From: Tom Lane <tgl@sss.pgh.pa.us>
To: Simon Riggs <simon@2ndQuadrant.com>
Cc: Heikki Linnakangas <heikki.linnakangas@enterprisedb.com>
Cc: List pgsql-patches <pgsql-patches@postgresql.org>
Subject: Re: WIP Join Removal
Date: Tue, 02 Sep 2008 12:02:05 -0400
Message-ID: <14800.1220371325@sss.pgh.pa.us> (raw)
In-Reply-To: <1220352898.4371.355.camel@ebony.2ndQuadrant>
References: <1220176372.4371.118.camel@ebony.2ndQuadrant>
<48BC4127.30706@enterprisedb.com>
<1220348487.4371.307.camel@ebony.2ndQuadrant>
<48BD1374.4030504@enterprisedb.com>
<1220352112.4371.349.camel@ebony.2ndQuadrant>
<48BD1852.1070904@enterprisedb.com>
<1220352898.4371.355.camel@ebony.2ndQuadrant>
Simon Riggs <simon@2ndQuadrant.com> writes:
>> Oh. How does the query look like after removing the join, then?
> Same answer, just slower. Removing the join makes the access to a into a
> SeqScan, whereas it was a two-table index plan when both tables present.
I don't really believe this: please show an actual case where the join
would be faster.
AFAICS, in the outer-join examples, it is not possible for a join to
enable some kind of indexscan on the outer table, because by definition
an outer join excludes none of the left-hand rows. So a seqscan on the
outer is optimal.
I also find all the worry about generating other plans for the inner
relation to be off the mark. You're not going to *use* any plan for the
inner rel, so who cares what plans it has?
regards, tom lane
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To: pgsql-patches@postgresql.org
Cc: tgl@sss.pgh.pa.us, simon@2ndQuadrant.com, heikki.linnakangas@enterprisedb.com
Subject: Re: WIP Join Removal
In-Reply-To: <14800.1220371325@sss.pgh.pa.us>
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