Re: Performance improvement for joins where outer side is unique

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/
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