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Re: osdl-dbt3 run results - puzzled by the execution plans

From: "Matt Clark" <matt(at)ymogen(dot)net>
To: "Jenny Zhang" <jenny(at)osdl(dot)org>,<pgsql-performance(at)postgresql(dot)org>
Subject: Re: osdl-dbt3 run results - puzzled by the execution plans
Date: 2003-09-18 23:19:00
Message-ID: LFEIJBEOKGPDHCEMDGNFOEEKCDAA.matt@ymogen.net (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-performance
> We thought the large effective_cache_size should lead us to better
> plans. But we found the opposite.

Maybe it's inappropriate for little old me to jump in here, but the plan
isn't usually that important compared to the actual runtime.  The links you
give show the output of 'explain' but not 'explain analyze', so it's not
clear wich plan is actually _faster_.

If you really do have only 8MB of FS cache, then either plan will run
slowly.  If you really do have 5GB of FS cache then either plan will run a
lot faster.  Why would you deliberately give the planner false information
about this?

PG obviously thinks plan 1 is 'better' when pages have to be fetched from
disk, and plan 2 is 'better' when they don't.  Which is really better
depends on whether those pages do have to be fetched from disk or not, and
PG can only know what you tell it about that, so changing ECS without
actually removing the RAM from the system seems a little pointless to me...

M





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