From: Stephan Szabo <sszabo@megazone23.bigpanda.com>
To: Tomasz Myrta <jasiek@klaster.net>
Cc: pgsql-sql@postgresql.org
Subject: Re: sub-select with aggregate
Date: Wed, 23 Oct 2002 08:27:45 -0700 (PDT)
Message-ID: <20021023082355.G5481-100000@megazone23.bigpanda.com> (raw)
In-Reply-To: <20021023081036.M5349-100000@megazone23.bigpanda.com>
On Wed, 23 Oct 2002, Stephan Szabo wrote:
> Did you see the other two queries I gave? On 7.3, both of those queries
> appear (according to explain output) to do the limiting of group_id
> inside the subquery rather than doing the subquery with all rows.
> The explanation above was why I believe it was different from your
> original query.
Replying to myself with a clarification.
The other forms only move the filtering clauses around. There's still
only a filter on the outer group_id equaling the inner group_id and
a filter on group_id=3. It's just a question of whether it's:
Scan users in subselect from group_id=3, group and aggregate them
and join with users on subselect's group_id = users.group_id
or
Scan users for group_id=3, scan users in subselect, group and
aggregate them and join these two on subselect's
group_id=users.group_id.
The latter is effectively what your original query gave, the former
appears to be what my two queries from a couple of mails ago gives.
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To: pgsql-sql@postgresql.org
Cc: sszabo@megazone23.bigpanda.com, jasiek@klaster.net
Subject: Re: sub-select with aggregate
In-Reply-To: <20021023082355.G5481-100000@megazone23.bigpanda.com>
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