Re: Question on trigger - Mailing list pgsql-general

On 4/11/24 07:31, veem v wrote:
> Hi, We used to use Oracle database in which we had audit 
> triggers(something as below) mandated for all tables by the control 
> team. Now we are going to use the postgresql 15.4 database for one of 
> our applications. So,wanted to understand if there exists any downside 
> of such audit trigger setup for all the tables? Will it impact the bulk 
> data insert/update/delete OR slowdown of any of the DML operations 
> significantly (and thus will not be advisable to use for all tables but 
> selected ones)?

Triggers are overhead in Postgres as they where in Oracle. If they 
didn't cause an issue in Oracle I would suspect that would also be the 
case in Postgres. To confirm you would need to create a test setup and 
run some common operations and see what the overhead is.

Some potential performance improvements:

https://www.postgresql.org/docs/current/sql-createtrigger.html

"...a trigger that is marked FOR EACH STATEMENT only executes once for 
any given operation, regardless of how many rows it modifies (in 
particular, an operation that modifies zero rows will still result in 
the execution of any applicable FOR EACH STATEMENT triggers)."

<...>

"The REFERENCING option enables collection of transition relations, 
which are row sets that include all of the rows inserted, deleted, or 
modified by the current SQL statement. This feature lets the trigger see 
a global view of what the statement did, not just one row at a time. 
This option is only allowed for an AFTER trigger that is not a 
constraint trigger; also, if the trigger is an UPDATE trigger, it must 
not specify a column_name list. OLD TABLE may only be specified once, 
and only for a trigger that can fire on UPDATE or DELETE; it creates a 
transition relation containing the before-images of all rows updated or 
deleted by the statement. Similarly, NEW TABLE may only be specified 
once, and only for a trigger that can fire on UPDATE or INSERT; it 
creates a transition relation containing the after-images of all rows 
updated or inserted by the statement."


As example:

https://www.postgresql.org/docs/current/plpgsql-trigger.html

Example 43.7. Auditing with Transition Tables

> 
> CREATE OR REPLACE TRIGGER TAB_AUD_TRG
>    BEFORE DELETE OR INSERT OR UPDATE
>    ON tab
>    FOR EACH ROW
> BEGIN
>        IF inserting THEN
>          :NEW.create_timestamp := systimestamp;
>          :NEW.create_userid  := sys_context('USERENV','SESSION_USER');
>          :NEW.update_timestamp := systimestamp;
>          :NEW.update_userid := sys_context('USERENV','SESSION_USER');
>        ELSIF updating THEN
>          IF  updating('create_userid') OR updating('create_timestamp') THEN
>              :new.create_userid   := :old.create_userid;
>              :new.create_timestamp  := :old.create_timestamp;
>          END IF;
>          :NEW.update_timestamp := systimestamp;
>          :NEW.update_userid := sys_context('USERENV','SESSION_USER');
>        END IF;
>    END;
> /
> 
> Regards
> Veem

-- 
Adrian Klaver
adrian.klaver@aklaver.com