From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
---|---|
To: | Marko Tiikkaja <marko(at)joh(dot)to> |
Cc: | Etsuro Fujita <fujita(dot)etsuro(at)lab(dot)ntt(dot)co(dot)jp>, Rukh Meski <rukh(dot)meski(at)gmail(dot)com>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: LIMIT for UPDATE and DELETE |
Date: | 2014-08-29 14:33:45 |
Message-ID: | 17866.1409322825@sss.pgh.pa.us |
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Lists: | pgsql-hackers |
Marko Tiikkaja <marko(at)joh(dot)to> writes:
> The LIMIT part *has* to happen after the rows have been locked or it
> will work very surprisingly under concurrency (sort of like how FOR
> SHARE / FOR UPDATE worked before 9.0).
Good point.
> So either it has to be inside
> ModifyTable or the ModifyTable has to somehow pass something to a Limit
> node on top of it
... or we add a LockRows node below the Limit node. Yeah, that would make
UPDATE/LIMIT a tad slower, but I think that might be preferable to what
you're proposing anyway. Raw speed of what is fundamentally a fringe
feature ought not trump every other concern.
> This is just my personal opinion, but what I think should happen is:
> 1) We put the LIMIT inside ModifyTable like this patch does. This
> doesn't prevent us from doing ORDER BY in the future, but helps numerous
> people who today have to
> 2) We allow ORDER BY on tables with no inheritance children using
> something similar to Rukh's previous patch.
> 3) Someone rewrites how UPDATE works based on Tom's suggestion here:
> http://www.postgresql.org/message-id/1598.1399826841@sss.pgh.pa.us,
> which allows us to support ORDER BY on all tables (or perhaps maybe not
> FDWs, I don't know how those work). The LIMIT functionality in this
> patch is unaffected.
I still think we should skip #2 and go directly to work on #3. Getting
rid of the unholy mess that is inheritance_planner would be a very nice
thing.
regards, tom lane
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