AskTOM Office Hours on Database Triggers

Copyright © 2019, Oracle and/or its affiliates. All rights reserved. |
PL/SQL Office Hours session begins soon…
Database Triggers:
You Say Stop, I Say Go
with
Chris Saxon, @ChrisRSaxon & @SQLDaily
https://www.youtube.com/c/TheMagicofSQL
https://blogs.oracle.com/sql
Steven Feuerstein, @sfonplsql
https://www.youtube.com/channel/PracticallyPerfectPLSQL
http://stevenfeuersteinonplsql.blogspot.com/
And
special
guests!

Welcome to Ask TOM Office Hours!

Copyright © 2019, Oracle and/or its affiliates. All rights reserved. | blogs.oracle.com/sql www.youtube.com/c/TheMagicOfSQL @ChrisRSaxon
Database Triggers
Some people say you should NEVER use them.
Some people hate using the word "NEVER."
Let's talk about it.
We'll hear from Toon Koppelaars, Jacek Gebal and Chris
Saxon. But we also want to hear from you.

Database Triggers
Broadly two different types of triggers:
• System triggers (database events like logon triggers,
DDL triggers, etc.)
• DML triggers (triggers on tables and views): simple,
compound, instead of

CREATE OR REPLACE TRIGGER stop_drop_trg
BEFORE DROP ON hr.SCHEMA
BEGIN
RAISE_APPLICATION_ERROR (
num => -20999,
msg => 'Cannot drop object');
END;
blogs.oracle.com/sql www.youtube.com/c/TheMagicOfSQL @ChrisRSaxon
System
trigger
example

Focusing In On DML Triggers
For the rest of this session, we will concentrate on
DML triggers – triggers created on tables or views.
They are the source of controversy and likely impact
on production applications.

Some thoughts from
Jacek Gabel (Fidelity, utPLSQL)
"Triggers are great. They can also
become a maintenance hell."

Some thoughts from
Toon Koppelaars on….
"Triggers Considered Harmful,
Considered Harmful"

Mostly Agree With #1 Influencer of Past Times
• http://tkyte.blogspot.com/2006/09/classic-example-of-why-i-despise.html
– “Triggers are so abused - and used so inappropriately, I'd rather live without them.”
– “There are no times triggers cannot be avoided. They are purely a convenience that is
overused, abused and improperly used.”
• http://tkyte.blogspot.com/2007/03/dreaded-others-then-null-strikes-
again.html
– [On why he doesn’t like triggers] “Because I hate being surprised or tricked. And
triggers are all about trickery and surprises.”
9
Still available at web.archive.org

Tom's Legacy
• Default response at community.oracle.com when someone poses
question related to triggers:
“Triggers should be avoided as much as possible“
“Don’t use them, they are bad”
“Triggers are considered harmful”
10

Four Use-Cases of Database Triggers
1. Modify :NEW columns
2. Execute changes outside transaction
3. Execute changes inside transaction
4. Query tables
11

Modify :NEW Columns
• Done in row-triggers
– :NEW.CREATED := SYSDATE
– :NEW.CREATOR := USER
– :NEW.ID := SEQ.NEXTVAL
– …
• What happens if I supply these column values?
– They get overwritten
– Maybe application error would be more appropriate?
12

Execute Changes Outside Transaction
• Examples
– Call REST service
– Talk to some endpoint over tcp/ip
– Send email
– Write to file
– Execute autonomous transaction
• What happens if my transaction rolls back?
– All of the above is outside your transaction and irreversable
13

Execute Changes Inside Transaction
• Examples
– Insert audit row in audit table
– Maintain redundant data in other table(s)
– Propagate more changes to other parts of database design
• Semantics of your insert, update or delete has changed significantly
14

Query Tables
• Very common use-case is to mandate cross-row constraints via
triggers
– These require queries
– E.g. check at most one president in EMPLOYEES table
– Mutating table error
– Serialization
• More complex than you think to get this right
– SQL assertions
15

Four Use-Cases of Database Triggers
1. Modify :NEW columns
2. Execute changes outside transaction
3. Execute changes inside transaction
4. Query tables
16
“Surprises and Trickery”
Too complex to do it right

CREATE OR REPLACE TRIGGER dg_workouts_bir
BEFORE INSERT ON dg_workouts FOR EACH ROW
DECLARE
BEGIN
IF :new.workout_id IS NULL
THEN
:new.workout_id := dg_workouts_seq.NEXTVAL;
END IF;
:new.created_on := SYSDATE;
:new.created_by := qdb_config.apex_user ();
:new.changed_on := SYSDATE;
:new.changed_by := qdb_config.apex_user ();
END;
What
do you
think of
this
trigger?

CREATE TABLE dg_workouts
(
workout_id INTEGER NOT NULL
NUMBER GENERATED ALWAYS AS IDENTITY,
...
created_by VARCHAR2 (132 BYTE) NOT NULL,
changed_by VARCHAR2 (132 BYTE) NOT NULL,
created_on DATE NOT NULL DEFAULT SYSDATE,
changed_on DATE NOT NULL DEFAULT SYSDATE
)
Fix the
table
DDL
or
Switch
to API
call.

CREATE OR REPLACE TRIGGER dg_workouts_bir
BEFORE INSERT ON dg_workouts FOR EACH ROW
DECLARE
PRAGMA AUTONOMOUS_TRANSACTION;
BEGIN
/* Change another table */
UPDATE ...;
/* Send email */
UTL_SMTP...;
COMMIT;
END;
What
do you
think of
this
trigger?

Trigger Restarts
“Our trigger fired twice!” We had a
one-row table with a BEFORE FOR
EACH ROW trigger on it. We
updated one row, yet the trigger
fired two times.
Tom: "If you have a trigger that does
anything non-transactional, trigger restarts
could be a fairly serious issue."

CREATE OR REPLACE PACKAGE BODY dg_workout_mgr
IS
PROCEDURE update_workout (...) IS
PRAGMA AUTONOMOUS_TRANSACTION;
BEGIN
UPDATE dg_workouts ...;
/* Change other table */
UPDATE ...;
/* Send email */
UTL_SMTP...;
COMMIT;
END;
What to
do?
Move logic
to PL/SQL
API, restrict
direct
access to
table

SQL> create or replace
2 trigger TRG_AUDIT_EMPLOYEES
3 after insert or update or delete on EMPLOYEES
4 for each row
5 declare
6 l_operation varchar2(1) :=
7 case when updating then 'U'
8 when deleting then 'D'
9 else 'I' end;
10 begin
11 if updating or inserting then
12 insert into AUDIT_EMPLOYEES
13 (aud_who, aud_when ...
What
about
this
trigger?

update ex_rates
set exchange_rate = 1
where to_currency_code = 'USD'
and from_current_code = 'GBP';

"Sell one million
dollars!"
Ryan McGuire / Gratisography

update ex_rates
set exchange_rate = 1.29
where to_currency_code = 'USD'
and from_current_code = 'GBP';

"Buy one million
dollars!"

"We need
auditing!"

select from_currency_code,
to_currency_code,
exchange_rate
from ex_rates
versions between timestamp
date'2018-01-01' and sysdate;
Limited
by undo

Flashback Data
Archive

create flashback archive change_history
tablespace users retention 7 year;

alter table ex_rates
flashback archive change_history;

select from_currency_code,
to_currency_code,
exchange_rate,
versions_operation,
versions_starttime,
versions_endtime
from ex_rates
versions between timestamp
date'2018-01-01' and sysdate;
FREE from
11.2.0.4

FROM_CODE TO_CODE EX_RATE OP STARTTIME ENDTIME
GBP USD 1.3 <null> <null> 17-SEP-2018 08.58.38
GBP USD 1.34 U 17-SEP-2018 08.58.38 17-SEP-2018 08.58.38
GBP USD 1.29 U 17-SEP-2018 08.58.38 17-SEP-2018 08.58.38
GBP USD 1 U 17-SEP-2018 08.58.38 17-SEP-2018 08.58.41
GBP USD 1.29 U 17-SEP-2018 08.58.41 17-SEP-2018 08.58.41
GBP USD 1.31 U 17-SEP-2018 08.58.41 <null>

Now it's over to you!

asktom.oracle.com
#AskTOMOfficeHours

AskTOM Office Hours on Database Triggers

More Related Content

What's hot

Similar to AskTOM Office Hours on Database Triggers

More from Steven Feuerstein

Recently uploaded

AskTOM Office Hours on Database Triggers