>> I would've expected once per query. There's no way the expressions can >> reference the row data, so there's no reason to evaluate them each >> time. > > I disagree - it is hypothetical situation but it is possible > > if somebody store documents like > > id, xml > ===== > id = 1, xml = <doc id = 1> ....<> > id = 2, xml = <doc id = 2> .... > > Then evaluating one per query doesn't allow to use any reference to other > columns, and doesn't to build expressions like PATH (...[@id= ' || id || ']
Referencing columns on the same evaluation level? I dunno about that. You're relying on strict order of evaluation which is pretty unusual for SQL.
I guess this is why full XQuery would be desirable, but that's a whole different business.
I would personally expect this sort of thing to be handled by a second pass; isn't that part of why it's so easy to return xml fields from xmltable?
Evaluating expressions each time seems likely to be bad for performance, but I guess it's not going to make a big difference compared to all the XML crud, so I don't have a super strong opinion here.
When expression will a constant, then the cost will be minimal - more, we can do preevaluation in parser/transform time, and if expression is some constant, then we should not to evaluate it later.
We can wait if some other people will have a opinion to this topic. This is important topic, but it is not to hard implement both variants, and more - this is corner case - it is not important for any example that I found on net.
Regards
Pavel
Either way, it's crucial that the behaviour be documented. > DEFAULT should be evaluated per output row - anybody can use volatile > function there - example: when I have not data - use some random there
That would be consistent with how we handle DEFAULT on a table, so I agree. It's a departure from what we do normally, but we didn't have table functions before either.
