From: | Alvaro Herrera <alvherre(at)commandprompt(dot)com> |
---|---|
To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | pgsql-hackers(at)postgreSQL(dot)org |
Subject: | Re: Improving planning of outer joins |
Date: | 2005-12-16 20:12:43 |
Message-ID: | 20051216201243.GA27602@surnet.cl |
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Lists: | pgsql-hackers |
Tom Lane wrote:
> I've spent some time looking into how we can improve our planning of outer
> joins. The current planner code slavishly follows the syntactic join
> order, which can lead to quite bad plans. The reason it does this is that
> in some cases altering the join order of outer joins can change the
> results. However, there are many cases where the results would not be
> changed, and we really need to start taking advantage of those cases.
I wonder if the code is already able to transform right joins to left
joins, like
(A rightjoin B on (Pab)) = (B leftjoin A on (Pab))
I haven't looked at the code but I vaguely remember it is possible with
some strings attached, like not being able to use not-mergejoinable
conditions or something. I imagine it shows up as a leftjoin node with
some flag set.
How does this affect this optimization? Does this hold:
(A rightjoin B on (Pab)) innerjoin C on (Pbc)
= (B leftjoin A on (Pab)) innerjoin C on (Pbc)
= (B innerjoin C on (Pbc)) leftjoin A on (Pab)
?
--
Alvaro Herrera http://www.CommandPrompt.com/
The PostgreSQL Company - Command Prompt, Inc.
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