Trigger for modification timestamp column

2010/7/7 Johan Andersson <warb@mail.com>:

Hello!

I am trying to write a trigger for updating a modification column and am
having some trouble getting it to behave as I want.

The trigger should set the column to the supplied value if it is set in the
UPDATE statement and to the current timestamp [NOW()] if it is not. The
problem is that I don't know how to check if the column is set or not. I can
check the column's value for NULL but that doesn't work if I want the column
to accept NULL values (meaning "unmodified").

I would like something like:
CREATE FUNCTION update_modified()
   RETURNS TRIGGER AS $$
       BEGIN
           IF NOT isset(NEW.modified) THEN
               NEW.modified = NOW();
           END IF;
           RETURN NEW;
       END;
   $$ LANGUAGE 'plpgsql';

Does anyone know how to do this?

sorry - it isn't possible - you cannot do it. You need one column more
for information if value is modified or not.

Regards

Pavel Stehule

Can't you check it using something like this:

IF OLD.modified = NEW.modified THEN
NEW.modified = NOW();
END IF;
RETURN NEW;

Susan
-----Original Message-----
From: pgsql-general-owner@postgresql.org [mailto:pgsql-general-owner@postgresql.org] On Behalf Of Johan Andersson
Sent: Wednesday, July 07, 2010 6:49 AM
To: pgsql-general@postgresql.org
Subject: [GENERAL] Trigger for modification timestamp column

Hello!

I am trying to write a trigger for updating a modification column and am
having some trouble getting it to behave as I want.

The trigger should set the column to the supplied value if it is set in the
UPDATE statement and to the current timestamp [NOW()] if it is not. The
problem is that I don't know how to check if the column is set or not. I can
check the column's value for NULL but that doesn't work if I want the column
to accept NULL values (meaning "unmodified").

I would like something like:
CREATE FUNCTION update_modified()
RETURNS TRIGGER AS $$
BEGIN
IF NOT isset(NEW.modified) THEN
NEW.modified = NOW();
END IF;
RETURN NEW;
END;
$$ LANGUAGE 'plpgsql';

Does anyone know how to do this?

Thanks in advance!

/ Johan
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2010/7/7 Susan Cassidy <scassidy@stbernard.com>:

Can't you check it using something like this:

IF OLD.modified = NEW.modified THEN
 NEW.modified = NOW();
END IF;
RETURN NEW;

but this take only case where column modified was changed. There are
not a functionality for detection if user explicitly updated column or
not

Pavel

On Wed, Jul 7, 2010 at 8:48 AM, Johan Andersson <warb@mail.com> wrote:

Hello!

I am trying to write a trigger for updating a modification column and am
having some trouble getting it to behave as I want.

The trigger should set the column to the supplied value if it is set in the
UPDATE statement and to the current timestamp [NOW()] if it is not. The
problem is that I don't know how to check if the column is set or not. I
can
check the column's value for NULL but that doesn't work if I want the
column
to accept NULL values (meaning "unmodified").

It isn't very clear just what modification it is you're tracking, a specific
column or the entire row?

Is this a column of type timestamp?

You can do a comparison between OLD.XXX and NEW.XXX in a before-update
trigger but you will need to take into account null values. For example, is
it possible for the modified flag go from non-null to null, and if so what
does that mean?

Suppose the update statement explicitly sets XXX to the same value it
already has, is that updating it or not?