|From:||Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>|
|To:||Konstantin Knizhnik <k(dot)knizhnik(at)postgrespro(dot)ru>|
|Cc:||PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>, David Rowley <david(dot)rowley(at)2ndquadrant(dot)com>, Alexander Korotkov <a(dot)korotkov(at)postgrespro(dot)ru>|
|Subject:||Re: Optimizer questions|
|Views:||Raw Message | Whole Thread | Download mbox|
Konstantin Knizhnik <k(dot)knizhnik(at)postgrespro(dot)ru> writes:
> I think that the best approach is to generate two different paths:
> original one, when projection is always done before sort and another one
> with postponed projection of non-trivial columns. Then we compare costs
> of two paths and choose the best one.
> Unfortunately, I do not understand now how to implement it with existed
> Do you think that it is possible?
After fooling with this awhile, I don't think it's actually necessary
to do that. See attached proof-of-concept patch.
Although this patch gets through our regression tests, that's only because
it's conservative about deciding to postpone evaluation; if it tried to
postpone evaluation in a query with window functions, it would fail
miserably, because pull_var_clause doesn't know about window functions.
I think that that's a clear oversight and we should extend it to offer
the same sorts of behaviors as it does for Aggrefs. But that would be
slightly invasive, there being a dozen or so callers; so I didn't bother
to do it yet pending comments on this patch.
I think it's probably also broken for SRFs in the tlist; we need to work
out what semantics we want for those. If we postpone any SRF to after
the Sort, we can no longer assume that a query LIMIT enables use of
bounded sort (because the SRF might repeatedly return zero rows).
I don't have a huge problem with that, but I think now would be a good
time to nail down some semantics.
Some other work that would be useful would be to refactor so that the
cost_qual_eval operations aren't so redundant. But that's just a small
time savings not a question of functionality.
And we'd have to document that this changes the behavior for volatile
functions. For the better, I think: this will mean that you get
consistent results no matter whether the query is implemented by
indexscan or seqscan-and-sort, which has never been true before.
But it is a change.
Do people approve of this sort of change in general, or this patch
approach in particular? Want to bikeshed the specific
when-to-postpone-eval policies implemented here? Other comments?
regards, tom lane
|Next Message||Alvaro Herrera||2016-03-09 22:57:56||Re: TAP / recovery-test fs-level backups, psql enhancements etc|
|Previous Message||Breen Hagan||2016-03-09 22:44:18||Re: BUG #13755: pgwin32_is_service not checking if SECURITY_SERVICE_SID is disabled|