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Set returning functions in select column list

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Can someone explain how set returning functions in a select clause work?

It seems that it is doing some sort of implicit cross join.

deliverance_development=# select id, generate_series(1, 3) from users;
 id | generate_series
----+-----------------
  0 |               1
  0 |               2
  0 |               3
  1 |               1
  1 |               2
  1 |               3
(6 rows)

But if multiple set returning functions that return the same number of rows are in the same select it doesn't further cross join it.

deliverance_development=# select id, generate_series(1, 3), generate_series(4, 6) from users;
 id | generate_series | generate_series
----+-----------------+-----------------
  0 |               1 |               4
  0 |               2 |               5
  0 |               3 |               6
  1 |               1 |               4
  1 |               2 |               5
  1 |               3 |               6
(6 rows)


But if the set returning functions return a different number of rows then it goes back to a cross join.

deliverance_development=# select id, generate_series(1, 3), generate_series(4, 5) from users;
 id | generate_series | generate_series
----+-----------------+-----------------
  0 |               1 |               4
  0 |               2 |               5
  0 |               3 |               4
  0 |               1 |               5
  0 |               2 |               4
  0 |               3 |               5
  1 |               1 |               4
  1 |               2 |               5
  1 |               3 |               4
  1 |               1 |               5
  1 |               2 |               4
  1 |               3 |               5
(12 rows)


I really don't understand what is going on here. I have checked Google and the PostgreSQL docs, but it appears either I do not know the key words to search for or it is sparsely documented.

--
Jack Christensen
jackc@xxxxxxxxxxxxxxxxx

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