Subject: Re: best way to auto-update a field when row is updated
From: Scott Marlowe <smarlowe@g2switchworks.com>
To: Henry Ortega <juandelacruz@gmail.com>
Cc: pgsql-sql@postgresql.org
In-Reply-To: <2bffcc3305031012092fde7dc2@mail.gmail.com>
References: <2bffcc3305031012092fde7dc2@mail.gmail.com>
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Date: Thu, 10 Mar 2005 14:35:48 -0600

On Thu, 2005-03-10 at 14:09, Henry Ortega wrote:
> I have the following table
> 
> FIELD_A        |          FIELD_B          |           TSTAMP
> --------------------------------------------------------------------------------------------------------
> x                              y                          2005-03-10
> 14:56:47.456431
> 
> TSTAMP = not null default now()
> 
> What's the best way to always auto-update TSTAMP to it's
> default value whenever the row gets updated?
> (e.g. update TABLENAME set FIELD_A='zzz' where FIELD_A='x';
> should automatically set TSTAMP to now)

Here's a simple trigger to do that for ya.

-- 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