* Andres Freund (and...@2ndquadrant.com) wrote: > On 2013-08-20 17:24:18 -0400, Tom Lane wrote: > > In a thread over in pgsql-performance, Tomas Vondra pointed out that > > choose_hashed_distinct was sometimes making different choices than > > choose_hashed_grouping, so that queries like these: > > > > select distinct x from ... ; > > select x from ... group by 1; > > > > might get different plans even though they should be equivalent. > > After some debugging it turns out that I omitted hash_agg_entry_size() > > from the hash table size estimate in choose_hashed_distinct --- I'm pretty > > sure I momentarily thought that this function must yield zero if there are > > no aggregates, but that's wrong. So we need a patch like this: > > > What I'm wondering is whether to back-patch this or leave well enough > > alone. The risk of back-patching is that it might destabilize plan > > choices that people like. [...] > > > A possible compromise is to back-patch into 9.3 (where the argument about > > destabilizing plan choices doesn't have much force yet), but not further. > > +1 for 9.3 only.
Yeah, I've been thinking about this a bit also and agree that 9.3 is
fine but not farther back.
Thanks,
Stephen
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