Here are general trigger functions provided as workable examples
of using SPI and triggers. "General" means that functions may be
used for defining triggers for any tables but you have to specify
table/field names (as described below) while creating a trigger.

1. refint.c - functions for implementing referential integrity.

check_primary_key () is to used for foreign keys of a table.
   
   You are to create trigger (BEFORE INSERT OR UPDATE) using this 
function on a table referencing another table. You are to specify
as function arguments: triggered table column names which correspond
to foreign key, referenced table name and column names in referenced
table which correspond to primary/unique key.
   You may create as many triggers as you need - one trigger for
one reference.

check_foreign_key () is to used for primary/unique keys of a table.

   You are to create trigger (BEFORE DELETE OR UPDATE) using this
function on a table referenced by another table(s). You are to specify
as function arguments: number of references for which function has to
performe checking, action if referencing key found ('cascade' - to delete
corresponding foreign key, 'restrict' - to abort transaction if foreign keys 
exist, 'setnull' - to set foreign key referencing primary/unique key
being deleted to null), triggered table column names which correspond
to primary/unique key, referencing table name and column names corresponding
to foreign key (, ... - as many referencing tables/keys as specified
by first argument).
   Note, that NOT NULL constraint and unique index have to be defined
by youself.

   There are examples in refint.example and regression tests 
(sql/triggers.sql).