PostgreSQL nested CTE and UNION

Question

I'm trying to learn SQL, using PostgreSQL 9.1.3. I would like to understand some behavior that strikes me as inconsistent. To wit:

This works:

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

I get this:

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

This works:

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

Result:

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

This also works:

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

I get this:

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

But this does not work:

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

Result:

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

To my way of thinking, either the last one should succeed or one of the other ones should fail. I don't see the pattern. Is there some general rule that would enable me to predict what combinations of nested CTEs and UNIONs will or will not work?

Answer 1

The mystery is solved: the behavior I was observing is a known bug. I sent the same original post to a PostgreSQL-specific list and got this answer:

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