sql - 具有不同列数的记录类型比较不会失败

Question

为什么以下查询在 PostgreSQL 11.6 中不会触发“无法比较具有不同列数的记录类型”错误？

with
s AS (SELECT 1)
, main AS (
SELECT (a) = (b) , (a) = (a), (b) = (b), a, b -- I expect (a) = (b) fails
FROM s
  , LATERAL (select 1 as x, 2 as y) AS a
  , LATERAL (select 5 as x) AS b
)
select * from main;

虽然这个是这样的:

with
x AS (SELECT 1)
, y AS (select 1, 2)
select (x) = (y) from x, y;

Answer 1

请参阅 the docs on row comparison 中的注释

Errors related to the number or types of elements might not occur if the comparison is resolved using earlier columns.

在本例中，因为 a.x=1 且 b.x=5，所以它返回 false，而不会注意到列数不匹配。将它们更改为匹配，您将得到相同的异常(这也是第二个查询确实有该异常的原因)。

testdb=# with
s AS (SELECT 1)
, main AS (
SELECT a = b , (a) = (a), (b) = (b), a, b -- I expect (a) = (b) fails
FROM s
  , LATERAL (select 5 as x, 2 as y) AS a
  , LATERAL (select 5 as x) AS b
)
select * from main;
ERROR:  cannot compare record types with different numbers of columns