Re: MERGE SQL Statement for PG11

On Fri, Oct 27, 2017 at 4:41 PM, Simon Riggs <simon@2ndquadrant.com> wrote:
> I didn't say it but my intention was to just throw an ERROR if no
> single unique index can be identified.
>
> It could be possible to still run MERGE in that situaton but we would
> need to take a full table lock at ShareRowExclusive. It's quite likely
> that such statements would throw duplicate update errors, so I
> wouldn't be aiming to do anything with that for PG11.

Like Peter, I think taking such a strong lock for a DML statement
doesn't sound like a very desirable way forward.  It means, for
example, that you can only have one MERGE in progress on a table at
the same time, which is quite limiting.  It could easily be the case
that you have multiple MERGE statements running at once but they touch
disjoint groups of rows and therefore everything works.  I think the
code should be able to cope with concurrent changes, if nothing else
by throwing an ERROR, and then if the user wants to ensure that
doesn't happen by taking ShareRowExclusiveLock they can do that via an
explicit LOCK TABLE statement -- or else they can prevent concurrency
by any other means they see fit.

Other problems with taking ShareRowExclusiveLock include (1) probable
lock upgrade hazards and (2) do you really want MERGE to kick
autovacuum off of your giant table?  Probably not.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company