Re: statement-level trigger sample out there?

Alvaro Herrera wrote:

>Gerardo Herzig escribió:
>
>
>>Stephen Cook wrote:
>>
>>
>>
>>>I am curious (coming from a MS SQL Server background, I just started
>>>playing with PostgreSQL recently).
>>>
>>>What type of situation would warrant a statement-level trigger that can't
>>>access the old and new values? Without that access, isn't the only
>>>information you get is the fact that an operation occurred on the table?
>>>Or am I missing something?
>>>
>>>
>>What about this. Suppose you have this table "planets":
>>planet_name | star_id|....
>>
>>There is a lot of stars, right? And if a very common query involves a
>>"select planet_name, count(*) from planets group by star_id"....Well, if
>>there is 1.000.000.000 of galaxies, and 1.000.000.000.000 of stars per
>>galaxy...Thats a lot of planets to count!!! So maybe you want a helper
>>table who maintains such of subtotals.
>>
>>Well, each time you discover a new galaxy, insert every planet in the
>>monster table, and *after* all the inserts, run a trigger for updating the
>>helper table.
>>
>>
>
>Right, but it would be much more useful if you can access the NEW set
>and instead of counting all the planets from scratch, you just take the
>current count and add the number of planets being added.
>
>You can do it with FOR EACH ROW triggers, but it's much worse because
>you need one UPDATE on the counter for each new planet.
>
>Perhaps the usefulness is that you store _in memory_ the number of
>planets added during the FOR EACH ROW trigger, and when that's done,
>call the FOR EACH STATEMENT trigger that does a single update adding the
>number in memory. This would work only if the FOR EACH STATEMENT
>trigger was promised to be executed after all the FOR EACH ROW triggers
>were called.
>
>
>
Shure. In that case, i will do the initial inserts into a temporary
table, do the counting, updating the helper table, and then insert into
the planets table.
I use that approach and works fine to me.

Gerardo