How to access full OLD data in SQL Trigger - sql

Question

I have a trigger whose purpose is to fire whenever there is a DELETE on a particular table and insert the deleted data into another table in json format.
The trigger works fine if I am specifying each column explicitly. Is there any way to access the entire table row?
This is my code.
TRIGGER1
AFTER DELETE
ON QUESTION
FOR EACH ROW
DECLARE
json_doc CLOB;
BEGIN
select json_arrayagg (
json_object ('code' VALUE :old.id,
'name' VALUE :old.text,
'description' VALUE :old.text) returning clob
) into json_doc
from dual;
PROCEDURE1(json_doc);
END;
This works fine. However, what I want is something like this. Instead of explicity specifying each column, I want to convert the entire :OLD data
TRIGGER1
AFTER DELETE
ON QUESTION
FOR EACH ROW
DECLARE
json_doc CLOB;
BEGIN
select json_arrayagg (
json_object (:old) returning clob
) into json_doc
from dual;
PROCEDURE1(json_doc);
END;
Any suggestion please.

Answer 1

The short and correct answer is you can't. We have a few tables in our application where we do this and the developer is responsible for updating the trigger when they add a column: this is enforced with code reviews and is probably the cleanest solution for this scenario.
The long answer is you can get close, but I wouldn't do this in production for several reasons:
Triggers are terrible for performance
Triggers are terrible for code clarity
This requires reading the row again using flashback query so
You aren't getting the values of this row from inside your current transaction: if you update the row in your transaction and then delete it the JSON will show what the values were BEFORE your update
There is a performance penalty for reading from UNDO
There is potential that UNDO won't be available and your trigger will fail
Your user needs permission to execute flashback queries
Your database needs to meet all the perquisites to support flashback queries
Deleting a lot of rows will cause the ROWID collection to get large and consume PGA
There are probably more reasons, but in the interest of "can it be done" here you go...
DROP TABLE t1;
DROP TABLE t2;
DROP TRIGGER t1_ad;
CREATE TABLE t1 (
id NUMBER,
name VARCHAR2(100),
description VARCHAR2(100)
);
CREATE TABLE t2 (
dt TIMESTAMP(9),
json_data CLOB
);
INSERT INTO t1 VALUES (1, 'A','aaaa');
INSERT INTO t1 VALUES (2, 'B','bbbb');
INSERT INTO t1 VALUES (3, 'C','cccc');
INSERT INTO t1 VALUES (4, 'D','dddd');
CREATE OR REPLACE TRIGGER t1_ad
FOR DELETE ON t1
COMPOUND TRIGGER
TYPE t_rowid_tab IS TABLE OF ROWID;
v_rowid_tab t_rowid_tab := t_rowid_tab();
AFTER EACH ROW IS
BEGIN
v_rowid_tab.extend;
v_rowid_tab(v_rowid_tab.last) := :old.rowid;
END AFTER EACH ROW;
AFTER STATEMENT IS
v_scn v$database.current_scn := dbms_flashback.get_system_change_number;
v_json_data CLOB;
v_sql CLOB;
BEGIN
FOR i IN 1 .. v_rowid_tab.count
LOOP
SELECT 'SELECT json_arrayagg(json_object(' ||
listagg('''' || lower(t.column_name) || ''' VALUE ' ||
lower(t.column_name),
', ') within GROUP(ORDER BY t.column_id) || ') RETURNING CLOB) FROM t1 AS OF SCN :scn WHERE rowid = :r'
INTO v_sql
FROM user_tab_columns t
WHERE t.table_name = 'T1';
EXECUTE IMMEDIATE v_sql
INTO v_json_data
USING v_scn, v_rowid_tab(i);
INSERT INTO t2
VALUES
(current_timestamp,
v_json_data);
END LOOP;
END AFTER STATEMENT;
END t1_ad;
/
UPDATE t1
SET NAME = 'zzzz' -- not captured
WHERE id = 2;
DELETE FROM t1 WHERE id < 3;
SELECT *
FROM t2;
-- 13-NOV-20 01.08.15.955426000 PM [{"id":1,"name":"A","description":"aaaa"}]
-- 13-NOV-20 01.08.15.969755000 PM [{"id":2,"name":"B","description":"bbbb"}]