The following bug has been logged online:

Bug reference: 2251
Logged by: Sergei Dubov
Email address: sdubov (AT) gmail (DOT) com
PostgreSQL version: 8.1.2
Operating system: Windows XP
Description: NOT IN clause is not working correctly
Details:

I have two tables, let's say A and B.

B is a child of a in one to many relationship. A contains records that are
not referenced by B.

I am running a query:

select * from A t1 where t1.id not in (select t2.A_id from B t2);

It returns 0 rows.

Now I run
(select t1.id from A t1) except (select t2.A_id from B t2);

And now Postgres correctly returns records from A that are not referenced by
B.

Thanks a lot for your consideration of this bug.

Serge.

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On Fri, 10 Feb 2006, Sergei Dubov wrote:

This may not be a bug, since these two queries will do different things if
there are any nulls in B.A_id according to spec AFAIK. The short form is
that rvc NOT IN (<subselect returning nulls>) will not return true.

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TIP 9: In versions below 8.0, the planner will ignore your desire to
choose an index scan if your joining column's datatypes do not
match