UPDATE ... SET (a, b, c) = (expr)

Hi,

I've been having a look around to see what would it take to implement
the syntax in the subject, completing a patch posted by Sussane Ebrecht
two years ago.

As far as I can tell, the first problem is in the grammar. The SQL spec
mandates <multiple value expression> as a production for the expression
at the right of the equal sign. This enters a *large* chain of
productions that can end up in a lot of things, and for which I don't
think we have a production in our grammar. In fact I think we have
special cases for many of the things that can be derived from here, so
maybe some things will have to be reworked in order to support all the
stuff that the standard mandates. I'll refrain from doing this
initially, and will focus on getting the simpler cases working.

But then there's the matter of how to represent it from the grammar
inwards. At first I thought of creating a new parse node type, say
MultiResTarget that would replace ResTarget in UpdateStmt, and get it
from there into the optimizer and executor. However, this seems to
require creating a new node for TargetEntry too, and duplicate logic for
all of that. As far as transformTargetList is concerned this wouldn't
be much of a problem, but from there onwards it's pretty complex -- it
requires a lot of duplicate logic. In fact I think it would cause the
current code which is common for UPDATE/INSERT/SELECT/DELETE to split in
two, one for UPDATE and one for the rest, which looks ungood.

So is this the right way to approach the problem, or am I missing some
simpler way?

Perhaps, for example, I should be treating normal updates as currently,
and add a new member to UpdateStmt that would hold multi-column
expressions. But I'm not sure if that would work at all.

--
Alvaro Herrera http://www.CommandPrompt.com/
The PostgreSQL Company - Command Prompt, Inc.