=> select (select 1 union select 2);
ERROR:  more than one row returned by a subquery used as an expression

但是如果我创建一个执行相同操作的函数,我会得到我想要的行为.

=> create or replace function onetwo() returns setof integer as $$
$> select 1 union select 2 
$> $$language 'sql' strict immutable;
CREATE FUNCTION
=> select onetwo();
 onetwo 
--------
      1
      2

为什么不同？