From: Tom Lane <tgl@sss.pgh.pa.us>
To: Stephan Szabo <sszabo@megazone23.bigpanda.com>
Cc: Nicolas Fertig <nfertig@swissnow.ch>
Cc: pgsql-sql@postgresql.org
Subject: Re: OUTER JOIN with filter
Date: Sun, 02 Mar 2003 02:09:17 -0500
Message-ID: <16210.1046588957@sss.pgh.pa.us> (raw)
In-Reply-To: <20030301170535.U21117-100000@megazone23.bigpanda.com>
References: <20030301170535.U21117-100000@megazone23.bigpanda.com>
Stephan Szabo <sszabo@megazone23.bigpanda.com> writes:
> On Sat, 1 Mar 2003, Nicolas Fertig wrote:
>> Can anyone help me with the problem bellow ?
>>
>> SELECT TM.some_field, TS.name
>> FROM table_main TM
>> LEFT OUTER JOIN (SELECT name FROM table_slave WHERE c1 = 'myvalue') TS
>> USING(id)
> select tm.some_field, ts.name
> from table_main tm left outer join table_slave ts
> on (tm.id=ts.id and ts.c1='myvalue');
Offhand I believe that these queries should generate identical plans.
They do not at the moment --- the second one generates a worse plan
(sorry Stephan ;-)) --- because the planner does not realize it could
push down the ts.c1='myvalue' JOIN condition into the scan of ts, even
though the join is OUTER. But AFAICS it would not change the results to
do so; ts rows failing ts.c1='myvalue' will not produce join output
anyway, but would allow outer-joined lefthand rows to be produced. (Can
anyone see a hole in that logic? It's on my to-do list to change it...)
I concur with the other comments that there's nothing wrong with the
given query ... other than such trivial details as it doesn't work ...
the sub-select doesn't produce an ID column. So I think Nicolas is
giving us an over-censored description of his real problem; but without
the omitted details, we're not likely to offer much useful help.
regards, tom lane
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To: pgsql-sql@postgresql.org
Cc: tgl@sss.pgh.pa.us, sszabo@megazone23.bigpanda.com, nfertig@swissnow.ch
Subject: Re: OUTER JOIN with filter
In-Reply-To: <16210.1046588957@sss.pgh.pa.us>
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