Question on how to conditionally commit or rollback a sql-request

Hi all,

Making an update to a db-table I want to check if it is the same data
that has already been updated within nearest 0.5 minute before
committing

The table contains a timestamp-field, registeredatdatetime, that I try
to use in a before-update-trigger like:

/* TRIGGER: objectdescription_bu */

CREATE OR REPLACE FUNCTION objectdescription_bu() RETURNS trigger AS $$

DECLARE

intval INTEGER;

BEGIN

intval = (current_timestamp - old.registereddatetime);

IF NEW.STATUS = OLD.STATUS or

NEW.POSITIONCODE = OLD.POSITIONCODE and

intval > (interval '30 seconds') then

Commit;

ELSE

Rollback;

END IF;

END;

$$ LANGUAGE plpgsql;

CREATE TRIGGER objectdescription_bu

BEFORE UPDATE

ON objectdescription

FOR EACH ROW

EXECUTE PROCEDURE objectdescription_bu();

When I try to do an update like:

update objectdescription

set status = 466

where

customerid = 25 AND

objectnumber = 4;

I get the following error:

ERROR: SPI execute_plan failed executing query "Commit":

SPI_ERROR_TRANSACTION

CONTEXT: Pl/pgSQL function "objectdescription_bu" line 9 at SQL
statement

What does this mean and what should I do to be able to do the desired
check

Regards
Staffan Bergbom