Re: So what's an "empty" array anyway?

On Wed, Nov 12, 2008 at 02:12:19PM +0200, Peter Eisentraut wrote:
> Tom Lane wrote:
> >Seems like we ought to clean this up. I'm not sure which way to jump
> >though: should we decree that arrays of no elements must always have
> >zero dimensions, or should we get rid of that and standardize on, say,
> >1-D array with lower bound 1 and upper bound 0?
>
> It was pointed out to me today that a zero-dimensional matrix is a
> scalar. This makes a bit of sense, if you say that
>
> '{{56}}' is of type int[][], 2 dimensions
> '{56}' is of type int[], 1 dimension
> '56' is of type int, 0 dimensions
>
> Notice that the number of brace pairs in the literal matches the number
> of bracket pairs in the type declaration.
>
> By that logic, '{}' has one dimension. I think this also works best in
> practice, for example with array concatenation.

I could argue a very convincing case that '{}' should be of *at least*
one dimension. The fact that you can concat it onto any other size
array suggests the current implementation of PG agrees---probably for
very different reasons though. '{{}}' would imply an array of at least
two dimensions, but doesn't appear to be currently supported.

Another weirdo is the fact that this works:

(ARRAY[ARRAY[3]])[1][1];

whereas this doesn't:

((ARRAY[ARRAY[3]])[1])[1];

which seems a little strange, although from a historical point of view
I can understand why it behaves the way it does. From the current
behavior, the following syntax would make more sense:

(ARRAY[ARRAY[3]])[1,1];

I.e. it's obvious that you're pulling stuff out of a two dimensional
array. With the current syntax, I'd expect to get slices of higher
dimensional arrays back:

(ARRAY[ARRAY[3,4]])[1];

would evaluate to:

ARRAY[3,4]

We then get onto issues of arrays currently having to be rectangular;
even though it's trivial to express things that don't want rectangular
arrays:

SELECT array(VALUES (ARRAY[1]),(ARRAY[2,3]));

I'd expect to get a 1D array of 1D arrays back from this---note that
this is distinct from a 2D array, which would indeed be rectangular and
wouldn't work at all. I'd expect the following to be invalid:

SELECT array(VALUES (ARRAY[1]),(ARRAY[ARRAY[2,3]]));

I could go on, but I'm not sure of any good ways of keeping this all
compatible with the current syntax or semantics.

Sam