Re: Weird (?) problem with order of conditions in SELECT

"Mark, Terry" <tmark@amgen.com> writes:
> I have encountered a weird problem I  can't seem to understand.  It involves
> a correlated subquery, where the rows returned seem to depend upon the order
> I specify my conditions.  I can't see why the order should be important
> (except maybe for performance)

> SELECT c.score FROM c
> WHERE c.score >= (SELECT MAX(score) AS score FROM c
>             WHERE a.name='nugget'
>             AND a.job='programmer'
>             AND a.a_id=b.a_id AND c.b_id = b.b_id)
> AND a.name='nugget'
> AND a.job='programmer'
> AND a.a_id=b.a_id
> AND c.b_id = b.b_id;

> SELECT c.score FROM c
> WHERE a.name='nugget'
> AND a.job='programmer'
> AND a.a_id=b.a_id
> AND c.b_id = b.b_id
> AND c.score >= (SELECT MAX(score) AS score FROM c
>             WHERE a.name='nugget'
>             AND a.job='programmer'
>             AND a.a_id=b.a_id
>             AND c.b_id = b.b_id);

This is a little less mysterious if you run it under 7.1, because 7.1
emits some warning notices:

NOTICE:  Adding missing FROM-clause entry in subquery for table "a"
NOTICE:  Adding missing FROM-clause entry in subquery for table "b"
NOTICE:  Adding missing FROM-clause entry for table "a"
NOTICE:  Adding missing FROM-clause entry for table "b"
 score
-------
  2500
(1 row)

NOTICE:  Adding missing FROM-clause entry for table "a"
NOTICE:  Adding missing FROM-clause entry for table "b"
 score
-------
   100
  2500
(2 rows)

From this we can infer that Postgres is actually interpreting the first
query as

SELECT c.score FROM a,b,c
WHERE c.score >= (SELECT MAX(score) AS score FROM a,b,c
            WHERE a.name='nugget'
            AND a.job='programmer'
            AND a.a_id=b.a_id AND c.b_id = b.b_id)
AND a.name='nugget'
AND a.job='programmer'
AND a.a_id=b.a_id
AND c.b_id = b.b_id;

whereas the second one is being interpreted as

SELECT c.score FROM a,b,c
WHERE a.name='nugget'
AND a.job='programmer'
AND a.a_id=b.a_id
AND c.b_id = b.b_id
AND c.score >= (SELECT MAX(score) AS score FROM c
            WHERE a.name='nugget'
            AND a.job='programmer'
            AND a.a_id=b.a_id
            AND c.b_id = b.b_id);

That is, in the second case the sub-select's references to A and B are
being taken as outer references to the current A and B rows of the outer
query, whereas in the first case the sub-select is interpreted as a
completely independent query.

I am not sure which interpretation you were actually intending.

This example shows one reason why the "implicit FROM item" feature of
Postgres is confusing and has come to be deprecated: it's not always
clear which FROM list an implicit item should be added to.  We've
started to emit a warning about use of this feature in 7.1, and perhaps
someday it will be removed entirely.

            regards, tom lane