>>> I think it would be best to disallow the double-quoted form...
Actually, on second thought that argument holds no water at all.
If we tried to extend SET to accept expressions, we'd have to break
compatibility with its acceptance of unquoted words, so breaking
the interpretation of double-quoted words too is no biggie. Certainly
there's no point in going through a lot of lexer and parser pushups
to disallow double-quoted words here.
So now my thought is not to document double-quote, but not to go out
of our way to disallow it either.
I still agree that all forms of SET should be consistent about what
they will take.
regards, tom lane