Is it ever possible to design an Oracle trigger that modifies same table (guaranteed not same row)? - oracle

Question

I need to write a TRIGGER that preserves the old value of a column before it is updated, by inserting or updating the old value into another row in the same table. (yes, I know).
The following MERGE/DUAL trickery has served me well,
but because in this case, I'm inserting into or updating the same table,
Oracle complains at runtime. Also, for some reason, I found it unusually difficult to write code that compiles without errors.
Two questions:
Is it ever possible to modify the same table that the trigger is on, even when I can guarantee that the trigger will never update the row that triggered the trigger? Or do I have to do something like (e.g.): insert pending changes into another table, so that a 2nd trigger can merge them back into the original table? (This table is a customer interface, so I can't re-architect this to use a second table for permanently storing old values.)
What's with the compiler errors that don't let me use :old.event_key, but do let me use :old.property_val in the MERGE statement? (declaring a variable old_event_key and assigning it to the value of :old.event_key seems to work) Is there some sort of hidden intermediate language that knows when a column is (part of) the primary key, and prevents you from referencing it via :old.?
Here is the offending code:
create or replace trigger remember_old_status
before update on event_properties
for each row
when (old.property_name = 'CURRENT-STATUS')
declare
old_event_key varchar2(20);
begin
old_event_key := :old.event_key;
merge into event_properties eprop
using (select 1 from dual) dummy
on ( eprop.event_key = old_event_key
AND eprop.property_name = 'PREVIOUS-STATUS')
when matched then
update set property_val = :old.property_val
when not matched then
insert (event_key, property_name, property_val)
values (old_event_key, 'PREVIOUS-STATUS', :old.property_val);
end;
And here's the table:
CREATE TABLE "CUST"."EVENT_PROPERTIES"
( "EVENT_KEY" VARCHAR2(20 BYTE) CONSTRAINT "NN_FLE_FLK" NOT NULL ENABLE,
"PROPERTY_NAME" VARCHAR2(20 BYTE) CONSTRAINT "NN_FLE_PN" NOT NULL ENABLE,
"PROPERTY_VAL" VARCHAR2(80 BYTE),
CONSTRAINT "PX_EVENT_PROPERTIES" PRIMARY KEY ("EVENT_KEY", "PROPERTY_NAME") DEFERRABLE
USING INDEX PCTFREE 10 INITRANS 2 MAXTRANS 255 COMPUTE STATISTICS
STORAGE(INITIAL 65536 NEXT 1048576 MINEXTENTS 1 MAXEXTENTS 2147483645
PCTINCREASE 0 FREELISTS 1 FREELIST GROUPS 1
BUFFER_POOL DEFAULT FLASH_CACHE DEFAULT CELL_FLASH_CACHE DEFAULT)
TABLESPACE "CUST_TS" ENABLE
) SEGMENT CREATION IMMEDIATE
PCTFREE 10 PCTUSED 40 INITRANS 1 MAXTRANS 255
NOCOMPRESS LOGGING
STORAGE(INITIAL 65536 NEXT 1048576 MINEXTENTS 1 MAXEXTENTS 2147483645
PCTINCREASE 0 FREELISTS 1 FREELIST GROUPS 1
BUFFER_POOL DEFAULT FLASH_CACHE DEFAULT CELL_FLASH_CACHE DEFAULT)
TABLESPACE "CUST_TS" ;
And here are the error messages:
ORA-04091: table CUST.EVENT_PROPERTIES is mutating, trigger/function may not see it
ORA-06512: at "CUST.REMEMBER_OLD_STATUS", line 5

Answer 1

You could use a compound trigger to do this, storing the old values in variables in a before each row section, and then merging in an after statement.
This assumes you'll only ever update one row at a time:
create or replace trigger remember_old_status
for update on event_properties
compound trigger
old_rec event_properties%rowtype;
before each row is
begin
if (:old.property_name = 'CURRENT-STATUS') then
old_rec.event_key := :old.event_key;
old_rec.property_name := :old.property_name;
old_rec.property_val := :old.property_val;
end if;
end before each row;
after statement is
begin
if (old_rec.property_name = 'CURRENT-STATUS') then
merge into event_properties eprop
using (
select old_rec.event_key as event_key,
'PREVIOUS-STATUS' as property_name,
old_rec.property_val as property_val
from dual
) dummy
on (eprop.event_key = dummy.event_key
and eprop.property_name = dummy.property_name)
when matched then
update set property_val = old_rec.property_val
when not matched then
insert (event_key, property_name, property_val)
values (dummy.event_key, dummy.property_name, dummy.property_val);
end if;
end after statement;
end remember_old_status;
/
Quick test:
insert into event_properties values('SOME_EVENT', 'CURRENT-STATUS', 'A');
1 row inserted.
update event_properties set property_val = 'B' where event_key = 'SOME_EVENT' and property_name = 'CURRENT-STATUS';
1 row updated.
select * from event_properties;
EVENT_KEY PROPERTY_NAME PROPERTY_VAL
-------------------- -------------------- --------------------------------------------------------------------------------
SOME_EVENT CURRENT-STATUS B
SOME_EVENT PREVIOUS-STATUS A
update event_properties set property_val = 'C' where event_key = 'SOME_EVENT' and property_name = 'CURRENT-STATUS';
1 row updated.
select * from event_properties;
EVENT_KEY PROPERTY_NAME PROPERTY_VAL
-------------------- -------------------- --------------------------------------------------------------------------------
SOME_EVENT CURRENT-STATUS C
SOME_EVENT PREVIOUS-STATUS B
If you want to deal with multiple updates on one statement then the before each row can populate a collection instead, and you can then use that in the after statement.
create or replace trigger remember_old_status
for update on event_properties
compound trigger
type t_type is table of event_properties%rowtype;
old_recs t_type := t_type();
before each row is
begin
if (:old.property_name = 'CURRENT-STATUS') then
old_recs.extend();
old_recs(old_recs.count).event_key := :old.event_key;
old_recs(old_recs.count).property_name := :old.property_name;
old_recs(old_recs.count).property_val := :old.property_val;
end if;
end before each row;
after statement is
begin
forall i in old_recs.first..old_recs.last
merge into event_properties eprop
using (
select old_recs(i).event_key as event_key,
'PREVIOUS-STATUS' as property_name,
old_recs(i).property_val as property_val
from dual
) dummy
on (eprop.event_key = dummy.event_key
and eprop.property_name = dummy.property_name)
when matched then
update set property_val = old_recs(i).property_val
when not matched then
insert (event_key, property_name, property_val)
values (dummy.event_key, dummy.property_name, dummy.property_val);
end after statement;
end remember_old_status;
/