How to force Postgresql "cartesian product" behavior when unnest'ing multiple arrays in select? - arrays

Question

Postgresql behaves strangely when unnesting multiple arrays in the select list:
select unnest('{1,2}'::int[]), unnest('{3,4}'::int[]);
unnest | unnest
--------+--------
1 | 3
2 | 4
vs when arrays are of different lengths:
select unnest('{1,2}'::int[]), unnest('{3,4,5}'::int[]);
unnest | unnest
--------+--------
1 | 3
2 | 4
1 | 5
2 | 3
1 | 4
2 | 5
Is there any way to force the latter behaviour without moving stuff to the from clause?
The SQL is generated by a mapping layer and it will be very much easier for me to implement the new feature I am adding if I can keep everything in the select.

Answer 1

https://www.postgresql.org/docs/10/static/release-10.html
Set-returning functions are now evaluated before evaluation of scalar
expressions in the SELECT list, much as though they had been placed in
a LATERAL FROM-clause item. This allows saner semantics for cases
where multiple set-returning functions are present. If they return
different numbers of rows, the shorter results are extended to match
the longest result by adding nulls. Previously the results were cycled
until they all terminated at the same time, producing a number of rows
equal to the least common multiple of the functions' periods.
(emphasis mine)

Answer 2

I tested with version 12.12 of Postgres and as mentioned by OP in a comment, having two unnest() works as expected when you move those to the FROM clause like so:
SELECT a, b
FROM unnest('{1,2}'::int[]) AS a,
unnest('{3,4}'::int[]) AS b;
Then you get the expected table:
a | b
--+--
1 | 3
1 | 4
2 | 3
2 | 4
As we can see, in this case the arrays have the same size.
In my case, the arrays come from a table. You can first name the name and then name column inside the unnest() calls like so:
SELECT a, b
FROM my_table,
unnest(col1) AS a,
unnest(col2) AS b;
You can, of course, select other columns as required. This is a normal cartesian product.