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Re: Merge Join vs Nested Loop

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Tobias Brox <tobias(at)nordicbet(dot)com>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: Merge Join vs Nested Loop
Date: 2006-09-26 22:09:56
Message-ID: 719.1159308596@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-performance
Tobias Brox <tobias(at)nordicbet(dot)com> writes:
> What causes the nested loops to be estimated so costly - or is it the
> merge joins that are estimated too cheaply?  Should I raise all the
> planner cost constants, or only one of them?

If your tables are small enough to fit (mostly) in memory, then the
planner tends to overestimate the cost of a nestloop because it fails to
account for cacheing effects across multiple scans of the inner table.
This is addressed in 8.2, but in earlier versions about all you can do
is reduce random_page_cost, and a sane setting of that (ie not less than
1.0) may not be enough to push the cost estimates where you want them.
Still, reducing random_page_cost ought to be your first recourse.

			regards, tom lane

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