Re: [HACKERS] MERGE SQL Statement for PG11

On Mon, Mar 26, 2018 at 12:17 PM, Simon Riggs <simon@2ndquadrant.com> wrote:
>> As far as I
>> know, the proposed MERGE patch has that issue an existing DML commands
>> don't; but someone else may have better information.
>
> I will look deeper and report back.

It's quite clear that the problem exists with the MERGE patch; the
simple fact that RangeVarGetRelidExtended() is called twice with the
same RangeVar argument shows this. However, the Oid cross-check seems
like a sufficient defense against an inconsistency that causes real
trouble, since the cross-check will only error-out when a concurrent
table creation (or maybe ALTER TABLE) makes a second table visible, in
a schema that appears earlier in the user's search_path. It's hard to
imagine any legitimate user truly preferring some alternative behavior
in this particular scenario, which makes it okay.

This cross-check workaround is ugly, but apparently there is a
precedent in copy.c. I didn't know that detail until Robert pointed it
out. That makes me feel a lot better about this general question of
how the target relation is represented, having two RTEs, etc.

-- 
Peter Geoghegan