Re: Planner not using column limit specified for one column for another column equal to first - Mailing list pgsql-performance

From Tom Lane
Subject Re: Planner not using column limit specified for one column for another column equal to first
Date
Msg-id 10157.1271432748@sss.pgh.pa.us
Whole thread Raw
In response to Re: Planner not using column limit specified for one column for another column equal to first  (Yeb Havinga)
List pgsql-performance
Yeb Havinga <> writes:
> New expensive planner infrastructure to support from a>b and b>c infer
> a>c, yes.

> But I wonder if something like Leibniz's principle of identity holds for
> members of the same equivalence class, e.g. like if x,y are both members
> of the same EC, then for every predicate P, P(x) iff P(y).

This could only be assumed to apply for predicates constructed from
operators that are in the equivalence operator's btree opfamily.
Now, that would certainly be a large enough set of cases to sometimes
give useful results --- but I stand by the opinion that it wouldn't
win often enough to justify the added planner overhead.

            regards, tom lane

pgsql-performance by date:

Previous
From: Tom Lane
Date:
Subject: Re: stats collector suddenly causing lots of IO
Next
From: Tom Lane
Date:
Subject: Re: stats collector suddenly causing lots of IO