| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Greg Smith <greg(at)2ndquadrant(dot)com> |
| Cc: | jd(at)commandprompt(dot)com, Ogden <lists(at)darkstatic(dot)com>, Jesper Krogh <jesper(at)krogh(dot)cc>, Kenneth Marshall <ktm(at)rice(dot)edu>, pgsql-performance(at)postgresql(dot)org |
| Subject: | Re: Query much faster with enable_seqscan=0 |
| Date: | 2010-09-21 23:24:47 |
| Message-ID: | 11632.1285111487@sss.pgh.pa.us |
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| Lists: | pgsql-performance |
Greg Smith <greg(at)2ndquadrant(dot)com> writes:
> and the query optimizer needs to be careful about what it does and
> doesn't pull from disk. If that's not the case, like here where there's
> 8GB of RAM and a 7GB database, dramatic reductions to both seq_page_cost
> and random_page_cost can make sense. Don't be afraid to think lowering
> below 1.0 is going too far--something more like 0.01 for sequential and
> 0.02 for random may actually reflect reality here.
If you are tuning for an all-in-RAM situation, you should set
random_page_cost equal to seq_page_cost (and usually set both smaller
than 1). By definition, those costs are equal if you're fetching from
RAM. If it's only mostly-in-RAM then keeping random_page_cost a bit
higher makes sense.
regards, tom lane
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