Ilja Golshtein wrote:
Well, it would obviously be better if PG could figure out it was safe,
but I'm not sure there's a general case where it is. You can see it's OK
because you know there's only one row in your SELECT result-set.
I think, it's OK because NULL can be compared with anything
with predictable result and no additional information about
types is necessary.
Is it correct vision?
Yes*, but you've not got a single NULL in your examples, you've got a
set of rows containing one unnamed column with an unspecified type. That
set happens to have only one row and that contains a NULL.
I agree it's hard to proceed your query with UNION
and some sort of error is reasonable here.
But from outside the brackets, they look the same.
What would happen ideally, is that PG would notice we have a single row
and column here and collapse this down to a single scalar value.
However, I'm not sure under what circumstances it can do so (or does),
or whether it is cost-effective.
[* Actually, I think NULLs are typed in SQL, which means you should be
able to get type violations. ]
--
Richard Huxton
Archonet Ltd
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