Thread: simple auto-updating timestamp ?

Hello list,

I suspect, this is a common issue for newbies.
Is there a simple way to have an auto-updating timestamp like mysql has ?

create table something (
   id                int4,
   sometext     text,
   update_ts    timestamp(0),
   primary key (id)
);

Everytime this table gets updated the timestamp should be automatically
refreshed to NOW() ?
I hope someone could point me to an example.

Andreas wrote:

> I suspect, this is a common issue for newbies.
> Is there a simple way to have an auto-updating timestamp like mysql has ?
>
> create table something (
>   id                int4,
>   sometext     text,
>   update_ts    timestamp(0),
>   primary key (id)
> );
>
> Everytime this table gets updated the timestamp should be
> automatically refreshed to NOW() ?
> I hope someone could point me to an example.

You can do this by adding a trigger to your table.  Just define the trigger
to be invoked on INSERT and UPDATE for your table.  The trigger definition
would look something like this:

    CREATE TRIGGER "trg_set_update_ts" BEFORE INSERT OR UPDATE
    ON "public.something" FOR EACH ROW
    EXECUTE PROCEDURE "public"."set_update_ts"();

Then, your function in PL/PGSQL that sets the update_ts to NOW() would look
something like this:

    CREATE FUNCTION "public"."set_update_ts" () RETURNS trigger AS'
    BEGIN
        NEW.update_ts = NOW();
        RETURN NEW;
    END;
    'LANGUAGE 'plpgsql' IMMUTABLE CALLED ON NULL INPUT SECURITY INVOKER;

Of course, this function would end up setting the update_ts to NOW() every
time you insert or update your table.  And you could never set the value
to anything other than NOW() because your trigger would catch it and set it
back to NOW() again.  If that's not exact, it'll at least point you in
the right direction.

Dante

----------
D. Dante Lorenso
dante@lorenso.com

D. Dante Lorenso wrote:

> You can do this by adding a trigger to your table.  Just define the
> trigger
> to be invoked on INSERT and UPDATE for your table.  The trigger
> definition
> would look something like this:  [...]

Thanks.
So far that works for one table.

Can I have this behaviour somehow inherited by child-tables ?
Like:
CREATE TABLE objects (
   id                      integer primary key,
   created_ts         timestamp(0)   DEFAULT LOCALTIMESTAMP,
   update_ts          timestamp(0),
   deleted_ts         timestamp(0),   -- things get ignored in normal
processing
...
);

Then create a trigger as in your example that updates this timestamp.

Every other table in the db would inherit (objects) to get those
standard fields that I'd like to have everywhere. It'd be nice not
having to bother about the "methods" of the objects-class for every
child-class.

>    CREATE FUNCTION "public"."set_update_ts" () RETURNS trigger AS'
>    BEGIN
>        NEW.update_ts = NOW();
>        RETURN NEW;
>    END;   'LANGUAGE 'plpgsql' IMMUTABLE CALLED ON NULL INPUT SECURITY
> INVOKER;

I entered your code into psql and checked it afterwards with pgadmin3.
pgadmin shows some parts different to the code that I pushed through psql :
1)  create OR REPLACE ...
2)  immuntable;    <-- End of line  What does this part behind
"immutable" do ?

> Of course, this function would end up setting the update_ts to NOW()
> every
> time you insert or update your table.  And you could never set the value
> to anything other than NOW() because your trigger would catch it and
> set it
> back to NOW() again.

That is what I had in mind. I'd like to see when there was the last
update to a record.


... Andreas

Andreas wrote:

> D. Dante Lorenso wrote:
>
>> You can do this by adding a trigger to your table.  Just define the
>> trigger
>> to be invoked on INSERT and UPDATE for your table.  The trigger
>> definition
>> would look something like this:  [...]
>
>
> Thanks.
> So far that works for one table.
>
> Can I have this behaviour somehow inherited by child-tables ?
> Like:
> CREATE TABLE objects (
>   id                      integer primary key,
>   created_ts         timestamp(0)   DEFAULT LOCALTIMESTAMP,
>   update_ts          timestamp(0),
>   deleted_ts         timestamp(0),   -- things get ignored in normal
> processing
> ...
> );
>
> Then create a trigger as in your example that updates this timestamp.
> Every other table in the db would inherit (objects) to get those
> standard fields that I'd like to have everywhere. It'd be nice not
> having to bother about the "methods" of the objects-class for every
> child-class.

Yeah I know what you mean.  Someone jump in here and correct me if I'm
wrong,
but I don't believe that triggers are inherited in PG.  Of course, you
already
have the 'set_update_ts' function defined, so you would only have to declare
the trigger for every child table (not the function).

Verify that this is true.  Last time I checked i think that's how it worked.

>>    CREATE FUNCTION "public"."set_update_ts" () RETURNS trigger AS'
>>    BEGIN
>>        NEW.update_ts = NOW();
>>        RETURN NEW;
>>    END;   'LANGUAGE 'plpgsql' IMMUTABLE CALLED ON NULL INPUT SECURITY
>> INVOKER;
>
>
> I entered your code into psql and checked it afterwards with pgadmin3.
> pgadmin shows some parts different to the code that I pushed through
> psql :
> 1)  create OR REPLACE ...
> 2)  immuntable;    <-- End of line  What does this part behind
> "immutable" do ?

You probably want to remove the 'IMMUTABLE CALLED ON NULL INPUT SECURITY
INVOKER'.
That was my cut-and-paste error.  I meant to strip that off for you.
Here's the
page that explains what all those do, though:

    http://www.postgresql.org/docs/7.4/static/sql-createfunction.html

Dante