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

On 27 March 2018 at 14:58, Dean Rasheed <dean(dot)a(dot)rasheed(at)gmail(dot)com> wrote:
> On 27 March 2018 at 01:36, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com> wrote:
>> 4) handling of NOT clauses in MCV lists (and in histograms)
>>
>> The query you posted does not fail anymore...
>>
> Ah, it turns out the previous query wasn't actually failing for the
> reason I thought it was -- it was failing because it had a
> ScalarArrayOpExpr that was being passed to
> mcv_clauselist_selectivity() because of the wrong list being passed to
> it. I could see from the code that a NOT clause would have tripped it
> up, but most NOT clauses actually get rewritten by negate_clause() so
> they end up not being NOT clauses.
>

Thinking about that some, I think that the only NOT clauses this needs
to actually worry about are NOTs of boolean Vars. Anything else that
this code supports will have been transformed into something other
than a NOT before reaching this point. Thus it might be much simpler
to handle that as a special case in statext_is_compatible_clause() and
mcv_update_match_bitmap(), rather than trying to support general NOT
clauses, and going through a recursive call to
mcv_update_match_bitmap(), and then having to merge bitmaps. NOT of a
boolean Var could then be treated just like var=false, setting the
appropriate attribute match entry if it's found in the MCV list. This
would allow clauses like (a=1 and NOT b) to be supported, which I
don't think currently works, because fullmatch won't get set.

Regards,
Dean