Question

The PostgreSQL Documentation on arrays provides an example using [-1] to access what appears to be the last element of an array; however while SELECT arr[2:3]; produces {5,9}, arr[2:-1] results in {}.

How can the last element of an array be obtained in PostgreSQL?

Edit: Windows, PostgreSQL v9.2.1

Answer 1

I think you’re misinterpreting the example. PostgreSQL arrays don’t have to be indexed from 1 to n, that’s just the default:

By default PostgreSQL uses a one-based numbering convention for arrays, that is, an array of n elements starts with array[1] and ends with array[n].

The example you’re looking at is this:

SELECT f1[1][-2][3] AS e1, f1[1][-1][5] AS e2
 FROM (SELECT '[1:1][-2:-1][3:5]={{{1,2,3},{4,5,6}}}'::int[] AS f1) AS ss;

But those negative numbers aren’t indexing from the end of the arrays as in languages such as Perl. In the FROM (SELECT ... part, they’re specifying the starting and ending indexes so the -1 in f1[1][-1][5] is just a plain old index. Consider this array_dims result:

=> SELECT array_dims('[1:1][-2:-1][3:5]={{{1,2,3},{4,5,6}}}'::int[]);
    array_dims     
-------------------
 [1:1][-2:-1][3:5]

If you’re using the default 1-based arrays then you can get the last element with a simple arr[array_length(arr, 1)]. If you’re not using the default [1:n] arrays then you’ll have to mess around with array_lower and array_upper to get the first and last elements; or, depending on the circumstances, you might be able to use unnest to unpack the array then work with the array as a rowset.