From: | Merlin Moncure <mmoncure(at)gmail(dot)com> |
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To: | Dave Crooke <dcrooke(at)gmail(dot)com> |
Cc: | Elias Ghanem <e(dot)ghanem(at)acteos(dot)com>, pgsql-performance(at)postgresql(dot)org |
Subject: | Re: Fwd: Dead lock |
Date: | 2010-06-14 20:44:26 |
Message-ID: | AANLkTik4XC9oxIQFX4ei_Xyez4lPb00NENkbukhYr0DH@mail.gmail.com |
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Lists: | pgsql-performance |
On Mon, Jun 14, 2010 at 11:58 AM, Dave Crooke <dcrooke(at)gmail(dot)com> wrote:
> If you're doing straight SQL bulk updates, then as someone suggested, you could use an ORDER BY on a subquery, but I don't know if that is a guarantee, if you're not actually displaying the results then the DB may be technically allowed to optimize it out from underneath you. The only way to be sure is a cursor / procedure.
'order by' should be safe if you use SELECT...FOR UPDATE. update
doesn't have an order by clause. Using cursor/procedure vs a query
is not the material point; you have to make sure locks are acquired in
a regular way.
update foo set x=x where id in (select * from bar order by x) does
look dangerous.
I think:
update foo set x=x where id in (select * from bar order by x for update)
should be ok. I don't usually do it that way.
merlin
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