Re: Updating column on row update

On Sun, Nov 22, 2009 at 1:32 PM, Thom Brown <thombrown(at)gmail(dot)com> wrote:
> 2009/11/22 Scott Marlowe <scott(dot)marlowe(at)gmail(dot)com>
>>
>> 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
>
> Thanks Scott.  It's a shame a function has to be used because it then has
> the dependency of plpgsql being loaded.  I'm attempting to write a database
> schema to accompany a PostgreSQL driver for a popular CMS, but I guess I
> could get it to load plpgsql in as a language.
> The problem now is if the the schema creation script is run against a
> database where the language is already installed, I would get an error
> saying it already exists.  Is there a way to get it to check for it first,
> and only create it if it isn't exist?  Bear in mind I'd want this to be
> compatible at least as far back as 8.1.

Try this:

select * from pg_language ;

Pretty sure that exists pretty far back.