Subject: Question on trigger

From: veem v <[email-masked]>
Date: Thu, 11 Apr 2024 20:01:23 +0530

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)?

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

---

From: Adrian Klaver <[email-masked]>
Date: Thu, 11 Apr 2024 09:30:32 -0700

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

-- 
Adrian Klaver
[email-masked]

---

From: veem v <[email-masked]>
Date: Sat, 13 Apr 2024 12:33:47 +0530

Thank you Adrian.

So it seems the heavy DML tables will see an impact if having triggers
(mainly for each row trigger) created on them.

And also the bulk DML/array based insert (which inserts multiple rows in
one short or one batch) , in those cases it seems the trigger will not make
that happen as it will force it to make it happen row by row, as the
trigger is row based. Will test anyway though.

---

From: Adrian Klaver <[email-masked]>
Date: Sat, 13 Apr 2024 09:14:44 -0700

On 4/13/24 00:03, veem v wrote:
> one short or one batch) , in those cases it seems the trigger will not 
> make that happen as it will force it to make it happen row by row, as 
> the trigger is row based. Will test anyway though.

You said you have triggers in the Oracle database and I assumed they 
worked and where not a show stopping issue there. What makes you think 
that would be different in Postgres?

What type of triggers where there in Oracle, per row, per statement or a 
mix?

> 
> On Thu, 11 Apr 2024 at 22:00, Adrian Klaver <[email-masked] 
> <mailto:[email-masked]>> wrote:
> 
>     <https://www.postgresql.org/docs/current/sql-createtrigger.html>
> 
>     <https://www.postgresql.org/docs/current/plpgsql-trigger.html>
> 
>     [email-masked] <mailto:[email-masked]>
>