| From: | Bruce Momjian <bruce(at)momjian(dot)us> |
|---|---|
| To: | Peter Geoghegan <pg(at)bowt(dot)ie> |
| Cc: | Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Andres Freund <andres(at)anarazel(dot)de>, David Rowley <dgrowleyml(at)gmail(dot)com>, Jeff Davis <pgsql(at)j-davis(dot)com>, Justin Pryzby <pryzby(at)telsasoft(dot)com>, Melanie Plageman <melanieplageman(at)gmail(dot)com>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Default setting for enable_hashagg_disk |
| Date: | 2020-06-29 17:31:40 |
| Message-ID: | 20200629173140.GE17529@momjian.us |
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| Lists: | pgsql-docs pgsql-hackers |
On Mon, Jun 29, 2020 at 10:20:14AM -0700, Peter Geoghegan wrote:
> I have no reason to believe that the planner is any more or any less
> likely to conclude that the hash table will fit in memory in v13 as
> things stand (I don't know if the BufFile issue matters).
>
> In general, grouping estimates probably aren't very good compared to
> join estimates. I imagine that in either v12 or v13 the planner is
> likely to incorrectly believe that it'll all fit in memory fairly
> often. v12 was much too permissive about what could happen. But v13 is
> too conservative.
FYI, we have improved planner statistics estimates for years, which must
have affected node spill behavior on many node types (except hash_agg),
and don't remember any complaints about it.
--
Bruce Momjian <bruce(at)momjian(dot)us> https://momjian.us
EnterpriseDB https://enterprisedb.com
The usefulness of a cup is in its emptiness, Bruce Lee
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