Re: MERGE vs REPLACE

See 'merge_db' in http://lnk.nu/postgresql.org/5sl.html

On Fri, Nov 11, 2005 at 10:07:07PM -0500, Rod Taylor wrote:
> On Fri, 2005-11-11 at 18:36 -0500, mark(at)mark(dot)mielke(dot)cc wrote:
> > On Fri, Nov 11, 2005 at 06:00:32PM -0500, Rod Taylor wrote:
> > > So? That is what save points are for. You can even skip the select for
> > > update if you don't mind dead tuples from the attempted insert.
> > > SELECT ... FOR UPDATE;
> > > IF not exists THEN
> > > SAVEPOINT;
> > > INSERT ;
> > > IF UNIQUE VIOLATION THEN
> > > /* Someone else inserted between the SELECT and our INSERT */
> > > ROLLBACK TO SAVEPOINT;
> > > UPDATE;
> > > ELSE
> > > RELEASE SAVEPOINT;
> > > FI
> > > ELSE
> > > UPDATE;
> > > FI
> >
> > Isn't there still a race between INSERT and UPDATE?
>
> I suppose there is although I hadn't noticed before. I've never run into
> it and always check to ensure the expected number of tuples were touched
> by the update or delete.
>
> Within the PostgreSQL backend you might get away with having your insert
> hold a lock on the index page and follow it up with a FOR UPDATE lock on
> the offending tuple thus ensuring that your update will succeed. If you
> hack index mechanisms for the support you don't need the SAVEPOINT
> either -- just don't throw an error when you run across the existing
> entry.
>
> For client side code one possibility is to repeat until successful.
>
> WHILE
> SELECT FOR UPDATE;
> IF NOT EXISTS THEN
> SAVEPOINT
> INSERT;
> IF UNIQUE VIOLATION THEN
> ROLLBACK TO SAVEPOINT;
> ELSE
> RELEASE SAVEPOINT
> EXIT;
> FI
> ELSE
> UPDATE;
> EXIT;
> END
>
> -- Check for infinite loop
> END
>
> --
>
>
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--
Jim C. Nasby, Sr. Engineering Consultant jnasby(at)pervasive(dot)com
Pervasive Software http://pervasive.com work: 512-231-6117
vcard: http://jim.nasby.net/pervasive.vcf cell: 512-569-9461