Re: Back-patch change in hashed DISTINCT estimation?

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.

Greetings,

Andres Freund

-- Andres Freund                       http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training &
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