java.sql.BatchUpdateException: ORA-04091 on BEFORE INSERT TRIGGER - oracle

Question

I'm getting a curious error on a BEFORE INSERT TRIGGER, which I can't understand. Even after reading multiple questions posted here with similar problems.
failed to process "method": category_id = 'foo' and request_id = '99'
error: java.sql.BatchUpdateException: ORA-04091: table SCHEMA.ANIMAL_TABLE
is mutating, trigger/function may not see it ORA-06512: at
"SCHEMA.TRIGGER_NAME", line 7 ORA-04088: error during execution of
trigger 'SCHEMA.TRIGGER_NAME'
Here is the trigger:
CREATE OR REPLACE TRIGGER TRIGGER_NAME
BEFORE INSERT ON animal_table FOR EACH ROW WHEN (NEW.animal_type = 'cats')
DECLARE base_animal_id NUMBER(19,0); base_amount NUMBER(19,0);
BEGIN
SELECT animal_nbr INTO base_animal_id
FROM animal_table
WHERE category_id = :NEW.category_id AND summary_id = :NEW.summary_id
AND animal_type = 'special';
SELECT animal_amount INTO base_amount
FROM animal_table
WHERE category_id = :NEW.category_id AND summary_id = :NEW.summary_id
AND animal_type = 'special';
IF :NEW.category_id = 'foo' THEN
:NEW.animal_info1 := base_animal_id;
:NEW.animal_info2 := base_amount;
:NEW.animal_info3 := '00';
END IF;
END;
I know the rules regarding modifications on the same table which the trigger is being held, but I also red something that it should work when changing new columns and only for the :NEW fields. I also thought it may be missing the UPDATE as trigger event, but that was not the case. Can anyone help me please? As I am new to triggers and PL/SQL.

Answer 1

The error message has nothing to do with updating the table. You cannot SELECT from the table that is currently being changed in a ROW level trigger.
The only workaround for this is to write the new rows into a intermediate table in the row level trigger. Then create a statement level trigger that processes all rows that have been written into the intermediate table (most probably only a single UPDATE statement with a subselect).
You might get away without the row level trigger and the intermediate table if you can identify the rows to be post-processed inside the statement level trigger e.g. by checking animal_type = 'cats' and category_id = 'foo'.
If that is the case, the following trigger (untested!!!) might do what you want (instead of the one you have)
CREATE OR REPLACE TRIGGER TRIGGER_NAME
AFTER INSERT ON animal_table
BEGIN
UPDATE animal_table
SET (animal_info1,
animal_info2,
animal_info3) = (SELECT animal_nbr, animal_amount, '00'
FROM animal_table t2
WHERE t2.category_id = animal_table.category_id
AND t2.sumary_id = animal_table.summary_id
AND t2.animal_type = 'special'
)
WHERE animal_type = 'cats'
AND category_id = 'foo'
END;
Another more general PL/SQL thing: You don't need to run one SELECT for each column you want to retrieve, you can do that in a single select statement if the conditions are the same:
SELECT animal_nbr, animal_amount
INTO base_animal_id, base_amount
FROM animal_table
WHERE category_id = :NEW.category_id
AND summary_id = :NEW.summary_id
AND animal_type = 'special';
(Note the two columns in the select and into list)