Re: [HACKERS] PATCH: multivariate histograms and MCV lists

On Sun, 10 Mar 2019 at 17:36, Tomas Vondra <tomas.vondra@2ndquadrant.com> wrote:
> On 3/10/19 2:09 PM, Dean Rasheed wrote:
> > 14). The attnums Bitmapset passed to
> > statext_is_compatible_clause_internal() is an input/output argument
> > that it updates. That should probably be documented. When it calls
> > itself recursively for AND/OR/NOT clauses, it could just pass the
> > original Bitmapset through to be updated (rather than creating a new
> > one and merging it), as it does for other types of clause.
>
> I don't think it's really possible, because the AND/OR/NOT clause is
> considered compatible only when all the pieces are compatible. So we
> can't tweak the original bitmapset directly in case the incompatible
> clause is not the very first one.
>

In the case where the overall clause is incompatible, you don't
actually care about the attnums returned. Right now it will return an
empty set (NULL). With this change it would return all the attnums
encountered before the incompatible piece, but that wouldn't matter.
In fact, you could easily preserve the current behaviour just by
having the outer statext_is_compatible_clause() function set attnums
back to NULL if the result is false.

Regards,
Dean