Re: ARRAY() returning NULL instead of ARRAY[] resp. {}

Joe Conway <mail(at)joeconway(dot)com> writes:
> Markus Bertheau wrote:
>> why does SELECT ARRAY(SELECT 1 WHERE FALSE) return NULL instead of
>> ARRAY[] resp. '{}'?

> Why would you expect an empty array instead of a NULL?

I think he's got a good point, actually. We document the ARRAY-with-
parens-around-a-SELECT syntax as

The resulting one-dimensional array will have an element for
each row in the subquery result, with an element type matching
that of the subquery's output column.

To me, that implies that a subquery result of no rows generates a
one-dimensional array of no elements, not a null array.

This is not the same as

SELECT ARRAY[(SELECT 1 WHERE FALSE)];

We define a scalar subquery that returns no rows as returning null, so
this is equivalent to

SELECT ARRAY[NULL];

which *ought* to yield an array containing a single NULL element,
but since we can't yet handle arrays containing nulls we punt and
return a null array value. That's wrong too ... but it's a different
issue. The point Markus is complaining about seems like it should
be easily fixable.

regards, tom lane