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Re: ask for review of MERGE

From: Greg Stark <gsstark(at)mit(dot)edu>
To: Martijn van Oosterhout <kleptog(at)svana(dot)org>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Greg Smith <greg(at)2ndquadrant(dot)com>, Marko Tiikkaja <marko(dot)tiikkaja(at)cs(dot)helsinki(dot)fi>, Boxuan Zhai <bxzhai2010(at)gmail(dot)com>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: ask for review of MERGE
Date: 2010-10-24 23:11:51
Message-ID: AANLkTimyoPqJ58Nz=aCvj2GAJURLVexU4LEwqytD61wQ@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Sun, Oct 24, 2010 at 2:50 AM, Martijn van Oosterhout
<kleptog(at)svana(dot)org> wrote:
> Can we please not get MERGE hung up on trying to make it atomic. The
> standard requires no such thing and there are plenty of uses of MERGE
> that don't involve high concurrency updates of the same row by
> different processes. If we want an atomic UPSERT, make that a seperate
> command, MERGE without atomicity is useful enough already.

Really? I don't really see any point in a non-atomic MERGE. Nor in a
non-atomic UPDATE or INSERT or any other operation. The A in ACID is
as important as any other letter.

For that matter if you don't care about atomicity then this is a
problem already solvable easily solvable in the application. Why
bother providing a special command for it. The whole reason to want a
special command is precisely because the database can implement it
atomically more easily and more efficiently than any application
implementation.

Moreover having a case which is non-atomic and allows inconsistent
results or errors in the face of concurrent updates is a foot-gun.
Someone will come along and try to use it and it will appear to work
in their application but introduce nasty hidden race conditions.

-- 
greg

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