Hey, It looks quite nice. Personally I'd like to also have the returning statement, and have the number of deleted and inserted rows as separate numbers in the output message.
Earlier thoughts on how this could/could not be done were sometimes imprecise or inaccurate, so I have gone through the command per SQL:2011 spec and produced a definitive spec in the form of an SGML ref page. This is what I intend to deliver for PG11.
MERGE will use the same mechanisms as INSERT ON CONFLICT, so concurrent behavior does not require further infrastructure changes, just detailed work on the statement itself.
I'm building up the code from scratch based upon the spec, rather than trying to thwack earlier attempts into shape. This looks more likely to yield a commitable patch.
Major spanners or objections, please throw them in now cos I don't see any.
Questions?
-- Simon Riggs http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services