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Commit407bd1c

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Add MIN/MAX LIMIT/OFFSET mention.
1 parentaad4cc7 commit407bd1c

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+21
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lines changed

2 files changed

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‎doc/FAQ

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Frequently Asked Questions (FAQ) for PostgreSQL
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Last updated:Thu Jun20 22:00:57 EDT 2002
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Last updated:Sun Jun23 17:16:13 EDT 2002
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Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)
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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 selects
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only a small percentage of the rows in the table. This is because the
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random disk access caused by an index scanis sometimes slower than a
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random disk access caused by an index scancan be slower than a
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straight read through the table, or sequential scan.
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To determine if an index should be used, PostgreSQL must have
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sequential scan followed by an explicit sort is usually faster than an
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index scan of a large table.
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However, LIMIT combined with ORDER BY often will use an index because
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only a small portion of the table is returned.
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only a small portion of the table is returned. In fact, though MAX()
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and MIN() don't use indexes, it is possible to retrieve such values
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using an index with ORDER BY and LIMIT:
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SELECT col
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FROM tab
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ORDER BY col
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LIMIT 1
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When using wild-card operators such as LIKE or ~, indexes can only be
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used if the beginning of the search is anchored to the start of the
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string. Therefore, to use indexes, LIKE patterns must not start with

‎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:Thu Jun20 22:00:57 EDT 2002</P>
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<P>Last updated:Sun Jun23 17:16:13 EDT 2002</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>)<BR>
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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 scanis sometimes
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because the random disk access caused by an index scancan be
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slower than a straight read through the table, or sequential scan.
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<P>To determine if an index should be used, PostgreSQL must have
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usually faster than an index scan of a large table.</P>
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However,<SMALL>LIMIT</SMALL> combined with<SMALL>ORDER BY</SMALL>
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often will use an index because only a small portion of the table
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is returned.
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is returned. In fact, though MAX() and MIN() don't use indexes,
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it is possible to retrieve such values using an index with ORDER BY
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and LIMIT:
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<PRE>
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SELECT col
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FROM tab
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ORDER BY col
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LIMIT 1
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</PRE>
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<P>When using wild-card operators such as<SMALL>LIKE</SMALL> or
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<I>~</I>, indexes can only be used if the beginning of the search

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