Support for negative index values in array fetching

From: "Valtonen, Hannu" <hannu(dot)valtonen(at)hut(dot)fi>
To: pgsql-hackers(at)postgresql(dot)org
Subject: Support for negative index values in array fetching
Date: 2011-01-02 10:45:29
Message-ID: 4D205749.6040704@hut.fi
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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

Attachment Content-Type Size
array.diff text/plain 2.6 KB

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