The following documentation comment has been logged on the website: Page: https://www.postgresql.org/docs/12/queries-union.html Description:
Apologies if this has been raised previously - I searched the archives and did not find anything. Regarding section 7.4 of the documentation. I was curious about the behavior when combining different set operations in one query, e.g, query1 op1 query2 op2 query3; where op1 and op2 are one of UNION, INTERSECT, EXCEPT. The documentation suggests that this is equivalent to (query1 op1 query2) op2 query3; but only states it for the case when op1 = op2 = UNION. (Which really doesn't matter anyway given that set union is associative.) In fact, mixing INTERSECT and UNION does not exhibit the expected behavior (at least in version 12 - I have not tested in later versions). Here's my counterexample: CREATE TABLE test1 (x INTEGER); CREATE TABLE test2 (x INTEGER); CREATE TABLE test3 (x INTEGER); INSERT INTO test1 VALUES (1), (2); INSERT INTO test2 VALUES (2), (3); INSERT INTO test3 VALUES (3), (4); SELECT * FROM test1 UNION SELECT * FROM test2 INTERSECT SELECT * FROM test3; (SELECT * FROM test1 UNION SELECT * FROM test2) INTERSECT SELECT * FROM test3; The observed behavior suggests that INTERSECT is applied before UNION. I haven't tested to figure out other interactions (e.g., with EXCEPT or the ALL variations).
