Ordering guarantees on UNNEST

Question

I am a bit confused around the ordering guarantees of UNNEST(some_array)) . I have read different posts on SO where it is mentioned that Postgres does not guarantee that the order of the input and output will be the same.

For example, on this query select unnest(array[1, 2, 3]), unnest(array[4, 5, 6]); The output is:

orestis=# select unnest(array[1, 2, 3]), unnest(array[4, 5, 6]);
 unnest | unnest
--------+--------
      1 |      4
      2 |      5
      3 |      6
(3 rows)

Is this, also, a possible outcome?:

orestis=# select unnest(array[1, 2, 3]), unnest(array[4, 5, 6]);
 unnest | unnest
--------+--------
      1 |      5
      2 |      4
      3 |      6
(3 rows)

Answer 1

Interesting. The documentation explains:

The array's elements are read out in storage order.

This would imply that the two are aligned.

I would instead recommend:

select *
from unnest(array[1, 2, 3], array[4, 5, 6]);

The multiple argument form aligns the values.