Postgresql 9.2 trigger to log changes to data in a

I have several tables and want to log when changes are made to them, what the change was and who made the change. Postgresql 9.2

CREATE TABLE unitsref (
unitsrefid serial primary key,
units varchar,
unitname varchar,
inuse boolean,
systemuse varchar,
keynotes integer,
linkid integer
);

Is the best practise to use OLD.* IS DISTINCT FROM NEW.* ?

CREATE TRIGGER log_unitsref
    AFTER UPDATE ON unitsref
    FOR EACH ROW
    WHEN (OLD.* IS DISTINCT FROM NEW.*)
EXECUTE PROCEDURE log_unitsref();

I am only really interested in the three fields:

units varchar,
unitname varchar,
inuse boolean,

I want to record these changes in a table eventlog with the fields:

recordtype varchar,
recordkey varchar,
changetype varchar,
personid integer,
changedate date,
changetime time,
changefrom varchar,
changeto varchar,

What is the best syntax to write a function to do this? In Progress Openedge I would write

create  EventLog.
assign  EventLog.PersonId    = glb-Personid
        EventLog.RecordType  = "UnitsRef"
        EventLog.RecordKey   = UnitsRef.Units
        EventLog.ChangeType  = "Create"
        EventLog.changeFrom  = ""
        EventLog.changeTo    = ""
        EventLog.changeDate  = today
        EventLog.changeTime  = time

but I don`t know the best method in Postgresql

I am only really interested in the three fields

Then it should be more efficient to only call the trigger after changes to these fields:


CREATE TRIGGER log_unitsref
AFTER UPDATE OF units, unitname, inuse
ON unitsref
FOR EACH ROW
WHEN (OLD.units, OLD.unitname, OLD.inuse) IS DISTINCT FROM
     (NEW.units, NEW.unitname, NEW.inuse)
EXECUTE PROCEDURE log_unitsref();

I quote the manual on CREATE TRIGGER:

UPDATE OF ...

The trigger will only fire if at least one of the listed columns is mentioned as a target of the UPDATE command.

WHEN ...

A Boolean expression that determines whether the trigger function will actually be executed.

Note that these two elements are closely related but neither mutually exclusive nor redundant.

It is much cheaper not to fire the trigger at all, if no column of interest is involved.
It is much cheaper not to execute the trigger function if no column of interest was actually altered.