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Add MIN/MAX LIMIT/OFFSET mention.

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Bruce Momjian 2002-06-23 21:16:29 +00:00
parent aad4cc7d0d
commit 407bd1c29b
2 changed files with 21 additions and 7 deletions
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doc/FAQ
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Frequently Asked Questions (FAQ) for PostgreSQL Frequently Asked Questions (FAQ) for PostgreSQL
Last updated: Thu Jun 20 22:00:57 EDT 2002 Last updated: Sun Jun 23 17:16:13 EDT 2002
Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us) Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)
@ -723,7 +723,7 @@
Indexes are not automatically used by every query. Indexes are only Indexes are not automatically used by every query. Indexes are only
used if the table is larger than a minimum size, and the query selects used if the table is larger than a minimum size, and the query selects
only a small percentage of the rows in the table. This is because the only a small percentage of the rows in the table. This is because the
random disk access caused by an index scan is sometimes slower than a random disk access caused by an index scan can be slower than a
straight read through the table, or sequential scan. straight read through the table, or sequential scan.
To determine if an index should be used, PostgreSQL must have 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 sequential scan followed by an explicit sort is usually faster than an
index scan of a large table. index scan of a large table.
However, LIMIT combined with ORDER BY often will use an index because However, LIMIT combined with ORDER BY often will use an index because
only a small portion of the table is returned. only a small portion of the table is returned. In fact, though MAX()
and MIN() don't use indexes, it is possible to retrieve such values
using an index with ORDER BY and LIMIT:
SELECT col
FROM tab
ORDER BY col
LIMIT 1
When using wild-card operators such as LIKE or ~, indexes can only be When using wild-card operators such as LIKE or ~, indexes can only be
used if the beginning of the search is anchored to the start of the used if the beginning of the search is anchored to the start of the
string. Therefore, to use indexes, LIKE patterns must not start with string. Therefore, to use indexes, LIKE patterns must not start with

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doc/src/FAQ/FAQ.html
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alink="#0000ff"> alink="#0000ff">
<H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1> <H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1>
<P>Last updated: Thu Jun 20 22:00:57 EDT 2002</P> <P>Last updated: Sun Jun 23 17:16:13 EDT 2002</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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Indexes are not automatically used by every query. Indexes are only Indexes are not automatically used by every query. Indexes are only
used if the table is larger than a minimum size, and the query used if the table is larger than a minimum size, and the query
selects only a small percentage of the rows in the table. This is selects only a small percentage of the rows in the table. This is
because the random disk access caused by an index scan is sometimes because the random disk access caused by an index scan can be
slower than a straight read through the table, or sequential scan. slower than a straight read through the table, or sequential scan.
<P>To determine if an index should be used, PostgreSQL must have <P>To determine if an index should be used, PostgreSQL must have
@ -940,7 +940,15 @@
usually faster than an index scan of a large table.</P> usually faster than an index scan of a large table.</P>
However, <SMALL>LIMIT</SMALL> combined with <SMALL>ORDER BY</SMALL> However, <SMALL>LIMIT</SMALL> combined with <SMALL>ORDER BY</SMALL>
often will use an index because only a small portion of the table often will use an index because only a small portion of the table
is returned. is returned. In fact, though MAX() and MIN() don't use indexes,
it is possible to retrieve such values using an index with ORDER BY
and LIMIT:
<PRE>
SELECT col
FROM tab
ORDER BY col
LIMIT 1
</PRE>
<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 if the beginning of the search <I>~</I>, indexes can only be used if the beginning of the search