The following bug has been logged online:

Bug reference: 2412
Logged by: Sidar Lopez Cruz
Email address: sidarlopez (AT) hotmail (DOT) com
PostgreSQL version: 8.1.3
Operating system: Ubuntu 6.04
Description: Foreing key accept nulls
Details:

I think that when I create table that have a foreing key to another one
these FK may not accept nulls by default.

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TIP 5: don't forget to increase your free space map settings

On Fri, 28 Apr 2006, Sidar Lopez Cruz wrote:

The SQL spec seems to say otherwise. A foreign key constraint with match
simple (unspecified) is considered satisfied if any component of the key
is null or there is a match in the referenced table. A foreign key
constraint with match full is considered satisified if all components of
the key is null or there is a match in the referenced table. For a
single column key, this means NULLs are acceptable unless the column has
other constraints (like not null).



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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