Get values from varying columns in a generic trigger

I am new to PostgreSQL and found a trigger which serves my purpose completely except for one little thing. The trigger is quite generic and runs across different tables and logs different field changes. I found here.

What I now need to do is test for a specific field which changes as the tables change on which the trigger fires. I thought of using substr as all the column will have the same name format e.g. XXX_cust_no but the XXX can change to 2 or 4 characters. I need to log the value in theXXX_cust_no field with every record that is written to the history_ / audit table. Using a bunch of IF / ELSE statements to accomplish this is not something I would like to do.

The trigger as it now works logs the table_name, column_name, old_value, new_value. I however need to log the XXX_cust_no of the record that was changed as well.

Erwin Brandstetter

Basically you need dynamic SQL for dynamic column names. format helps to format the DML command. Pass values from NEW and OLD with the USING clause.

Given these tables:

CREATE TABLE tbl (
  t_id serial PRIMARY KEY
 ,abc_cust_no text
);

CREATE TABLE log (
  id          int
 ,table_name  text
 ,column_name text
 ,old_value   text
 ,new_value   text
);

It could work like this:

CREATE OR REPLACE FUNCTION trg_demo()
  RETURNS TRIGGER AS
$func$
BEGIN

EXECUTE format('
   INSERT INTO log(id, table_name, column_name, old_value, new_value)
   SELECT ($2).t_id
         , $3
         , $4
         ,($1).%1$I
         ,($2).%1$I', TG_ARGV[0])
USING OLD, NEW, TG_RELNAME, TG_ARGV[0];

RETURN NEW;

END
$func$ LANGUAGE plpgsql;

CREATE TRIGGER demo
BEFORE UPDATE ON tbl
FOR EACH ROW EXECUTE PROCEDURE trg_demo('abc_cust_no'); -- col name here.

SQL Fiddle.