Re: support for MERGE

On 2022-Aug-01, Justin Pryzby wrote:

> On Tue, Jun 14, 2022 at 11:27:06AM +0200, Álvaro Herrera wrote:
> > @@ -448,9 +448,9 @@ COMMIT;
> >      and execute the first one that succeeds.
> >      If <command>MERGE</command> attempts an <command>INSERT</command>
> >      and a unique index is present and a duplicate row is concurrently
> > -    inserted, then a uniqueness violation is raised.
> > -    <command>MERGE</command> does not attempt to avoid the
> > -    error by executing an <command>UPDATE</command>.
> > +    inserted, then a uniqueness violation error is raised;
> > +    <command>MERGE</command> does not attempt to avoid such
> > +    errors by evaluating <literal>MATCHED</literal> conditions.
> 
> This was a portion of a chang that was committed as ffffeebf2.
> 
> But I don't understand why this changed from "does not attempt to avoid the
> error by executing an <command>UPDATE</command>." to "...by evaluating
> <literal>MATCHED</literal> conditions."
> 
> Maybe it means to say "..by re-starting evaluation of match conditions".

Yeah, my thought there is that it may also be possible that the action
that would run if the conditions are re-run is a DELETE or a WHEN
MATCHED THEN DO NOTHING; so saying "by executing an UPDATE" it leaves
out those possibilities.  IOW if we're evaluating NOT MATCHED INSERT and
we find a duplicate, we do not go back to MATCHED.  We have this comment
in ExecMerge:

     * ExecMergeMatched takes care of following the update chain and
     * re-finding the qualifying WHEN MATCHED action, as long as the updated
     * target tuple still satisfies the join quals, i.e., it remains a WHEN
     * MATCHED case. If the tuple gets deleted or the join quals fail, it
     * returns and we try ExecMergeNotMatched. Given that ExecMergeMatched
     * always make progress by following the update chain and we never switch
     * from ExecMergeNotMatched to ExecMergeMatched, there is no risk of a
     * livelock.

(Another change there is the period to semicolon.  I did that to make it
clear that the last phrase applies to only that part and not phrases
earlier in the same paragraph.)

Your proposed rewording might be a clearer way to express the same idea.

Any other opinions?

> Sorry to re-raise this 6 weeks later..

No worries.  As the Zen of Python says, now is better than never.

-- 
Álvaro Herrera               48°01'N 7°57'E  —  https://www.EnterpriseDB.com/
"All rings of power are equal,
But some rings of power are more equal than others."
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