On Mon, May 06, 2019 at 11:18:28PM -0400, Tom Lane wrote:
Tomas Vondra <tomas.von...@2ndquadrant.com> writes:
Do we actually check how many duplicates are there during planning?
Certainly that's part of the planner's cost estimates ... but it's
only as good as the planner's statistical knowledge.
I'm looking at the code, and the only place where I see code dealing with
MCVs (probably the best place for info about duplicate values) is
estimate_hash_bucketsize in final_cost_hashjoin. That's not quite what I
had in mind - I was thinking more about something along the lines "See the
larget group of duplicate values, disable hash join if it can't fit into
work_mem at all."
Of course, if the input estimates are off, that may not work too well. It
would certainly not help the query failing with OOM, because that was a
case of severe underestimate.
Or did you mean some other piece of code that I have missed.
regards
--
Tomas Vondra http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
