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Re: Seqscan rather than Index

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Greg Stark <gsstark(at)mit(dot)edu>
Cc: Richard Huxton <dev(at)archonet(dot)com>,David Brown <time(at)bigpond(dot)net(dot)au>, pgsql-performance(at)postgresql(dot)org
Subject: Re: Seqscan rather than Index
Date: 2004-12-17 18:36:52
Message-ID: 21610.1103308612@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-performance
Greg Stark <gsstark(at)mit(dot)edu> writes:
> Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> writes:
>> I think the one effect that's not being modeled is amortization of index
>> fetches across successive queries.  

> And across multiple fetches in a single query, such as with a nested loop.

Right, that's effectively the same problem.  You could imagine making a
special-purpose solution for nestloop queries but I think the issue is
more general than that.

> It seems like the effective_cache_size parameter should be having some
> influence here.

But it doesn't :-(.  e_c_s is currently only used to estimate
amortization of repeated heap-page fetches within a single indexscan.

			regards, tom lane

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