具有不同列数的记录类型比较不会失败

Question

为什么以下查询不会在 PostgreSQL 11.6 中触发 "cannot compare record types with different numbers of columns" 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：

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

Answer 1

参见 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.

在这种情况下，因为 a.x=1 和 b.x=5，它 returns 错误而没有注意到列数不匹配。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

具有不同列数的记录类型比较不会失败

Record type comparison with different numbers of columns isn't failing

sql

postgresql

comparison

record