Andrey Lepikhov <a.lepikhov@postgrespro.ru> writes:
> Can we include the CustomScan node in the list of nodes that do not
> support projection?
That seems like a pretty bad idea. Maybe it's a good thing for whatever
unspecified extension you have in mind right now, but it's likely to be
a net negative for many more. As an example, if some custom extension has
a better way to calculate some output expression than the core code does,
a restriction like this would prevent that from being implemented.
> Reason is that custom node can contain quite arbitrary logic that does
> not guarantee projection support.
I don't buy this for a minute. Where do you think projection is
going to happen? There isn't any existing node type that *couldn't*
support projection if we insisted that that be done across-the-board.
I think it's mostly just a legacy thing that some don't.
regards, tom lane