| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Josh Berkus <josh(at)agliodbs(dot)com> |
| Cc: | Robert Haas <robertmhaas(at)gmail(dot)com>, pgsql-hackers(at)postgresql(dot)org |
| Subject: | Re: from_collapse_limit vs. geqo_threshold |
| Date: | 2009-05-21 11:50:56 |
| Message-ID: | 8847.1242906656@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
Josh Berkus <josh(at)agliodbs(dot)com> writes:
> Robert,
>> It appears that this statement has been in our documentation since Tom
>> Lane added FROM_COLLAPSE_LIMIT (back then, it was capitalized) on
>> January 25, 2003 (9bf97ff426de9), but I can't find any justification
>> for it anywhere. I think we either need to justify this advice, or
>> remove it.
> ... trying to remember why I wrote that ... what would happen if
> FROM_COLLAPSE_LIMIT was *more* than GEQO_THRESHOLD?
I think I wrote it, not you. The point of the advice is to keep
subquery collapsation (hm, what's the right noun form? Need caffeine)
from turning a non-GEQO query into a GEQO one, and thus subjecting
you to unpredictable plans. Maybe the resulting plans would be better
on average, or maybe they wouldn't, but in any case they'd be
unpredictable.
regards, tom lane
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