Tom Lane wrote:
> Tomas Vondra <tomas.vondra@2ndquadrant.com> writes:
> >> Although I've not done anything about it here, I'm not happy about the
> >> handling of dependencies for stats objects.
>
> > Yeah, it's a bit frankensteinian ...
>
> I'm prepared to create a fix for that, but it'd be easier to commit the
> current patch first, to avoid merge conflicts.
It seems we're mostly in agreement regarding the parts I was touching.
Do you want to push your version of the patch?
> >> Lastly, I tried the example given in the CREATE STATISTICS reference page,
> >> and it doesn't seem to work.
>
> > I assume you're talking about the functional dependencies and in that
> > case that's expected behavior, because f. dependencies do assume the
> > conditions are "consistent" with the functional dependencies.
>
> Hm. OK, but then that example is pretty misleading, because it leads
> the reader to suppose that the planner can tell the difference between
> the selectivities of the two queries. Maybe what's lacking is an
> explanation of how you'd use this statistics type.
I think we should remove the complex example from that page, and instead
refer the reader to chapters 14 and 69. The CREATE STATISTICS page can
just give some overview of what the syntax looks like, and all
explanations of complex topics such as "what does it mean for a query to
be consistent with the functional dependencies" can be given at length
elsewhere.
--
Álvaro Herrera https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
