Re: WIP Join Removal

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Simon Riggs <simon(at)2ndQuadrant(dot)com>
Cc: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>, List pgsql-patches <pgsql-patches(at)postgresql(dot)org>
Subject: Re: WIP Join Removal
Date: 2008-09-02 16:02:05
Message-ID: 14800.1220371325@sss.pgh.pa.us
Views: Raw Message | Whole Thread | Download mbox | Resend email
Thread:
Lists: pgsql-patches

Simon Riggs <simon(at)2ndQuadrant(dot)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

In response to

Responses

Browse pgsql-patches by date

  From Date Subject
Next Message Gregory Stark 2008-09-02 16:03:15 Re: WIP Join Removal
Previous Message Tom Lane 2008-09-02 15:39:29 Re: rmgr hooks and contrib/rmgr_hook