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| -rw-r--r-- | doc/FAQ | 14 | ||||
| -rw-r--r-- | doc/src/FAQ/FAQ.html | 14 |
2 files changed, 21 insertions, 7 deletions
@@ -1,7 +1,7 @@ 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) @@ -723,7 +723,7 @@ 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 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. To determine if an index should be used, PostgreSQL must have @@ -738,8 +738,14 @@ sequential scan followed by an explicit sort is usually faster than an index scan of a large table. 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 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 diff --git a/doc/src/FAQ/FAQ.html b/doc/src/FAQ/FAQ.html index 08e9c9d98eb..b6de1a6d499 100644 --- a/doc/src/FAQ/FAQ.html +++ b/doc/src/FAQ/FAQ.html @@ -14,7 +14,7 @@ alink="#0000ff"> <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= "mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)<BR> @@ -923,7 +923,7 @@ 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 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. <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> However, <SMALL>LIMIT</SMALL> combined with <SMALL>ORDER BY</SMALL> 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 <I>~</I>, indexes can only be used if the beginning of the search |