| From: | Bruce Momjian <bruce(at)momjian(dot)us> |
|---|---|
| To: | Justin Pryzby <pryzby(at)telsasoft(dot)com> |
| Cc: | David Rowley <dgrowleyml(at)gmail(dot)com>, Jeff Davis <pgsql(at)j-davis(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Melanie Plageman <melanieplageman(at)gmail(dot)com>, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>, pgsql-hackers(at)postgresql(dot)org |
| Subject: | Re: Default setting for enable_hashagg_disk |
| Date: | 2020-06-24 17:08:34 |
| Message-ID: | 20200624170834.GA17842@momjian.us |
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| Lists: | pgsql-docs pgsql-hackers |
On Wed, Jun 24, 2020 at 07:38:43AM -0500, Justin Pryzby wrote:
> On Wed, Jun 24, 2020 at 05:06:28AM -0400, Bruce Momjian wrote:
> > It would seem merge join has almost the same complexities as the new
> > hash join code, since it can spill to disk doing sorts for merge joins,
> > and adjusting work_mem is the only way to control that spill to disk. I
> > don't remember anyone complaining about spills to disk during merge
> > join, so I am unclear why we would need a such control for hash join.
>
> It loooks like merge join was new in 8.3. I don't think that's a good analogy,
> since the old behavior was still available with enable_mergejoin=off.
Uh, we don't gurantee backward compatibility in the optimizer. You can
turn off hashagg if you want. That doesn't get you to PG 13 behavior,
but we don't gurantee that.
--
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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