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Re: Odd number of rows expected

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: "Jim C(dot) Nasby" <decibel(at)decibel(dot)org>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: Odd number of rows expected
Date: 2005-01-23 22:39:12
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-performance
"Jim C. Nasby" <decibel(at)decibel(dot)org> writes:
> In english, each bucket defines a specific time period, and no two
> buckets can over-lap (though there's no constraints defined to actually
> prevent that). So reality is that each row in page_log.log will in fact
> only match one row in bucket (at least for each value of rrs_id).

> Given that, would the optimizer make a better choice if it knew that
> (since it means a much smaller result set).

Given that the join condition is not an equality, there's no hope of
using hash or merge join; so the join itself is about as good as you're
gonna get.  With a more accurate rows estimate for the join result, it
might have decided to use HashAggregate instead of Sort/GroupAggregate,
but AFAICS that would not have made a huge difference ... at best maybe
25% of the total query time.

> Is there any way to tell the
> optimizer this is the case?

Nope.  This gets back to the old problem of not having any cross-column
(cross-table in this case) statistics.

			regards, tom lane

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