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Bruce Momjian 2003-02-14 04:07:46 +00:00
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Frequently Asked Questions (FAQ) for PostgreSQL Frequently Asked Questions (FAQ) for PostgreSQL
Last updated: Mon Jan 27 01:24:42 EST 2003 Last updated: Thu Feb 13 23:07:35 EST 2003
Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us) Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)

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<H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1> <H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1>
<P>Last updated: Thu Dec 5 00:47:26 EST 2002</P> <P>Last updated: Thu Feb 13 23:07:35 EST 2003</P>
<P>Current maintainer: Bruce Momjian (<A href= <P>Current maintainer: Bruce Momjian (<A href=
"mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)<BR> "mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)<BR>
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<H4><A name="1.7">1.7</A>) What is the latest release?</H4> <H4><A name="1.7">1.7</A>) What is the latest release?</H4>
<P>The latest release of PostgreSQL is version 7.2.3.</P> <P>The latest release of PostgreSQL is version 7.3.1.</P>
<P>We plan to have major releases every four months.</P> <P>We plan to have major releases every four months.</P>
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interface?</H4> interface?</H4>
Yes, there are several graphical interfaces to PostgreSQL available. Yes, there are several graphical interfaces to PostgreSQL available.
These include PgAccess <a href="http://www.pgaccess.com"> These include PgAccess <a href="http://www.pgaccess.org">
http://www.pgaccess.com</a>), PgAdmin II (<a http://www.pgaccess.org</a>), PgAdmin II (<a
href="http://www.pgadmin.org">http://www.pgadmin.org</a>, href="http://www.pgadmin.org">http://www.pgadmin.org</a>,
Win32-only), RHDB Admin (<a Win32-only), RHDB Admin (<a
href="http://sources.redhat.com/rhdb/">http://sources.redhat.com/rhdb/ href="http://sources.redhat.com/rhdb/">http://sources.redhat.com/rhdb/
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http://phppgadmin.sourceforge.net/ </a>), a web-based interface to http://phppgadmin.sourceforge.net/ </a>), a web-based interface to
PostgreSQL. PostgreSQL.
<P>We have a nice graphical user interface called PgAccess which can
also be used as a report generator. The Web page is
<A href="http://www.pgaccess.org/">http://www.pgaccess.org/</A>.</P>
<H4><A name="2.4">2.4</A>) What languages are able to communicate with <H4><A name="2.4">2.4</A>) What languages are able to communicate with
PostgreSQL?</H4> PostgreSQL?</H4>
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the number of allowed backend processes is so your system won't run the number of allowed backend processes is so your system won't run
out of resources.</P> 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="3.9">3.9</A>) What is in the <I>pgsql_tmp</I> directory?</H4> <H4><A name="3.9">3.9</A>) What is in the <I>pgsql_tmp</I> directory?</H4>
<P>This directory contains temporary files generated by the query <P>This directory contains temporary files generated by the query
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LIMIT 1; LIMIT 1;
</PRE> </PRE>
<P>If you believe the optimizer is incorrect in choosing a
sequential scan, use <CODE>SET enable_seqscan TO 'off'</CODE> and
run tests to see if an index scan is indeed faster.</P>
<P>When using wild-card operators such as <SMALL>LIKE</SMALL> or <P>When using wild-card operators such as <SMALL>LIKE</SMALL> or
<I>~</I>, indexes can only be used in certain circumstances:</P> <I>~</I>, indexes can only be used in certain circumstances:</P>
<UL> <UL>
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<PRE> <PRE>
Type Internal Name Notes Type Internal Name Notes
-------------------------------------------------- --------------------------------------------------
"char" char 1 character
CHAR(n) bpchar blank padded to the specified fixed length
VARCHAR(n) varchar size specifies maximum length, no padding VARCHAR(n) varchar size specifies maximum length, no padding
CHAR(n) bpchar blank padded to the specified fixed length
TEXT text no specific upper limit on length TEXT text no specific upper limit on length
"char" char one character
BYTEA bytea variable-length byte array (null-byte safe) BYTEA bytea variable-length byte array (null-byte safe)
</PRE> </PRE>
@ -1056,11 +1051,13 @@ BYTEA bytea variable-length byte array (null-byte safe)
stored out-of-line by <SMALL>TOAST</SMALL>, so the space on disk stored out-of-line by <SMALL>TOAST</SMALL>, so the space on disk
might also be less than expected.</P> might also be less than expected.</P>
<P><SMALL>CHAR(n)</SMALL> is best when storing strings that are <SMALL>VARCHAR(n)</SMALL> is best when storing variable-length
usually the same length. <SMALL>VARCHAR(n)</SMALL> is best when strings and it limits how long a string can be. <SMALL>TEXT</SMALL>
storing variable-length strings but it limits how long a string can is for strings of unlimited length, with a maximum of one gigabyte.
be. <SMALL>TEXT</SMALL> is for strings of unlimited length, maximum <P><SMALL>CHAR(n)</SMALL> is for storing strings that are all the
1 gigabyte. <SMALL>BYTEA</SMALL> is for storing binary data, same length. <SMALL>CHAR(n)</SMALL> pads with blanks to the specified
length, while <SMALL>VARCHAR(n)</SMALL> only stores the characters
supplied. <SMALL>BYTEA</SMALL> is for storing binary data,
particularly values that include <SMALL>NULL</SMALL> bytes. These particularly values that include <SMALL>NULL</SMALL> bytes. These
types have similar performance characteristics.</P> types have similar performance characteristics.</P>
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</PRE> </PRE>
For this to be fast, <CODE>subcol</CODE> should be an indexed column. For this to be fast, <CODE>subcol</CODE> should be an indexed column.
We hope to fix this limitation in a future release. This preformance problem will be fixed in 7.4.
<H4><A name="4.23">4.23</A>) How do I perform an outer join?</H4> <H4><A name="4.23">4.23</A>) How do I perform an outer join?</H4>