Re: declare column update expression

On Thu, Sep 11, 2008 at 1:35 AM, Chris Velevitch
<chris(dot)velevitch(at)gmail(dot)com> wrote:
> In 7.4, how do I declare that a column in a table is to be
> automatically set to the value of some fixed expression whenever a row
> is updated?
>
> Eg column last_modified is always set to current_timestamp

A trigger as Pavel said. writing them in plpgsql seems a bit hard at
first, but it's a simple language and it's pretty easy to write stuff
like this in.

Here's a simple example of last modified trigger using plpgsql from way back:

-- 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

>
>
>
> Chris
> --
> Chris Velevitch
> Manager - Adobe Platform Users Group, Sydney
> m: 0415 469 095
> www.apugs.org.au
>
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