Date: Thu, 2 Nov 2017 12:51:45 -0700
From: Peter Geoghegan <pg@bowt.ie>
To: Nico Williams <nico@cryptonector.com>
Cc: Simon Riggs <simon@2ndquadrant.com>,
	Robert Haas <robertmhaas@gmail.com>,
	Michael Paquier <michael.paquier@gmail.com>,
	PostgreSQL-development <pgsql-hackers@postgresql.org>
Subject: Re: MERGE SQL Statement for PG11
Message-ID: <20171102195145.GB27644@marmot>
References: 
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 <CANP8+jKNXUjW4FT=1me5FzTpvMhwWkK4-Ea3F1_Vw9vE0rkt0A@mail.gmail.com>
 <20171101182032.GA7104@marmot>
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Nico Williams <nico@cryptonector.com> wrote:
>If you want to ignore conflicts arising from concurrency you could
>always add an ON CONFLICT DO NOTHING to the INSERT DML in the mapping I
>proposed earlier.  Thus a MERGE CONCURRENTLY could just do that.
>
>Is there any reason not to map MERGE as I proposed?

Performance, for one. MERGE generally has a join that can be optimized
like an UPDATE FROM join.

I haven't studied this question in any detail, but FWIW I think that
using CTEs for merging is morally equivalent to a traditional MERGE
implementation. It may actually be possible to map from CTEs to a MERGE
statement, but I don't think that that's a good approach to implementing
MERGE.

Most of the implementation time will probably be spent doing things like
making sure MERGE behaves appropriately with triggers, RLS, updatable
views, and so on. That will take quite a while, but isn't particularly
technically challenging IMV.

--
Peter Geoghegan


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