Re: Use extended statistics to estimate (Var op Var) clauses

Hi,

Here is a slightly updated version of the patch - rebased to current 
master and fixing some minor issues to handle expressions (and not just 
the Var nodes as before).

The changes needed to support (Expr op Expr) are mostly mechanical, 
though I'm sure the code needs some cleanup. The main issue I ran into 
is the special case clauselist_selectivity, which does

     if (list_length(clauses) == 1)
         return clause_selectivity_ext(...);

which applies to cases like "WHERE a < b" which can now be handled by 
extended statistics, thanks to this patch. But clause_selectivity_ext 
only used to call restriction_selectivity for these clauses, which does 
not use extended statistics, of course.

I considered either getting rid of the special case, passing everything 
through extended stats, including cases with a single clause. But that 
ends up affecting e.g. OR clauses, so I tweaked clause_selectivity_ext a 
bit, which seems like a better approach.


regards

-- 
Tomas Vondra
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