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Re: Tuning planner cost estimates

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: "Jim C(dot) Nasby" <decibel(at)decibel(dot)org>
Cc: Josh Berkus <josh(at)agliodbs(dot)com>,pgsql-performance(at)postgresql(dot)org, testperf-general(at)pgfoundry(dot)org
Subject: Re: Tuning planner cost estimates
Date: 2005-05-20 23:06:51
Message-ID: 4415.1116630411@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-performance
"Jim C. Nasby" <decibel(at)decibel(dot)org> writes:
> Does this sound like a reasonable approach? Also, how important do
> people think it is to use explain analyze output instead of just doing
> SELECT count(*) FROM (query you actually want to test)? (The select
> count(*) wrapper is just a means to throw away the results since we
> don't really want to worry about data transfer times, etc). The testing
> I've done (http://stats.distributed.net/~decibel/base.log) shows explain
> analyze to be almost 5x slower than select count(*), so it seems a big
> gain if we can avoid that.

I'd go with the select count(*) --- I can't imagine that we will be
trying to model the behavior of anything so complex that we really need
explain analyze output.  (On the other hand, recording explain output is
a good idea to make sure you are testing what you think you are testing.)

Actually, it might be worth using "select count(null)", which should
avoid the calls to int8inc.  I think this doesn't matter so much in CVS
tip, but certainly in existing releases the palloc overhead involved is
noticeable.

BTW, 5x is an awful lot; I've not noticed overheads more than about 2x.

			regards, tom lane

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