PostgreSQL 9.4 is going to be shipped with a feature of the SQL standard called WITH ORDINALITY. It has been introduced by this commit:
commit c62736cc37f6812d1ebb41ea5a86ffe60564a1f0
Author: Greg Stark <stark@mit.edu>
Date: Mon Jul 29 16:38:01 2013 +0100
Add SQL Standard WITH ORDINALITY support for UNNEST (and any other SRF)
Author: Andrew Gierth, David Fetter
Reviewers: Dean Rasheed, Jeevan Chalke, Stephen Frost
When those keywords are appended after a function returning a set of rows in a FROM clause, an additional bigint column is added in the result, containing a counter beginning at 1 and incremented for each row returned by the function. The original commit implementing unnest() actually missed this feature, so that's a great addition.
Here is how it works with a simple function returning rows.
=# SELECT * FROM generate_series(4,1,-1) WITH ORDINALITY;
generate_series | ordinality
-----------------+------------
4 | 1
3 | 2
2 | 3
1 | 4
(4 rows)
The default column name is called "ordinality", but it is possible to associate an alias to it, like that for example:
=# SELECT * FROM json_object_keys('\{"a1":"1","a2":"2","a3":"3"\}')
WITH ORDINALITY AS t(keys, n);
keys | n
------+---
a1 | 1
a2 | 2
a3 | 3
(3 rows)
This feature is actually pretty useful when used with arrays when decomposing them with unnest().
=# SELECT * from unnest('\{\{14,41,7\},\{54,9,49\}\}'::int[])
WITH ORDINALITY AS t(elts, num);
elts | num
------+-----
14 | 1
41 | 2
7 | 3
54 | 4
9 | 5
49 | 6
(6 rows)
And it is actually far more interesting with the new feature called ROWS FROM (or multi-argument unnest), because you can associate a counter usable for some ORDER BY operations easily with that.
=# SELECT * FROM unnest('\{1,2,3\}'::int[], '\{4,5,6,7\}'::int[])
WITH ORDINALITY AS t(a1, a2, num) ORDER BY t.num DESC;
a1 | a2 | num
------+----+-----
null | 7 | 4
3 | 6 | 3
2 | 5 | 2
1 | 4 | 1
(4 rows)
Original, no?