On Fri, Oct 17, 2014 at 11:03:04PM -0400, Tom Lane wrote: > Bruce Momjian <br...@momjian.us> writes: > > On Fri, Oct 17, 2014 at 06:15:37PM -0400, Tom Lane wrote: > >> Those stats were perfectly valid: what the planner is looking for is > >> accurate minimum and maximum values for the index's leading column, and > >> that's what it got. You're correct that a narrower index could have given > >> the same results with a smaller disk footprint, but the planner got the > >> results it needed from the index you provided for it to work with. > > > Uh, why is the optimizer looking at the index on a,b,c and not just the > > stats on column a, for example? I am missing something here. > > Because it needs up-to-date min/max values in order to avoid being > seriously misled about selectivities of values near the endpoints. > See commit 40608e7f949fb7e4025c0ddd5be01939adc79eec.
Oh, I had forgotten we did that. It is confusing that there is no way via EXPLAIN to see the access, making the method of consulting pg_stat_* and using EXPLAIN unreliable. Should we document this somewhere? -- Bruce Momjian <br...@momjian.us> http://momjian.us EnterpriseDB http://enterprisedb.com + Everyone has their own god. + -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
