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Commit25deba3

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Update LIMIT/FETCH FAQ item.
1 parent0a92c58 commit25deba3

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‎doc/FAQ‎

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Frequently Asked Questions (FAQ) for PostgreSQL
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Last updated: Mon Jan 3119:18:44 EST 2005
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Last updated: Mon Jan 3120:35:58 EST 2005
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Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)
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PostgreSQL releases?
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The PostgreSQL team makes only small changes between minor releases,
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so upgrading from 7.2 to 7.2.1 does not require a dump and restore.
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However, major releases (e.g. from 7.2 to 7.3) often change the
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so upgrading from 7.4 to 7.4.1 does not require a dump and restore.
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However, major releases (e.g. from 7.3 to 7.4) often change the
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internal format of system tables and data files. These changes are
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often complex, so we don't maintain backwardcompatability for data
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often complex, so we don't maintain backwardcompatibility for data
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files. A dump outputs data in a generic format that can then be loaded
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in using the new internal format.
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In releases where the on-disk format does not change, the pg_upgrade
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script can be used to upgrade without a dump/restore. The release
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notes mention whether pg_upgrade is available for the release.
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3.8) What computer hardware should I use?
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Because PC hardware is mostly compatible, people tend to believe that
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4.1) How do I SELECT only the first few rows of a query? A random row?
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See the FETCH manual page, or use SELECT ... LIMIT....
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The entire query may have to be evaluated, even if you only want the
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first few rows. Consider using a query that has an ORDER BY. If there
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is an index that matches the ORDER BY, PostgreSQL may be able to
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evaluate only the first few records requested, or the entire query may
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have to be evaluated until the desired rows have been generated.
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To retrieve only a few rows, if you know at the number of rows needed
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at the time of the SELECT use LIMIT . If an index matches the ORDER BY
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it is possible the entire query does not have to be executed. If you
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don't know the number of rows at SELECT time, use a cursor and FETCH.
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To SELECT a random row, use:
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SELECT col

‎doc/src/FAQ/FAQ.html‎

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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: Mon Jan 3119:18:44 EST 2005</P>
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<P>Last updated: Mon Jan 3120:35:58 EST 2005</P>
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<P>Current maintainer: Bruce Momjian (<Ahref=
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"mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)
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to upgrade between major PostgreSQL releases?</H4>
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<P>The PostgreSQL team makes only small changes between minor releases,
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so upgrading from 7.2 to 7.2.1 does not require a dump and restore.
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However, major releases (e.g. from 7.2 to 7.3) often change the internal
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so upgrading from 7.4 to 7.4.1 does not require a dump and restore.
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However, major releases (e.g. from 7.3 to 7.4) often change the internal
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format of system tables and data files. These changes are often complex,
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so we don't maintain backwardcompatability for data files. A dump outputs
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so we don't maintain backwardcompatibility for data files. A dump outputs
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data in a generic format that can then be loaded in using the new internal
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format.</P>
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<P>In releases where the on-disk format does not change, the
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<I>pg_upgrade</I> script can be used to upgrade without a dump/restore.
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The release notes mention whether<I>pg_upgrade</I> is available for the
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release.</P>
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<H4><Aname="3.8">3.8</A>) What computer hardware should I use?</H4>
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<P>Because PC hardware is mostly compatible, people tend to believe that
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<H4><Aname="4.1">4.1</A>) How do I<SMALL>SELECT</SMALL> only the
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first few rows of a query? A random row?</H4>
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<P>See the<SMALL>FETCH</SMALL> manual page, or use
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<SMALL>SELECT</SMALL> ...<SMALL>LIMIT</SMALL>....</P>
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<P>The entire query may have to be evaluated, even if you only want
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the first few rows. Consider using a query that has an<SMALL>ORDER
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BY</SMALL>. If there is an index that matches the<SMALL>ORDER
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BY</SMALL>, PostgreSQL may be able to evaluate only the first few
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records requested, or the entire query may have to be evaluated
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until the desired rows have been generated.</P>
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<P>To retrieve only a few rows, if you know at the number of rows
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needed at the time of the<SMALL>SELECT</SMALL> use
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<SMALL>LIMIT</SMALL> . If an index matches the<SMALL>ORDER
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BY</SMALL> it is possible the entire query does not have to be
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executed. If you don't know the number of rows at
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<SMALL>SELECT</SMALL> time, use a cursor and
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<SMALL>FETCH</SMALL>.</P>
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<P>To<SMALL>SELECT</SMALL> a random row, use:
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<PRE>

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