Re: GEQO randomness?

Eric Schwarzenbach wrote:
> This is in a sense a followup to my post with subject "Wildly erratic
> query performance". The more I think about it the only thing that makes
> sense of my results is if the query planner really WAS choosing my join
> order truly randomly each time. I went digging into the manual and
> Section 49.3.1. "Generating Possible Plans with GEQO" says
>
> "In the initial stage, the GEQO code simply generates some possible join
> sequences at random."
>
> Now ordinarily I would interpret this use of the word random loosely, to
> mean "arbitrarily" or using some non-meaningful selection criteria. But
> given what I am seeing, this leads me to consider that "random" is meant
> literally, and that it uses a random number generate to pick paths. Can
> someone confirm that this is the case?

Yes, "random" means random.

> Is this really a good idea? Is non-deterministic behavior really
> acceptable? I would think it would be much more sensible to have it
> operate deterministically (such as with some predetermined random
> sequence of numbers used repeatedly).

Uh, no one has ever asked for that.

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ If your life is a hard drive, Christ can be your backup. +