Thread: [MASSMAIL]Question on trigger

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

On 4/13/24 00:03, veem v wrote:
> 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.

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 <adrian.klaver@aklaver.com 
> <mailto:adrian.klaver@aklaver.com>> wrote:
> 
>     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
>     <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
>     <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 <mailto:adrian.klaver@aklaver.com>
> 

-- 
Adrian Klaver
adrian.klaver@aklaver.com

On 4/16/24 12:39, veem v wrote:
> 
> 
> On Sat, 13 Apr 2024 at 21:44, Adrian Klaver <adrian.klaver@aklaver.com 
> <mailto:adrian.klaver@aklaver.com>> wrote:
> 
>     On 4/13/24 00:03, veem v wrote:
>      > 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.
> 
>     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?
> 
> 
> Actually we have row level triggers  in oracle which are running for 
> smaller volume DML and are making the direct path inserts to happen in 
> conventional row by row insert, in presence of trigger. So was wondering 

Not sure what the above means, you will need to provide a more detailed 
description. Though any DML you are doing on table that has any sort of 
constraint, index, trigger, foreign key, default values, etc is going to 
have more overhead then into an unencumbered table. FYI, some of the 
preceding are system triggers, for example foreign keys.

> if it postgres we will be encountering a similar issue and batch inserts 
> may be converted back to row by row automatically. And here we are going 
> to process higher volume DMLS in postgresql database.
> 

Hard to say with the information provided. Easiest way to find out is 
create a test setup  and run the code. Though I guess, as I have not 
actually tried this, you could have a per row trigger and per statement 
trigger for the same action and disable the per row and enable the per 
statement trigger for batch operations. Then once the batch operation is 
done reverse the process. Again something to test to verify.


-- 
Adrian Klaver
adrian.klaver@aklaver.com