Re: [BUGS] BUG #2913: Subscript on multidimensional array yields no value

Tom Lane wrote:
> Bruce Momjian <bruce(at)momjian(dot)us> writes:
> > Tom Lane wrote:
> >> This is not a bug, this is a definitional disagreement, and your TODO
> >> entry presupposes an answer that I don't particularly agree with.
>
> > Well, our documentation suggests thaat [1] is the same as [1:1]:
> > http://www.postgresql.org/docs/8.2/static/arrays.html#AEN5791
>
> It says absolutely no such thing. A subscript expression involving m:n
> produces a "slice", hence an array of different dimensionality from the
> original, whereas a subscript expression not involving any colon
> produces a single element --- that is, not an array at all.
>
> You could make a fair case that the (ARRAY[[1,2],[3,4]])[1] example
> should throw an error instead of returning null. But to claim it is
> the same as a slice expression is a typing violation.

[ moved to docs list ]

Well, I don't understand our array documentation, so odds are others
don't either. What are we saying here:

An array slice is denoted by writing lower-bound:upper-bound for one or
more array dimensions. For example, this query retrieves the first item
on Bill's schedule for the first two days of the week:

SELECT schedule[1:2][1:1] FROM sal_emp WHERE name = 'Bill';

schedule
------------------------
{{meeting},{training}}
(1 row)

We could also have written

SELECT schedule[1:2][1] FROM sal_emp WHERE name = 'Bill';

with the same result. An array subscripting operation is always taken to
represent an array slice if any of the subscripts are written in the
form lower:upper. A lower bound of 1 is assumed for any subscript where
only one value is specified, as in this example:

What is the difference between 'lower-bound:upper-bound' and
'lower:upper'? Here are the items that confuse me:

test=> SELECT (ARRAY[[1,2],[3,4]])[1][1];
array
-------
1
(1 row)

test=> SELECT (ARRAY[[1,2],[3,4]])[1:1];
array
---------
{{1,2}}
(1 row)

test=> SELECT (ARRAY[[1,2],[3,4]])[1];
array
-------

(1 row)

The first two make sense to me, but the last one does not. If someone
explains it, I can update our documentation.

--
Bruce Momjian bruce(at)momjian(dot)us
EnterpriseDB http://www.enterprisedb.com

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