| From: | Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us> |
|---|---|
| To: | Martijn van Oosterhout <kleptog(at)svana(dot)org> |
| Cc: | Rick Gigger <rick(at)alpinenetworking(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Christopher Kings-Lynne <chriskl(at)familyhealth(dot)com(dot)au>, "Jim C(dot) Nasby" <jnasby(at)pervasive(dot)com>, josh(at)agliodbs(dot)com, pgsql-hackers(at)postgresql(dot)org, Jaime Casanova <systemguards(at)gmail(dot)com>, Peter Eisentraut <peter_e(at)gmx(dot)net> |
| Subject: | Re: MERGE vs REPLACE |
| Date: | 2005-11-16 17:59:36 |
| Message-ID: | 200511161759.jAGHxaT00877@candle.pha.pa.us |
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| Lists: | pgsql-hackers |
Martijn van Oosterhout wrote:
-- Start of PGP signed section.
> On Wed, Nov 16, 2005 at 11:37:46AM -0500, Bruce Momjian wrote:
> >
> > Interesting approach. Actually, we could tell the user they have to use
> > BEGIN;LOCK tab before doing MERGE, and throw an error if we don't
> > already have a table lock.
>
> The bit I'm still missing is why there needs to be a lock at all. The
> SQL standard doesn't say anywhere that concurrent MERGE operations
> can't conflict. It seems to me that standard visibility rules apply. If
> neither MERGE statement can see the results of the other, then they
> will both INSERT. If you don't have a UNIQUE constraint to prevent this
> then what's the problem?
I assume they want MERGE because they don't want duplicates. If they
don't care, they would have used INSERT.
--
Bruce Momjian | http://candle.pha.pa.us
pgman(at)candle(dot)pha(dot)pa(dot)us | (610) 359-1001
+ If your life is a hard drive, | 13 Roberts Road
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