There are three match types: MATCH FULL,MATCH PARTIAL,and MATCH SIMPLE
(which is the default). MATCH FULL will not allow one column of
a multicolumn foreign key to be null unless all foreign key columns
are null; if they are all null,the row is not required to have a
match in the referenced table. MATCH SIMPLE allows any of the foreign
key columns to be null; if any of them are null,the row is not
required to have a match in the referenced table. MATCH PARTIAL is not
yet implemented. (Of course,NOT NULL constraints can be applied to
the referencing column(s) to prevent these cases from arising.)

另外,在chapter on Foreign Keys：

Normally,a referencing row need not satisfy the foreign key
constraint if any of its referencing columns are null. If MATCH FULL
is added to the foreign key declaration,a referencing row escapes
satisfying the constraint only if all its referencing columns are null
(so a mix of null and non-null values is guaranteed to fail a MATCH FULL
constraint). If you don’t want referencing rows to be able to
avoid satisfying the foreign key constraint,declare the referencing
column(s) as NOT NULL.

请务必参考当前的手册. (或者与您的安装相匹配的版本.)不要过时的Google链接指向过时的版本.