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

On Sun, Jun 13, 2021 at 1:29 PM Tomas Vondra <tomas(dot)vondra(at)enterprisedb(dot)com>
wrote:

> 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
> EnterpriseDB: http://www.enterprisedb.com
> The Enterprise PostgreSQL Company
>
Hi,

- for (i = 0; i < mcvlist->nitems; i++)
+ if (cst) /* Expr op Const */

It seems the Const op Expr is also covered by this if branch. Hence the
comment should include this case.

Cheers