On Mon, Aug 15, 2022 at 8:53 AM jian he <jian.universality@xxxxxxxxx> wrote:
select * from
(
(select 2 as v )
union all
(select 3 as v)
) as q1
cross join lateral
( (select * from
((select 4 as v) union all
(select 5 as v)) as q3
)
union all
(select q1.v)
) as q2;I thought q1 will be materialized as a constant set and will be equivalent as select 2 union all select 3;Then It will have 8 (2 * 4) rows total. Then It will be like {2,3} cross join with {2,3,4,5}But Here the actual result(return 6 rows) feels like two separate queries(A,B) then union together.QueryA: (select 2 as v ) cross join lateral (.....)QueryB: (select 3 as v ) cross join lateral (.....)Query A 3 row + Query B 3 row. So the total is 6 rows.Then I feel a little bit confused.
Lateral is literally a FOR EACH row construct. So q2 is evaluated for the first row in q1, then it is evaluated for the second row of q1. Which produces 6 rows (4 from q2 literal rows plus two more by copying the current row of q1 into a new row within q2 - twice).
CROSS JOIN here is a mis-nomer, and I personally avoid using it for that reason. You are really doing an inner join between a single row from q1 and each and every row produced by evaluating q2 in the context of that q1 row (it's a bit easier to understand if you have a function lateral as opposed to a subquery, but the effects are identical).
q1 INNER JOIN LATERAL AS q2 ON true
Where q2 can use the columns of q1 in producing its output.
David J.
