Question about CONSTRAINT TRIGGER

[ZM Yang <zmpgzm@xxxxxxxxx>]

Hi folks,

I'm confused about the usage of CONSTRAINT TRIGGER. More specifically, the documentation says that the name of another table referenced by the constraint can be specified in a FROM clause:

"The (possibly schema-qualified) name of another table referenced by the constraint. This option is used for foreign-key constraints and is not recommended for general use. This can only be specified for constraint triggers."

(1)

How can the table name specified in the the FROM clause be used? It seems that it cannot be referenced in the WHERE clause. (Please correct me if I'm wrong) 

(2)

How is the table specified in the FROM clause used for FK constraints?

Thanks,

ZM