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Diffstat (limited to 'doc/FAQ')
| -rw-r--r-- | doc/FAQ | 14 |
1 files changed, 10 insertions, 4 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 |