Re: Index Scans become Seq Scans after VACUUM ANALYSE - Mailing list pgsql-hackers

From Bruce Momjian
Subject Re: Index Scans become Seq Scans after VACUUM ANALYSE
Date
Msg-id 200204172204.g3HM49n08760@candle.pha.pa.us
Whole thread Raw
In response to Re: Index Scans become Seq Scans after VACUUM ANALYSE  (mlw <markw@mohawksoft.com>)
List 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@candle.pha.pa.us               |  (610)
853-3000+  If your life is a hard drive,     |  830 Blythe Avenue +  Christ can be your backup.        |  Drexel Hill,
Pennsylvania19026
 


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