sql - PostgreSQL 嵌套 CTE 和 UNION

Question

我正在尝试使用 PostgreSQL 9.1.3 学习 SQL。我想了解一些让我觉得前后矛盾的行为。即:

这个有效:

WITH innermost AS (SELECT 2)
SELECT * FROM innermost
UNION SELECT 3;

我明白了:

 ?column? 
----------
        2
        3

这个有效:

WITH outmost AS (
        (WITH innermost AS (SELECT 2)
         SELECT * FROM innermost)
)                                
SELECT * FROM outmost;

结果:

?column? 
----------
        2

这也有效:

WITH outmost AS (
  SELECT 1
  UNION (WITH innermost AS (SELECT 2)
         SELECT * FROM innermost)
)
SELECT * FROM outmost;

我明白了:

 ?column? 
----------
        1
        2

但这不有效:

WITH outmost AS (
  SELECT 1
  UNION (WITH innermost as (SELECT 2)
         SELECT * FROM innermost
         UNION SELECT 3)
)
SELECT * FROM outmost;

结果:

ERROR:  relation "innermost" does not exist
LINE 4:          SELECT * FROM innermost

按照我的想法，要么最后一个应该成功，要么另一个应该失败。我没有看到模式。是否有一些通用规则可以让我预测嵌套 CTE 和 UNION 的哪些组合有效或无效？

Answer 1

谜底已解:我观察到的行为是一个已知错误。我将相同的原始帖子发送到特定于 PostgreSQL 的列表并得到了这个答案:

This is a bug :-(. The parse analysis code seems to think that WITH can only be attached to the top level or a leaf-level SELECT within a set operation tree; but the grammar follows the SQL standard which says no such thing. The WITH gets accepted, and attached to the intermediate-level UNION which is where syntactically it should go, and then it's entirely ignored during parse analysis. Will see about fixing it.

      regards, tom lane

http://archives.postgresql.org/pgsql-novice/2012-07/msg00113.php