Re: Index doesn't appear to be working.

Is there any plan of making Postgresql collecting statistics a little more
automatically?

-----Original Message-----
From: pgsql-sql-owner(at)postgresql(dot)org
[mailto:pgsql-sql-owner(at)postgresql(dot)org]On Behalf Of Bruce Momjian
Sent: Sunday, March 03, 2002 11:03 AM
To: Stephan Szabo
Cc: John Oakes; pgsql-sql(at)postgresql(dot)org
Subject: Re: [SQL] Index doesn't appear to be working.

> > Why isn't it using the index? This query returns about 5,600 of 10,000
> > records. It is faster to just do a seq scan because it returns such a
high
> > percentage or records? Thanks, I appreciate the help!
>
> Usually the seq scan will be faster in this kind of situation. Because
> the system needs to load the matching rows anyway, it's likely to result
> in reading all the blocks of the table and paying a penalty for seeking
> around the file.

We are getting this "no index use" question over and over again, and I
was wondering why the FAQ item was not answering this question for
people. It turns out the wording was not very clear and I have updated
this FAQ item to more clearly answer the question. Seems I kept adding
to that item without restructuring the information:

<H4><A name="4.8">4.8</A>) My queries are slow or don't make use of
the indexes. Why?</H4>

Indexes are not automatically used by every query. Indexes are only
used if the table is larger than a minimum size, and the index
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 straight read through the table, or sequential scan.

<P>To determine if an index should be used, PostgreSQL must have
statistics about the table. These statistics are collected using
<SMALL>VACUUM ANALYZE</SMALL>, or simply <SMALL>ANALYZE</SMALL>.
Using statistics, the optimizer knows how many rows are in the
table, and can better determine if indexes should be used.
Statistics are also valuable in determining optimal join order and
join methods. Statistics collection should be performed periodically
as the contents of the table change.

<P>Indexes are normally not used for <SMALL>ORDER BY</SMALL> or to
perform joins. A sequential scan followed by an explicit sort is
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.

<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 is
anchored to the start of the string. Therefore, to use indexes,
<SMALL>LIKE</SMALL> patterns must not start with <I>%</I>, and
<I>~</I>(regular expression) patterns must start with <I>^</I>.</P>

--
Bruce Momjian | http://candle.pha.pa.us
pgman(at)candle(dot)pha(dot)pa(dot)us | (610) 853-3000
+ If your life is a hard drive, | 830 Blythe Avenue
+ Christ can be your backup. | Drexel Hill, Pennsylvania 19026

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