Hi,
I ran into the problem of getting the last n elements out of an array
and while some workarounds do exist:
(http://stackoverflow.com/questions/2949881/getting-the-last-element-of-a-postgres-array-declaratively)
I was still annoyed that I couldn't just ask for the last n values in an
array Python/Perl style.
Here's a patch to add support for negative index values in fetching
elements from an array.
i.e.
postgres=# CREATE TABLE blah (a int[]);
CREATE TABLE
Time: 11.357 ms
postgres=# INSERT INTO blah (a) VALUES (ARRAY[1,2,3,4,5,6,7,8,9,10]);
INSERT 0 1
Time: 1.282 ms
postgres=# SELECT a[-1] FROM blah;
a
----
10
(1 row)
Time: 0.450 ms
postgres=# SELECT a[-5:10] FROM blah;
a
--------------
{6,7,8,9,10}
(1 row)
Time: 0.949 ms
While testing this I BTW ran into funny behaviour in setting array
slices, as in:
postgres=# update blah set a[-5] = 12;
UPDATE 1
Time: 1.500 ms
postgres=# select * from blah;
a
------------------------------------------------------------
[-5:10]={12,NULL,NULL,NULL,NULL,NULL,1,2,3,4,5,6,7,8,9,10}
(1 row)
Time: 0.431 ms
And since this negative array expansion behaviour totally surprised me,
I haven't changed that in this patch at all.
--
Hannu Valtonen
