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@momjian.us> https://momjian.us
EnterpriseDB https://enterprisedb.com
The usefulness of a cup is in its emptiness, Bruce Lee