From 407bd1c29b66f7e007288455982f428e7f26b7f9 Mon Sep 17 00:00:00 2001
From: Bruce Momjian <bruce@momjian.us>
Date: Sun, 23 Jun 2002 21:16:29 +0000
Subject: [PATCH] Add MIN/MAX LIMIT/OFFSET mention.
---
doc/FAQ | 14 ++++++++++----
doc/src/FAQ/FAQ.html | 14 +++++++++++---
2 files changed, 21 insertions(+), 7 deletions(-)
diff --git a/doc/FAQ b/doc/FAQ
index a60063eac18..a1f9d150ba7 100644
--- a/doc/FAQ
+++ b/doc/FAQ
@@ -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
--
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