Re: Updating column on row update

On Sun, Nov 22, 2009 at 12:50 PM, Thom Brown <thombrown(at)gmail(dot)com> wrote:
> Hi,
> This should be simple, but for some reason I'm not quite sure what the
> solution is.  I want to be able to update the value of a column for rows
> that have been updated.  More specifically, if a row is updated, I want it's
> modified_date column to be populated with the current time stamp.  I've
> looked at triggers and rules, and it looks like I'd need to create a
> function just to achieve this which seems incredibly clumsy and unnecessary.
>  Could someone enlighten me?

Well, you DO have to create a function, but it's not all that clumsy
really. Also it's quite flexible so you can do lots of complex stuff
and hide it away in a trigger function.

Example:

-- FUNCTION --

CREATE FUNCTION modtime () RETURNS opaque AS '
BEGIN
new.lm :=''now'';
RETURN new;
END;
' LANGUAGE 'plpgsql';

-- TABLE --

CREATE TABLE dtest (
id int primary key,
fluff text,
lm timestamp without time zone
);

--TRIGGER --

CREATE TRIGGER dtest
BEFORE UPDATE or INSERT ON dtest FOR EACH ROW EXECUTE PROCEDURE
modtime(lm);

-- SQL TESTS --

INSERT INTO dtest (id, fluff) VALUES (1,'this is a test');
INSERT INTO dtest (id, fluff) VALUES (2,'this is another test');
SELECT * FROM dtest;
1 | this is a test | 2003-04-02 10:33:12.577089
2 | this is another test | 2003-04-02 10:33:18.591148
UPDATE dtest SET id=3 WHERE id=1;
3 | this is a test | 2003-04-02 10:34:52.219963 [1]
UPDATE dtest SET fluff='now is the time' WHERE id=2;
SELECT * FROM dtest WHERE id=2;
2 | now is the time | 2003-04-02 10:38:06.259443 [2]
UPDATE dtest SET lm='2003-04-02 08:30:00' WHERE id=3;
SELECT * FROM dtest WHERE id=3;
3 | this is a test | 2003-04-02 10:36:15.45687 [3]

[1] The timestamp has changed for this record when we changed the id field.
[2] The timestamp also changes for the fluff field.
[3] We tried to set lm, but the trigger on that field in dtest
intercepted the change and forced it