| From: | Mladen Gogala <mladen(dot)gogala(at)vmsinfo(dot)com> |
|---|---|
| To: | Neil Whelchel <neil(dot)whelchel(at)gmail(dot)com> |
| Cc: | "pgsql-performance(at)postgresql(dot)org" <pgsql-performance(at)postgresql(dot)org> |
| Subject: | Re: Slow count(*) again... |
| Date: | 2010-10-11 23:50:36 |
| Message-ID: | 4CB3A2CC.9070009@vmsinfo.com |
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| Lists: | pgsql-hackers pgsql-performance |
On 10/11/2010 3:54 PM, Neil Whelchel wrote:
> 1. A faster count(*), or something like my proposed estimate(*).
> 2. A way to get the total rows matched when using LIMIT and OFFSET before
> LIMIT and OFFSET are applied.
The biggest single problem with "select count(*)" is that it is
seriously overused. People use that idiom to establish existence, which
usually leads to a performance disaster in the application using it,
unless the table has no more than few hundred records. SQL language, of
which PostgreSQL offers an excellent implementation, offers [NOT]
EXISTS clause since its inception in the Jurassic era. The problem is
with the sequential scan, not with counting. I'd even go as far as to
suggest that 99% instances of the "select count(*)" idiom are probably
bad use of the SQL language.
--
Mladen Gogala
Sr. Oracle DBA
1500 Broadway
New York, NY 10036
(212) 329-5251
www.vmsinfo.com
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