Re: MERGE SQL Statement for PG11

From: Peter Geoghegan <pg(at)bowt(dot)ie>
To: Simon Riggs <simon(at)2ndquadrant(dot)com>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, Michael Paquier <michael(dot)paquier(at)gmail(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: MERGE SQL Statement for PG11
Date: 2017-11-02 19:16:36
Message-ID: 20171102191636.GA27644@marmot
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Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
>So if I understand you correctly, in your view MERGE should just fail
>with an ERROR if it runs concurrently with other DML?

That's certainly my opinion on the matter. It seems like that might be
the consensus, too.

Obviously there are things that you as a user can do about this on your
own, like opt to use a higher isolation level, or manually LOCK TABLE.
For some use cases, including bulk loading for OLAP, users might just
know that there isn't going to be concurrent activity because it's not
an OLTP system.

If this still seems odd to you, then consider that exactly the same
situation exists with UPDATE. A user could want their UPDATE to affect a
row where no row version is actually visible to their MVCC snapshot,
because they have an idea about reliably updating the latest row. UPDATE
doesn't work like that, of course. Is this unacceptable because the user
expects that it should work that way?

Bear in mind that ON CONFLICT DO UPDATE *can* actually update a row when
there is no version of it visible to the snapshot. It can also update a
row where there is a concurrent DELETE + INSERT, and the tuples with the
relevant unique index values end up not even being part of the same
update chain in each case (MVCC-snapshot-visible vs. latest). IOW, you
may end up updating a completely different logical row to the row with
the conflicting value that is visible to your MVCC snapshot!

>i.e. if a race condition between the query and an INSERT runs
>concurrently with another INSERT
>We have no interest in making that work?

Without meaning to sound glib: we already did make it work for a
special, restricted case that is important enough to justify introducing
a couple of kludges -- ON CONFLICT DO UPDATE/upsert.

I do agree that what I propose for MERGE will probably cause confusion;
just look into Oracle's MERGE implementation for examples of this. We
ought to go out of our way to make it clear that MERGE doesn't provide
these guarantees.

Peter Geoghegan

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