Re: PoC/WIP: Extended statistics on expressions

On Tue, 8 Dec 2020 at 12:44, Tomas Vondra <tomas(dot)vondra(at)enterprisedb(dot)com> wrote:
>
> Possibly. But I don't think it's worth the extra complexity. I don't
> expect people to have a lot of overlapping stats, so the amount of
> wasted space and CPU time is expected to be fairly limited.
>
> So I don't think it's worth spending too much time on this now. Let's
> just do what you proposed, and revisit this later if needed.
>

Yes, I think that's a reasonable approach to take. As long as the
documentation makes it clear that building MCV stats also causes
standard expression stats to be built on any expressions included in
the list, then the user will know and can avoid duplication most of
the time. I don't think there's any need for code to try to prevent
that -- just as we don't bother with code to prevent a user building
multiple indexes on the same column.

The only case where duplication won't be avoidable is where there are
multiple MCV stats sharing the same expression, but that's probably
quite unlikely in practice, and it seems acceptable to leave improving
that as a possible future optimisation.

Regards,
Dean