Message-ID: <54EF5C6C.6030001@2ndquadrant.com>
Date: Thu, 26 Feb 2015 18:48:28 +0100
From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Organization: 2ndQuadrant
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To: Tom Lane <tgl@sss.pgh.pa.us>
CC: David Rowley <dgrowleyml@gmail.com>,
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Subject: Re: Performance improvement for joins where outer side
 is unique
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On 26.2.2015 18:34, Tom Lane wrote:
> Tomas Vondra <tomas.vondra@2ndquadrant.com> writes:
>> FWIW this apparently happens because the code only expect that
>> EquivalenceMembers only contain Var, but in this particular case that's
>> not the case - it contains RelabelType (because oprcode is regproc, and
>> needs to be relabeled to oid).
> 
> If it thinks an EquivalenceMember must be a Var, it's outright
> broken; I'm pretty sure any nonvolatile expression is possible.

I came to the same conclusion, because even with the RelabelType fix
it's trivial to crash it with a query like this:

    SELECT 1 FROM pg_proc p JOIN pg_operator o
                              ON oprcode = (p.oid::int4 + 1);

I think fixing this (e.g. by restricting the optimization to Var-only
cases) should not be difficult, although it might be nice to handle
generic expressions too, and something like examine_variable() should do
the trick. But I think UNIQUE indexes on expressions are not all that
common.

-- 
Tomas Vondra                http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services


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