| From: | Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us> |
|---|---|
| To: | mlw <markw(at)mohawksoft(dot)com> |
| Cc: | Andrew Sullivan <andrew(at)libertyrms(dot)info>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
| Subject: | Re: Index Scans become Seq Scans after VACUUM ANALYSE |
| Date: | 2002-04-17 22:04:09 |
| Message-ID: | 200204172204.g3HM49n08760@candle.pha.pa.us |
| Views: | Raw Message | Whole Thread | Download mbox | Resend email |
| Thread: | |
| Lists: | pgsql-hackers |
mlw wrote:
> Bruce Momjian wrote:
>
> >
> > OK, yes, sequential scan _can_ be as slow as index scan, but sometimes
> > it is faster. Can you provide reasoning why index scan should be
> > preferred, other than the admin created it, which I already addressed?
>
> If you have a choice between two or more sub-plans, similar in cost, say within
> 20% of one another. Choosing a plan which uses an index has a chance of
> improved performance if the estimates are wrong where as choosing the
> sequential scan will always have the full cost.
And the chance of reduced performance if the estimate was too low.
--
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
| From | Date | Subject | |
|---|---|---|---|
| Next Message | Bruce Momjian | 2002-04-17 22:05:54 | Re: Index Scans become Seq Scans after VACUUM ANALYSE |
| Previous Message | Dann Corbit | 2002-04-17 22:03:25 | Re: Index Scans become Seq Scans after VACUUM ANALYSE |