From: | Bruce Momjian <bruce(at)momjian(dot)us> |
---|---|
To: | Bruce Momjian <bruce(at)momjian(dot)us> |
Cc: | Robert Haas <robertmhaas(at)gmail(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Hannu Krosing <hannu(at)2ndquadrant(dot)com>, Peter Hussey <peter(at)labkey(dot)com>, pgsql-performance <pgsql-performance(at)postgresql(dot)org> |
Subject: | Re: Questions on query planner, join types, and work_mem |
Date: | 2011-02-01 20:24:05 |
Message-ID: | 201102012024.p11KO6U20810@momjian.us |
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Lists: | pgsql-performance |
Bruce Momjian wrote:
> Robert Haas wrote:
> > On Wed, Aug 11, 2010 at 9:42 PM, Bruce Momjian <bruce(at)momjian(dot)us> wrote:
> > > This confused me. ?If we are assuing the data is in
> > > effective_cache_size, why are we adding sequential/random page cost to
> > > the query cost routines?
> >
> > See the comments for index_pages_fetched(). We basically assume that
> > all data starts uncached at the beginning of each query - in fact,
> > each plan node. effective_cache_size only measures the chances that
> > if we hit the same block again later in the execution of something
> > like a nested-loop-with-inner-indexscan, it'll still be in cache.
> >
> > It's an extremely weak knob, and unless you have tables or indices
> > that are larger than RAM, the only mistake you can make is setting it
> > too low.
>
> The attached patch documents that there is no assumption that data
> remains in the disk cache between queries. I thought this information
> might be helpful.
Applied.
--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com
+ It's impossible for everything to be true. +
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