|
|
|
|
|
|
<H4><A name="3.9">3.9</A>) I get 'Sorry, too many clients' when
|
|
|
|
|
trying to connect. Why?</H4>
|
|
|
|
|
|
|
|
|
|
<P>You need to increase the <I>postmaster's</I> limit on how many
|
|
|
|
|
concurrent backend processes it can start.</P>
|
|
|
|
|
|
|
|
|
|
<P>In PostgreSQL 6.5 and up, the default limit is 32 processes. You
|
|
|
|
|
can increase it by restarting the <I>postmaster</I> with a suitable
|
|
|
|
|
<I>-N</I> value. With the default configuration you can set
|
|
|
|
|
<I>-N</I> as large as 1024. If you need more, increase
|
|
|
|
|
<SMALL>MAXBACKENDS</SMALL> in <I>include/config.h</I> and rebuild.
|
|
|
|
|
You can set the default value of <I>-N</I> at configuration time,
|
|
|
|
|
if you like, using <I>configure's</I> <I>--with-maxbackends</I>
|
|
|
|
|
switch.</P>
|
|
|
|
|
|
|
|
|
|
<P>Note that if you make <I>-N</I> larger than 32, you must also
|
|
|
|
|
increase <I>-B</I> beyond its default of 64; <I>-B</I> must be at
|
|
|
|
|
least twice <I>-N,</I> and probably should be more than that for
|
|
|
|
|
best performance. For large numbers of backend processes, you are
|
|
|
|
|
also likely to find that you need to increase various Unix kernel
|
|
|
|
|
configuration parameters. Things to check include the maximum size
|
|
|
|
|
of shared memory blocks, <SMALL>SHMMAX;</SMALL> the maximum number
|
|
|
|
|
of semaphores, <SMALL>SEMMNS</SMALL> and <SMALL>SEMMNI;</SMALL> the
|
|
|
|
|
maximum number of processes, <SMALL>NPROC;</SMALL> the maximum
|
|
|
|
|
number of processes per user, <SMALL>MAXUPRC;</SMALL> and the
|
|
|
|
|
maximum number of open files, <SMALL>NFILE</SMALL> and
|
|
|
|
|
<SMALL>NINODE.</SMALL> The reason that PostgreSQL has a limit on
|
|
|
|
|
the number of allowed backend processes is so your system won't run
|
|
|
|
|
out of resources.</P>
|
|
|
|
|
|
|
|
|
|
<P>In PostgreSQL versions prior to 6.5, the maximum number of
|
|
|
|
|
backends was 64, and changing it required a rebuild after altering
|
|
|
|
|
the MaxBackendId constant in
|
|
|
|
|
<I>include/storage/sinvaladt.h.</I></P>
|
|
|
|
|
|
|
|
|
|
<H4><A name="3.10">3.10</A>) What are the <I>pg_sorttempNNN.NN</I>
|
|
|
|
|
files in my database directory?</H4>
|
|
|
|
|
|
|
|
|
|
<P>They are temporary files generated by the query executor. For
|
|
|
|
|
example, if a sort needs to be done to satisfy an <SMALL>ORDER
|
|
|
|
|
BY,</SMALL> and the sort requires more space than the backend's
|
|
|
|
|
<I>-S</I> parameter allows, then temporary files are created to
|
|
|
|
|
hold the extra data.</P>
|
|
|
|
|
|
|
|
|
|
<P>The temporary files should be deleted automatically, but might
|
|
|
|
|
not if a backend crashes during a sort. If you have no backends
|
|
|
|
|
running at the time, it is safe to delete the pg_tempNNN.NN
|
|
|
|
|
files.</P>
|
|
|
|
|
<HR>
|
|
|
|
|
|
|
|
|
|
<CENTER>
|
|
|
|
|
<H2>Operational Questions</H2>
|
|
|
|
|
</CENTER>
|
|
|
|
|
|
|
|
|
|
<H4><A name="4.1">4.1</A>) Why is system confused about commas,
|
|
|
|
|
decimal points, and date formats.</H4>
|
|
|
|
|
|
|
|
|
|
<P>Check your locale configuration. PostgreSQL uses the locale
|
|
|
|
|
setting of the user that ran the <I>postmaster</I> process. There
|
|
|
|
|
are postgres and psql <SMALL>SET</SMALL> commands to control the
|
|
|
|
|
date format. Set those accordingly for your operating
|
|
|
|
|
environment.</P>
|
|
|
|
|
|
|
|
|
|
<H4><A name="4.2">4.2</A>) What is the exact difference between
|
|
|
|
|
binary cursors and normal cursors?</H4>
|
|
|
|
|
|
|
|
|
|
<P>See the <SMALL>DECLARE</SMALL> manual page for a
|
|
|
|
|
description.</P>
|
|
|
|
|
|
|
|
|
|
<H4><A name="4.3">4.3</A>) How do I <SMALL>SELECT</SMALL> only the
|
|
|
|
|
first few rows of a query?</H4>
|
|
|
|
|
|
|
|
|
|
<P>See the <SMALL>FETCH</SMALL> manual page, or use
|
|
|
|
|
<SMALL>SELECT</SMALL> ... <SMALL>LIMIT</SMALL>....</P>
|
|
|
|
|
|
|
|
|
|
<P>The entire query may have to be evaluated, even if you only want
|
|
|
|
|
the first few rows. Consider a query that has an <SMALL>ORDER
|
|
|
|
|
BY.</SMALL> If there is an index that matches the <SMALL>ORDER
|
|
|
|
|
BY</SMALL>, PostgreSQL may be able to evaluate only the first few
|
|
|
|
|
records requested, or the entire query may have to be evaluated
|
|
|
|
|
until the desired rows have been generated.</P>
|
|
|
|
|
|
|
|
|
|
<H4><A name="4.4">4.4</A>) How do I get a list of tables or other
|
|
|
|
|
things I can see in <I>psql?</I><BR>
|
|
|
|
|
</H4>
|
|
|
|
|
|
|
|
|
|
<P>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 psql'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.</P>
|
|
|
|
|
|
|
|
|
|
<H4><A name="4.5">4.5</A>) How do you remove a column from a
|
|
|
|
|
table?</H4>
|
|
|
|
|
|
|
|
|
|
<P>We do not support <SMALL>ALTER TABLE DROP COLUMN,</SMALL> but do
|
|
|
|
|
this:</P>
|
|
|
|
|
<PRE>
|
|
|
|
|
SELECT ... -- select all columns but the one you want to remove
|
|
|
|
|
INTO TABLE new_table
|
|
|
|
|
FROM old_table;
|
|
|
|
|
DROP TABLE old_table;
|
|
|
|
|
ALTER TABLE new_table RENAME TO old_table;
|
|
|
|
|
</PRE>
|
|
|
|
|
|
|
|
|
|
<H4><A name="4.6">4.6</A>) What is the maximum size for a row,
|
|
|
|
|
table, database?</H4>
|
|
|
|
|
|
|
|
|
|
<P>These are the limits:</P>
|
|
|
|
|
<PRE>
|
|
|
|
|
Maximum size for a database? unlimited (60GB databases exist)
|
|
|
|
|
Maximum size for a table? unlimited on all operating systems
|
|
|
|
|
Maximum size for a row? unlimited in 7.1 and later
|
|
|
|
|
Maximum number of rows in a table? unlimited
|
|
|
|
|
Maximum number of columns in a table? unlimited
|
|
|
|
|
Maximum number of indexes on a table? unlimited
|
|
|
|
|
</PRE>
|
|
|
|
|
Of course, these are not actually unlimited, but limited to
|
|
|
|
|
available disk space.
|
|
|
|
|
|
|
|
|
|
<H4><A name="4.7">4.7</A>) How much database disk space is required
|
|
|
|
|
to store data from a typical text file?<BR>
|
|
|
|
|
</H4>
|
|
|
|
|
|
|
|
|
|
<P>A PostgreSQL database may need six-and-a-half times the disk
|
|
|
|
|
space required to store the data in a flat file.</P>
|
|
|
|
|
|
|
|
|
|
<P>Consider a file of 300,000 lines with two integers on each line.
|
|
|
|
|
The flat file is 2.4MB. The size of the PostgreSQL database file
|
|
|
|
|
containing this data can be estimated at 14MB:</P>
|
|
|
|
|
<PRE>
|
|
|
|
|
36 bytes: each row header (approximate)
|
|
|
|
|
+ 8 bytes: two int fields @ 4 bytes each
|
|
|
|
|
+ 4 bytes: pointer on page to tuple
|
|
|
|
|
----------------------------------------
|
|
|
|
|
48 bytes per row
|
|
|
|
|
|
|
|
|
|
The data page size in PostgreSQL is 8192 bytes (8 KB), so:
|
|
|
|
|
|
|
|
|
|
8192 bytes per page
|
|
|
|
|
------------------- = 171 rows per database page (rounded up)
|
|
|
|
|
48 bytes per row
|
|
|
|
|
|
|
|
|
|
300000 data rows
|
|
|
|
|
-------------------- = 1755 database pages
|
|
|
|
|
171 rows per page
|
|
|
|
|
|
|
|
|
|
1755 database pages * 8192 bytes per page = 14,376,960 bytes (14MB)
|
|
|
|
|
</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>
|
|
|
|
|
|
|
|
|
|
<H4><A name="4.8">4.8</A>) How do I find out what tables or
|
|
|
|
|
indexes are defined in the database?</H4>
|
|
|
|
|
|
|
|
|
|
<P><I>psql</I> has a variety of backslash commands to show such
|
|
|
|
|
information. Use \? to see them.</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>
|
|
|
|
|
|
|
|
|
|
<H4><A name="4.9">4.9</A>) My queries are slow or don't make use of
|
|
|
|
|
the indexes. Why?</H4>
|
|
|
|
|
|
|
|
|
|
<P>PostgreSQL does not automatically maintain statistics.
|
|
|
|
|
V<SMALL>ACUUM</SMALL> must be run to update the statistics. After
|
|
|
|
|
statistics are updated, the optimizer knows how many rows in the
|
|
|
|
|
table, and can better decide if it should use indices. Note that
|
|
|
|
|
the optimizer does not use indices in cases when the table is small
|
|
|
|
|
because a sequential scan would be faster.</P>
|
|
|
|
|
|
|
|
|
|
<P>For column-specific optimization statistics, use <SMALL>VACUUM
|
|
|
|
|
ANALYZE.</SMALL> <SMALL>VACUUM ANALYZE</SMALL> is important for
|
|
|
|
|
complex multijoin queries, so the optimizer can estimate the number
|
|
|
|
|
of rows returned from each table, and choose the proper join order.
|
|
|
|
|
The backend does not keep track of column statistics on its own, so
|
|
|
|
|
<SMALL>VACUUM ANALYZE</SMALL> must be run to collect them
|
|
|
|
|
periodically.</P>
|
|
|
|
|
|
|
|
|
|
<P>Indexes are usually not used for <SMALL>ORDER BY</SMALL>
|
|
|
|
|
operations: a sequential scan followed by an explicit sort is
|
|
|
|
|
faster than an indexscan of all tuples of a large table, because it
|
|
|
|
|
takes fewer disk accesses.</P>
|
|
|
|
|
|
|
|
|
|
<P>When using wild-card operators such as <SMALL>LIKE</SMALL> or
|
|
|
|
|
<I>~,</I> indices can only be used if the beginning of the search
|
|
|
|
|
is anchored to the start of the string. So, to use indices,
|
|
|
|
|
<SMALL>LIKE</SMALL> searches should not begin with <I>%,</I> and
|
|
|
|
|
<I>~</I>(regular expression searches) should start with
|
|
|
|
|
<I>^.</I></P>
|
|
|
|
|
|
|
|
|
|
<H4><A name="4.10">4.10</A>) How do I see how the query optimizer
|
|
|
|
|
is evaluating my query?</H4>
|
|
|
|
|
|
|
|
|
|
<P>See the <SMALL>EXPLAIN</SMALL> manual page.</P>
|
|
|
|
|
|
|
|
|
|
<H4><A name="4.11">4.11</A>) What is an R-tree index?</H4>
|
|
|
|
|
|
|
|
|
|
<P>An R-tree index is used for indexing spatial data. A hash index
|
|
|
|
|
can't handle range searches. A B-tree index only handles range
|
|
|
|
|
searches in a single dimension. R-tree's can handle
|
|
|
|
|
multi-dimensional data. For example, if an R-tree index can be
|
|
|
|
|
built on an attribute of type <I>point,</I> the system can more
|
|
|
|
|
efficiently answer queries such as "select all points within a
|
|
|
|
|
bounding rectangle."</P>
|
|
|
|
|
|
|
|
|
|
<P>The canonical paper that describes the original R-tree design
|
|
|
|
|
is:</P>
|
|
|
|
|
|
|
|
|
|
<P>Guttman, A. "R-trees: A Dynamic Index Structure for Spatial
|
|
|
|
|
Searching." Proc of the 1984 ACM SIGMOD Int'l Conf on Mgmt of Data,
|
|
|
|
|
45-57.</P>
|
|
|
|
|
|
|
|
|
|
<P>You can also find this paper in Stonebraker's "Readings in
|
|
|
|
|
Database Systems".</P>
|
|
|
|
|
|
|
|
|
|
<P>Built-in R-trees can handle polygons and boxes. In theory,
|
|
|
|
|
R-trees can be extended to handle higher number of dimensions. In
|
|
|
|
|
practice, extending R-trees requires a bit of work and we don't
|
|
|
|
|
currently have any documentation on how to do it.</P>
|
|
|
|
|
|
|
|
|
|
<H4><A name="4.12">4.12</A>) What is the Genetic Query
|
|
|
|
|
Optimizer?</H4>
|
|
|
|
|
|
|
|
|
|
<P>The <SMALL>GEQO</SMALL> module speeds query optimization when
|
|
|
|
|
joining many tables by means of a Genetic Algorithm (GA). It allows
|
|
|
|
|
the handling of large join queries through nonexhaustive
|
|
|
|
|
search.</P>
|
|
|
|
|
|
|
|
|
|
<H4><A name="4.13">4.13</A>) How do I perform regular expression
|
|
|
|
|
searches and case-insensitive regular expression searches?</H4>
|
|
|
|
|
|
|
|
|
|
<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>
|
|
|
|
|
|
|
|
|
|
<H4><A name="4.14">4.14</A>) In a query, how do I detect if a field
|
|
|
|
|
is <SMALL>NULL</SMALL>?</H4>
|
|
|
|
|
|
|
|
|
|
<P>You test the column with <SMALL>IS NULL</SMALL>IS NOT NULL.</P>
|
|
|
|
|
|
|
|
|
|
<H4><A name="4.15">4.15</A>) What is the difference between the
|
|
|
|
|
various character types?</H4>
|
|
|
|
|
<PRE>
|
|
|
|
|
Type Internal Name Notes
|
|
|
|
|
--------------------------------------------------
|
|
|
|
|
"char" char 1 character
|
|
|
|
|
CHAR(#) bpchar blank padded to the specified fixed length
|
|
|
|
|
VARCHAR(#) varchar size specifies maximum length, no padding
|
|
|
|
|
TEXT text no specific upper limit on length
|
|
|
|
|
BYTEA bytea variable-length byte array (null-safe)
|
|
|
|
|
</PRE>
|
|
|
|
|
|
|
|
|
|
<P>You will see the internal name when examining system catalogs
|
|
|
|
|
and in some error messages.</P>
|
|
|
|
|
|
|
|
|
|
<P>The last four types above are "varlena" types (i.e., the first
|
|
|
|
|
four bytes on disk are the length, followed by the data). Thus the
|
|
|
|
|
actual space used is slightly greater than the declared size.
|
|
|
|
|
However, these data types are also subject to compression or being
|
|
|
|
|
stored out-of-line by <SMALL>TOAST</SMALL>, so the space on disk
|
|
|
|
|
might also be less than expected.</P>
|
|
|
|
|
|
|
|
|
|
<H4><A name="4.16.1">4.16.1</A>) How do I create a
|
|
|
|
|
serial/auto-incrementing field?</H4>
|
|
|
|
|
|
|
|
|
|
<P>PostgreSQL supports a <SMALL>SERIAL</SMALL> data type. It
|
|
|
|
|
auto-creates a sequence and index on the column. For example,
|
|
|
|
|
this:</P>
|
|
|
|
|
<PRE>
|
|
|
|
|
CREATE TABLE person (
|
|
|
|
|
id SERIAL,
|
|
|
|
|
name TEXT
|
|
|
|
|
);
|
|
|
|
|
</PRE>
|
|
|
|
|
is automatically translated into this:
|
|
|
|
|
<PRE>
|
|
|
|
|
CREATE SEQUENCE person_id_seq;
|
|
|
|
|
CREATE TABLE person (
|
|
|
|
|
id INT4 NOT NULL DEFAULT nextval('person_id_seq'),
|
|
|
|
|
name TEXT
|
|
|
|
|
);
|
|
|
|
|
CREATE UNIQUE INDEX person_id_key ON person ( id );
|
|
|
|
|
</PRE>
|
|
|
|
|
See the <I>create_sequence</I> manual page for more information
|
|
|
|
|
about sequences. You can also use each row's <I>OID</I> field as a
|
|
|
|
|
unique value. However, if you need to dump and reload the database,
|
|
|
|
|
you need to use <I>pg_dump's -o</I> option or <SMALL>COPY WITH
|
|
|
|
|
OIDS</SMALL> option to preserve the <SMALL>OID</SMALL>s.
|
|
|
|
|
|
|
|
|
|
<P><A href="http://www.PostgreSQL.org/docs/aw_pgsql_book">Numbering
|
|
|
|
|
Rows.</A></P>
|
|
|
|
|
|
|
|
|
|
<H4><A name="4.16.2">4.16.2</A>) How do I get the value of a
|
|
|
|
|
<SMALL>SERIAL</SMALL> insert?</H4>
|
|
|
|
|
|
|
|
|
|
<P>One approach is to to retrieve the next <SMALL>SERIAL</SMALL>
|
|
|
|
|
value from the sequence object with the <I>nextval()</I> function
|
|
|
|
|
<I>before</I> inserting and then insert it explicitly. Using the
|
|
|
|
|
example table in <A href="#4.16.1">4.16.1</A>, that might look like
|
|
|
|
|
this:</P>
|
|
|
|
|
<PRE>
|
|
|
|
|
$newSerialID = nextval('person_id_seq');
|
|
|
|
|
INSERT INTO person (id, name) VALUES ($newSerialID, 'Blaise Pascal');
|
|
|
|
|
</PRE>
|
|
|
|
|
You would then also have the new value stored in
|
|
|
|
|
<CODE>$newSerialID</CODE> for use in other queries (e.g., as a
|
|
|
|
|
foreign key to the <CODE>person</CODE> table). Note that the name
|
|
|
|
|
of the automatically created <SMALL>SEQUENCE</SMALL> object will be
|
|
|
|
|
named <<I>table</I>>_<<I>serialcolumn</I>>_<I>seq</I>,
|
|
|
|
|
where <I>table</I> and <I>serialcolumn</I> are the names of your
|
|
|
|
|
table and your <SMALL>SERIAL</SMALL> column, respectively.
|
|
|
|
|
|
|
|
|
|
<P>Alternatively, you could retrieve the assigned
|
|
|
|
|
<SMALL>SERIAL</SMALL> value with the <I>currval</I>() function
|
|
|
|
|
<I>after</I> it was inserted by default, e.g.,</P>
|
|
|
|
|
<PRE>
|
|
|
|
|
INSERT INTO person (name) VALUES ('Blaise Pascal');
|
|
|
|
|
$newID = currval('person_id_seq');
|
|
|
|
|
</PRE>
|
|
|
|
|
Finally, you could use the <A href="#4.17"><SMALL>OID</SMALL></A>
|
|
|
|
|
returned from the <SMALL>INSERT</SMALL> statement to look up the
|
|
|
|
|
default value, though this is probably the least portable approach.
|
|
|
|
|
In Perl, using DBI with Edmund Mergl's DBD::Pg module, the oid
|
|
|
|
|
value is made available via <I>$sth->{pg_oid_status} after
|
|
|
|
|
$sth->execute().</I>
|
|
|
|
|
|
|
|
|
|
<H4><A name="4.16.3">4.16.3</A>) Don't <I>currval()</I> and
|
|
|
|
|
<I>nextval()</I> lead to a race condition with other users?</H4>
|
|
|
|
|
|
|
|
|
|
<P>No. This is handled by the backends.</P>
|
|
|
|
|
|
|
|
|
|
<H4><A name="4.17">4.17</A>) What is an <SMALL>OID</SMALL>? What is
|
|
|
|
|
a <SMALL>TID</SMALL>?</H4>
|
|
|
|
|
|
|
|
|
|
<P><SMALL>OID</SMALL>s are PostgreSQL's answer to unique row ids.
|
|
|
|
|
Every row that is created in PostgreSQL gets a unique
|
|
|
|
|
<SMALL>OID</SMALL>. All <SMALL>OID</SMALL>s generated during
|
|
|
|
|
<I>initdb</I> are less than 16384 (from
|
|
|
|
|
<I>backend/access/transam.h</I>). All user-created
|
|
|
|
|
<SMALL>OID</SMALL>s are equal to or greater than this. By default,
|
|
|
|
|
all these <SMALL>OID</SMALL>s are unique not only within a table or
|
|
|
|
|
database, but unique within the entire PostgreSQL installation.</P>
|
|
|
|
|
|
|
|
|
|
<P>PostgreSQL uses <SMALL>OID</SMALL>s in its internal system
|
|
|
|
|
tables to link rows between tables. These <SMALL>OID</SMALL>s can
|
|
|
|
|
be used to identify specific user rows and used in joins. It is
|
|
|
|
|
recommended you use column type <SMALL>OID</SMALL> to store
|
|
|
|
|
<SMALL>OID</SMALL> values. You can create an index on the
|
|
|
|
|
<SMALL>OID</SMALL> field for faster access.</P>
|
|
|
|
|
|
|
|
|
|
<P>O<SMALL>id</SMALL>s are assigned to all new rows from a central
|
|
|
|
|
area that is used by all databases. If you want to change the
|
|
|
|
|
<SMALL>OID</SMALL> to something else, or if you want to make a copy
|
|
|
|
|
of the table, with the original <SMALL>OID</SMALL>'s, there is no
|
|
|
|
|
reason you can't do it:</P>
|
|
|
|
|
<PRE>
|
|
|
|
|
CREATE TABLE new_table(old_oid oid, mycol int);
|
|
|
|
|
SELECT old_oid, mycol INTO new FROM old;
|
|
|
|
|
COPY new TO '/tmp/pgtable';
|
|
|
|
|
DELETE FROM new;
|
|
|
|
|
COPY new WITH OIDS FROM '/tmp/pgtable';
|
|
|
|
|
<!--
|
|
|
|
|
CREATE TABLE new_table (mycol int);
|
|
|
|
|
INSERT INTO new_table (oid, mycol) SELECT oid, mycol FROM old_table;
|
|
|
|
|
-->
|
|
|
|
|
</PRE>
|
|
|
|
|
|
|
|
|
|
<P>O<SMALL>ID</SMALL>s are stored as 4-byte integers, and will
|
|
|
|
|
overflow at 4 billion. No one has reported this ever happening, and
|
|
|
|
|
we plan to have the limit removed before anyone does.</P>
|
|
|
|
|
|
|
|
|
|
<P>T<SMALL>ID</SMALL>s are used to identify specific physical rows
|
|
|
|
|
with block and offset values. Tids change after rows are modified
|
|
|
|
|
or reloaded. They are used by index entries to point to physical
|
|
|
|
|
rows.</P>
|
|
|
|
|
|
|
|
|
|
<H4><A name="4.18">4.18</A>) What is the meaning of some of the
|
|
|
|
|
terms used in PostgreSQL?</H4>
|
|
|
|
|
|
|
|
|
|
<P>Some of the source code and older documentation use terms that
|
|
|
|
|
have more common usage. Here are some:</P>
|
|
|
|
|
|
|
|
|
|
<UL>
|
|
|
|
|
<LI>table, relation, class</LI>
|
|
|
|
|
|
|
|
|
|
<LI>row, record, tuple</LI>
|
|
|
|
|
|
|
|
|
|
<LI>column, field, attribute</LI>
|
|
|
|
|
|
|
|
|
|
<LI>retrieve, select</LI>
|
|
|
|
|
|
|
|
|
|
<LI>replace, update</LI>
|
|
|
|
|
|
|
|
|
|
<LI>append, insert</LI>
|
|
|
|
|
|
|
|
|
|
<LI><SMALL>OID</SMALL>, serial value</LI>
|
|
|
|
|
|
|
|
|
|
<LI>portal, cursor</LI>
|
|
|
|
|
|
|
|
|
|
<LI>range variable, table name, table alias</LI>
|
|
|
|
|
</UL>
|
|
|
|
|
|
|
|
|
|
<P>A list of general database terms can be found at: <A href=
|
|
|
|
|
"http://www.comptechnews.com/~reaster/dbdesign.html">http://www.comptechnews.com/~reaster/dbdesign.html</A></P>
|
|
|
|
|
|
|
|
|
|
<H4><A name="4.19">4.19</A>) Why do I get the error <I>"FATAL:
|
|
|
|
|
palloc failure: memory exhausted?"</I><BR>
|
|
|
|
|
</H4>
|
|
|
|
|
|
|
|
|
|
<P>It is possible you have run out of virtual memory on your
|
|
|
|
|
system, or your kernel has a low limit for certain resources. Try
|
|
|
|
|
this before starting the <I>postmaster:</I></P>
|
|
|
|
|
<PRE>
|
|
|
|
|
ulimit -d 65536
|
|
|
|
|
limit datasize 64m
|
|
|
|
|
</PRE>
|
|
|
|
|
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,
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and all subprocesses created after the command is run. If you are
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having a problem with the <SMALL>SQL</SMALL> client because the
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backend is returning too much data, try it before starting the
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client.
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<H4><A name="4.20">4.20</A>) How do I tell what PostgreSQL version
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I am running?<BR>
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</H4>
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<P>From <I>psql,</I> type <CODE>select version();</CODE></P>
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<H4><A name="4.21">4.21</A>) My large-object operations get
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<I>invalid large obj descriptor.</I> Why?<BR>
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</H4>
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<P>You need to put <CODE>BEGIN WORK</CODE> and <CODE>COMMIT</CODE>
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around any use of a large object handle, that is, surrounding
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<CODE>lo_open</CODE> ... <CODE>lo_close.</CODE></P>
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<P>Currently PostgreSQL enforces the rule by closing large object
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handles at transaction commit. So the first attempt to do anything
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with the handle will draw <I>invalid large obj descriptor.</I> So
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code that used to work (at least most of the time) will now
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generate that error message if you fail to use a transaction.</P>
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<P>If you are using a client interface like <SMALL>ODBC</SMALL> you
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may need to set <CODE>auto-commit off.</CODE></P>
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<H4><A name="4.22">4.22</A>) How do I create a column that will
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default to the current time?<BR>
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</H4>
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<P>Use <I>now()</I>:</P>
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<PRE>
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<CODE>CREATE TABLE test (x int, modtime timestamp DEFAULT now() );
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|
</CODE>
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</PRE>
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<H4><A name="4.23">4.23</A>) Why are my subqueries using
|
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<CODE><SMALL>IN</SMALL></CODE> so slow?<BR>
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</H4>
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<P>Currently, we join subqueries to outer queries by sequentially
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|
scanning the result of the subquery for each row of the outer
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|
query. A workaround is to replace <CODE>IN</CODE> with
|
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|
<CODE>EXISTS</CODE>:</P>
|
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|
<PRE>
|
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|
|
<CODE>SELECT *
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|
|
FROM tab
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|
WHERE col1 IN (SELECT col2 FROM TAB2)
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|
</CODE>
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</PRE>
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|
to:
|
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|
|
<PRE>
|
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|
|
<CODE>SELECT *
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|
FROM tab
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|
WHERE EXISTS (SELECT col2 FROM TAB2 WHERE col1 = col2)
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|
|
</CODE>
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|
</PRE>
|
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|
|
We hope to fix this limitation in a future release.
|
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|
|
<H4><A name="4.24">4.24</A>) How do I perform an outer join?<BR>
|
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|
|
</H4>
|
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|
|
|
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|
|
|
<P>PostgreSQL 7.1 and later supports outer joins using the SQL
|
|
|
|
|
standard syntax. Here are two examples:</P>
|
|
|
|
|
<PRE>
|
|
|
|
|
SELECT *
|
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|
|
|
FROM t1 LEFT OUTER JOIN t2 ON (t1.col = t2.col);
|
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|
|
</PRE>
|
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|
|
|
or
|
|
|
|
|
<PRE>
|
|
|
|
|
SELECT *
|
|
|
|
|
FROM t1 LEFT OUTER JOIN t2 USING (col);
|
|
|
|
|
</PRE>
|
|
|
|
|
These identical queries join t1.col to t2.col, and also return any
|
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|
|
|
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>
|
|
|
|
|
|
|
|
|
|
<P>In previous releases, outer joins can be simulated using
|
|
|
|
|
<SMALL>UNION</SMALL> and <SMALL>NOT IN</SMALL>. For example, when
|
|
|
|
|
joining <I>tab1</I> and <I>tab2,</I> the following query does an
|
|
|
|
|
<I>outer</I> join of the two tables:<BR>
|
|
|
|
|
<BR>
|
|
|
|
|
|
|
|
|
|
<PRE>
|
|
|
|
|
SELECT tab1.col1, tab2.col2
|
|
|
|
|
FROM tab1, tab2
|
|
|
|
|
WHERE tab1.col1 = tab2.col1
|
|
|
|
|
UNION ALL
|
|
|
|
|
SELECT tab1.col1, NULL
|
|
|
|
|
FROM tab1
|
|
|
|
|
WHERE tab1.col1 NOT IN (SELECT tab2.col1 FROM tab2)
|
|
|
|
|
ORDER BY col1
|
|
|
|
|
</PRE>
|
|
|
|
|
<HR>
|
|
|
|
|
|
|
|
|
|
<CENTER>
|
|
|
|
|
<H2>Extending PostgreSQL</H2>
|
|
|
|
|
</CENTER>
|
|
|
|
|
|
|
|
|
|
<H4><A name="5.1">5.1</A>) I wrote a user-defined function. When I
|
|
|
|
|
run it in <I>psql,</I> why does it dump core?</H4>
|
|
|
|
|
|
|
|
|
|
<P>The problem could be a number of things. Try testing your
|
|
|
|
|
user-defined function in a stand-alone test program first.</P>
|
|
|
|
|
|
|
|
|
|
<H4><A name="5.2">5.2</A>) What does the message
|
|
|
|
|
<I>"NOTICE:PortalHeapMemoryFree: 0x402251d0 not in alloc set!"</I>
|
|
|
|
|
mean?</H4>
|
|
|
|
|
|
|
|
|
|
<P>You are <I>pfree'</I>ing something that was not
|
|
|
|
|
<I>palloc'</I>ed. Beware of mixing <I>malloc/free</I> and
|
|
|
|
|
<I>palloc/pfree.</I></P>
|
|
|
|
|
|
|
|
|
|
<H4><A name="5.3">5.3</A>) How can I contribute some nifty new
|
|
|
|
|
types and functions to PostgreSQL?</H4>
|
|
|
|
|
|
|
|
|
|
<P>Send your extensions to the <I>pgsql-hackers</I> mailing list,
|
|
|
|
|
and they will eventually end up in the <I>contrib/</I>
|
|
|
|
|
subdirectory.</P>
|
|
|
|
|
|
|
|
|
|
<H4><A name="5.4">5.4</A>) How do I write a C function to return a
|
|
|
|
|
tuple?</H4>
|
|
|
|
|
|
|
|
|
|
<P>This requires wizardry so extreme that the authors have never
|
|
|
|
|
tried it, though in principle it can be done.</P>
|
|
|
|
|
|
|
|
|
|
<H4><A name="5.5">5.5</A>) I have changed a source file. Why does
|
|
|
|
|
the recompile not see the change?</H4>
|
|
|
|
|
|
|
|
|
|
<P>The <I>Makefiles</I> do not have the proper dependencies for
|
|
|
|
|
include files. You have to do a <I>make clean</I> and then another
|
|
|
|
|
<I>make</I>. If you are using <SMALL>GCC</SMALL> you can use the
|
|
|
|
|
<I>--enable-depend</I> option of <I>configure</I> to have the
|
|
|
|
|
compiler compute the dependencies automatically.</P>
|
|
|
|
|
</BODY>
|
|
|
|
|
</HTML>
|
