Re: Multicolumn hash indexes

On Tue, Sep 26, 2017 at 11:32 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> There is a facility in the planner to require a condition for the first
> column of an index before considering an indexscan plan. We could perhaps
> extend that to require a condition for each column of the index, though
> I'm not sure how much work is involved directly in that. The bigger
> picture here though is that it puts a premium on *not* throwing away
> "unnecessary" qual conditions, which is directly antithetical to a bunch
> of other planner goals.
>
> User: Why won't the planner use my multicolumn hash index?
> I have query conditions constraining all the columns!
> Us: Well, one of your conditions was discarded because it was
> constant-true after constant simplification, or redundant with
> a partition qual or CHECK constraint, or implied by an index
> predicate, or treated as a join condition instead of a
> restriction condition, or absorbed into an equivalence class
> and then the planner chose to emit some other equivalence
> condition instead, or possibly two or three other things.
> User: WAAAAH!

Ah. Yeah, that's a problem.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company