postgres/contrib/spi/README.MAX

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).

   To CREATE FUNCTIONs use refint.sql (will be made by gmake from
refint.source).




# Excuse me for my bad english. Massimo Lambertini
#
#
# New check foreign key 
#
I think that cascade mode is to be considered like that the operation over 
main table is to be made also in referenced table .
When i Delete , i must delete from referenced table , 
but when i update , i update referenced table and not delete like unmodified refint.c .

I made a new version of refint.c  that when i update it check the type of modified key ( if is a text , char() i
added '') and then create a update query that do the right thing .

For my point of view that policy is helpfull because i do not have in referenced table
loss of information .


In preprocessor subdir i have placed a little utility that from a SQL92 table definition,
it create all trigger for foreign key .


the schema that i use to analyze the problem is this 

create table 
A 
( key int4 not null primary key ,...,
) ;

create table
REFERENCED_B
( key int 4 , ... , 
foreign key ( key ) references A -- 
);


--
-- Trigger for REFERENCED_B
--

CREATE INDEX I_REFERENCED_B_KEY ON REFERENCED_B  ( KEY ) ;

CREATE TRIGGER T_P_REFERENCED_B_A BEFORE INSERT OR UPDATE ON REFERENCED_B  FOR EACH ROW
EXECUTE PROCEDURE
check_primary_key('KEY','A','KEY' );

CREATE TRIGGER T_F_D_A_REFERENCED_B  BEFORE DELETE ON A FOR EACH ROW
EXECUTE PROCEDURE
check_foreign_key(1,'cascade','KEY','REFERENCED_B ','KEY' );

CREATE TRIGGER T_F_U_A_REFERENCED_B  AFTER UPDATE ON A FOR EACH ROW
EXECUTE PROCEDURE
check_foreign_key(1,'cascade','KEY','REFERENCED_B ','KEY' );

-- ********************************

I write   TRIGGER T_F_U_A_REFERENCED_B ( AFTER ) and not BEFORE because if i set
BEFORE , when i try to modify ( update ) a key of A , i start a execution of   TRIGGER T_P_REFERENCED_B_A
( check_primary_key) before the real modification of key in A , then the execution of    ( check_primary_key) return
not ok.
With AFTER Clausole i modify first key of A then a update the value of referenced table REFERENCED_B.

Try also the new_example.sql to view the modified policy.
I wish that my explain of problem is quite clear .
If there is miss understanding ( cause my bad english ) please send email to massimo.lambertini@everex.it