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Mega-clarifications from Joachim Wieland.
This commit is contained in:
Bruce Momjian
@ -10,7 +10,7 @@
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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: Sat Nov 27 00:14:59 EST 2004</P>
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<P>Last updated: Sat Nov 27 23:55:37 EST 2004</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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serial/auto-incrementing field?<BR>
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<A href="#4.15.2">4.15.2</A>) How do I get the value of a
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<SMALL>SERIAL</SMALL> insert?<BR>
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<A href="#4.15.3">4.15.3</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="#4.15.3">4.15.3</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="#4.15.4">4.15.4</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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@ -395,10 +395,9 @@
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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>Please visit the PostgreSQL BugTool page at <A href=
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"http://www.PostgreSQL.org/bugs/bugs.php">http://www.PostgreSQL.org/bugs/bugs.php</A>,
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which gives guidelines and directions on how to submit a
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bug report.</P>
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<P>Visit the PostgreSQL bug form at <A href=
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"http://www.postgresql.org/bugform.html">
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http://www.postgresql.org/bugform.html</A>.</P>
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<P>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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@ -546,7 +545,9 @@
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http://www.pgaccess.org</a>), PgAdmin III (<a
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href="http://www.pgadmin.org">http://www.pgadmin.org</a>, RHDB Admin (<a
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href="http://sources.redhat.com/rhdb/">http://sources.redhat.com/rhdb/
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</a>) and Rekall (<a href="http://www.thekompany.com/products/rekall/">
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</a>), TORA (<a href="http://www.globecom.net/tora/">http://www.globecom.net/tora/
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(partly commercial)</a>, and Rekall
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(<a href="http://www.thekompany.com/products/rekall/">
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http://www.thekompany.com/products/rekall/</a>, proprietary). There is
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also PhpPgAdmin (<a href="http://phppgadmin.sourceforge.net/">
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http://phppgadmin.sourceforge.net/ </a>), a web-based interface to
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@ -606,10 +607,9 @@
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how many buffers and backend processes you configure for
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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 ~1 MB. See the <A
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href=
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"http://www.PostgreSQL.org/docs/view.php?version=current&idoc=1&file=kernel-resources.html">PostgreSQL
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Administrator's Guide</A> for more detailed information about
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shared memory and semaphores.</P>
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href="http://www.postgresql.org/docs/current/static/kernel-resources.html">PostgreSQL
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Administrator's Guide/Server Run-time Environment/Managing Kernel Resources</A>
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section for more detailed information about shared memory and semaphores.</P>
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<H4><A name="3.4">3.4</A>) When I try to start <I>postmaster</I>, I
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get <I>IpcSemaphoreCreate</I> errors. Why?</H4>
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better performance?</H4>
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<P>Certainly, indexes can speed up queries. The
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<SMALL>EXPLAIN ANALYZE</SMALL> command allows you to see how PostgreSQL is
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interpreting your query, and which indexes are being used.</P>
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<SMALL>EXPLAIN ANALYZE</SMALL> command allows you to see how
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PostgreSQL is interpreting your query, and which indexes are
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being used.</P>
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<P>If you are doing many <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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@ -657,22 +658,23 @@
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reduces the transaction overhead. Also, consider dropping and
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recreating indexes when making large data changes.</P>
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<P>There are several tuning options. You can disable <I>fsync()</I>
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by starting <I>postmaster</I> with a <I>-o -F</I> option. This will
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<P>There are several tuning options in the <a href=
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"http://www.postgresql.org/docs/current/static/runtime.html">
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Administration Guide/Server Run-time Environment/Run-time Configuration</a>.
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You can disable <I>fsync()</I> by using <i>fsync</I> option. This will
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prevent <I>fsync()</I>s 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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<P>You can use the <I>shared_buffers</I> option to
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increase the number of shared memory buffers used by the backend
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processes. If you make this parameter too high, the
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<I>postmaster</I> may not start because you have exceeded your
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kernel's limit on shared memory space. Each buffer is 8K and the
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default is 64 buffers.</P>
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default is 1000 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 (i.e. 512K).</P>
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<P>You can also use the <I>sort_mem</I> and <I>work_mem</I> options
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to increase the maximum amount of memory used by the backend processes
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for each temporary sort. The default is 1024 (i.e. 1MB).</P>
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<P>You can also use the <SMALL>CLUSTER</SMALL> command to group
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data in tables to match an index. See the <SMALL>CLUSTER</SMALL>
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<P>If <I>postmaster</I> is running, start <I>psql</I> in one
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window, then find the <SMALL>PID</SMALL> of the <I>postgres</I>
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process used by <I>psql</I>. Use a debugger to attach to the
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<I>postgres</I> <SMALL>PID</SMALL>. You can set breakpoints in the
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debugger and issue queries from <I>psql</I>. If you are debugging
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<I>postgres</I> startup, you can set PGOPTIONS="-W n", then start
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<I>psql</I>. This will cause startup to delay for <I>n</I> seconds
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so you can attach to the process with the debugger, set any
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breakpoints, and continue through the startup sequence.</P>
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process used by <I>psql</I> using <pre>SELECT pg_backend_pid()</pre>.
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Use a debugger to attach to the <I>postgres</I> <SMALL>PID</SMALL>.
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You can set breakpoints in the debugger and issue queries from
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<I>psql</I>. If you are debugging <I>postgres</I> startup, you can
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set PGOPTIONS="-W n", then start <I>psql</I>. This will cause startup
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to delay for <I>n</I> seconds so you can attach to the process with
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the debugger, set any breakpoints, and continue through the startup
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sequence.</P>
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<P>The <I>postgres</I> program has <I>-s, -A</I>, and <I>-t</I>
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options that can be very useful for debugging and performance
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measurements.</P>
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<P>There are several <pre>log_*</pre> server configuration variables
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that enable printing of process statistics which can be very useful
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for debugging and performance measurements.</P>
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<P>You can also compile with profiling to see what functions are
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taking execution time. The backend profile files will be deposited
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@ -947,7 +950,8 @@
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<H4><A name="4.8">4.8</A>) My queries are slow or don't make use of
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the indexes. Why?</H4>
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Indexes are not automatically used by every query. Indexes are only
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<P>Indexes are not automatically used by every query. Indexes are only
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used if the table is larger than a minimum size, and the query
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selects only a small percentage of the rows in the table. This is
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because the random disk access caused by an index scan can be
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<I>~*</I> do not utilise indexes. Instead, use functional
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indexes, which are described in section <a href="#4.12">4.12</a>.</LI>
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<LI>The default <I>C</I> locale must be used during
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<i>initdb</i>.</LI>
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<i>initdb</i> because it is not possible to know the next-greater
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character in a non-C locale. You can create a special
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<PRE>text_pattern_ops</PRE> index for such cases that work only
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for <PRE>LIKE</PRE> indexing.
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</LI>
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</UL>
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<P>
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@ -1169,8 +1177,8 @@ BYTEA bytea variable-length byte array (null-byte safe)
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value is made available via <I>$sth->{pg_oid_status}</I> after
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<I>$sth->execute()</I>.
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<H4><A name="4.15.3">4.15.3</A>) Don't <I>currval()</I> and
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<I>nextval()</I> lead to a race condition with other users?</H4>
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<H4><A name="4.15.3">4.15.3</A>) Doesn't <I>currval()</I>
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lead to a race condition with other users?</H4>
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<P>No. <I>currval()</I> returns the current value assigned by your
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backend, not by all users.</P>
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