Best practice for updating column specific triggers - oracle

Question

Welcome Oracle pro's
In an Oracle 12 database (upgrade is already scheduled ;-)) we have a setup of different tables updating a common base table via "after update" triggers like following:
Search_Flat
ID
Field_A
Field_B
Field_C
Now table1 contains n columns where let's say 2 out of n are relevant for the Search_Flat table. As the update of table1 may only affect columns not relevant for Seach_Flat we want to add checks to the trigger. So our first approach is like following:
CREATE OR REPLACE TRIGGER tr_tbl_1_au_search
AFTER UPDATE OF
field_a,
field_b
ON schemauser.search_flat
FOR EACH ROW
BEGIN
IF :new.field_a <> :old.field_a THEN
UPDATE schemauser.search_flat SET field_a = :new.field_a WHERE id = :new.ID;
END IF;
IF :new.field_b <> :old.field_b THEN
UPDATE schemauser.search_flat SET field_b = :new.field_b WHERE id = :new.ID;
END IF;
END;
Alternatively we could also setup the trigger like following:
CREATE OR REPLACE TRIGGER tr_tbl_1_au_search
AFTER UPDATE OF
field_a,
field_b
ON schemauser.search_flat
FOR EACH ROW
BEGIN
IF :new.field_a <> :old.field_a OR :new.field_b <> :old.field_b THEN
UPDATE schemauser.search_flat
SET field_a = :new.field_a,
field_b = :new.field_b
WHERE id = :new.ID;
END IF;
END;
The question now is about the setup of the triggers themselves. Which approach is the better with respect to:
locking time of search_flat rows
overall performance of affected components (i.e., table_1, trigger and search_flat)
In production we are talking about 4 tables with 10 fields each considered in the triggers. And we have independent app servers accessing the shared database updating the 4 tables simultaneously. From time to time we detect the following error which is the reason we wan't to optimize the triggers:
ORA-02049: timeout: distributed transaction waiting for lock
Sidenote: This setup has been chosen instead of a view or materialized view due to performance reasons as the base table is used in gui with the requirement to be instantly updated and the number of records of the 4 feeding tables are too high for updating materialized view on update.
I'm looking forward to the discussion and your thoughts.

Answer 1

As I understand your post, you have 4 live tables (called "table1", "table2", etc.) that you want to search on, but querying from them is too slow, so you want to maintain a single, flattened table to search on instead and have triggers to keep that flattened table always up-to-date.
You want to know which of two trigger approaches is better.
I think the answer is "neither", since both are prone to deadlocks. Imagine this scenario
User 1 -
UPDATE table1
SET field_a = 500
WHERE <condition effecting 200 distinct IDs>
User 2 at about the same time -
UPDATE table1
SET field_b = 700
WHERE <condition effecting 200 distinct IDs>
Triggers start processing. You cannot control the order in which the rows are updated. Maybe it goes like this:
User 1's trigger, time index 100 ->
UPDATE search_flat SET field_a = 500 WHERE id = 90;
User 2's trigger, time index 101 ->
UPDATE search_flat SET field_b = 700 WHERE id = 91;
User 1's trigger, time index 102 ->
UPDATE search_flat SET field_a = 500 WHERE id = 91; (waits on user 2's session)
User 2's trigger, time index 103 ->
UPDATE search_flat SET field_b = 700 WHERE id = 90; (deadlock error)
User 2's original update fails and rolls back.
You have multiple concurrent processes all updating the same set of rows in search_flat with no control over the processing order. That is a recipe for deadlocks.
If you wanted to do this safely, you should consider neither of the FOR EACH ROW trigger approaches you outlines. Rather, make a compound trigger to do this.
Here's some sample code to illustrate the idea. Be sure to read the comments.
-- Aside: consider setting this at the system level if on 12.2 or later
-- alter system set temp_undo_enabled=false;
CREATE GLOBAL TEMPORARY TABLE table1_updates_gtt (
id NUMBER,
field_a VARCHAR2(80),
field_b VARCHAR2(80)
) ON COMMIT DELETE ROWS;
CREATE GLOBAL TEMPORARY TABLE table2_updates_gtt (
id NUMBER,
field_a VARCHAR2(80)
) ON COMMIT DELETE ROWS;
-- .. so on for table3 and 4.
CREATE OR REPLACE TRIGGER table1_search_maint_trg
FOR INSERT OR UPDATE OR DELETE ON table1 -- with similar compound triggers for table2, 3, 4.
COMPOUND TRIGGER
AFTER EACH ROW IS
BEGIN
-- Update the table-1 specific GTT with the changes.
CASE WHEN INSERTING OR UPDATING THEN
-- Assumes ID is immutable primary key
INSERT INTO table1_updates_gtt (id, field_a) VALUES (:new.id, :new.field_a);
WHEN DELETING THEN
INSERT INTO table1_updates_gtt (id, field_a) VALUES (:old.id, null); -- or figure out what you want to do about deletes.
END CASE;
END AFTER EACH ROW;
AFTER STATEMENT IS
BEGIN
-- Write the data from the GTT to the search_flat table.
-- NOTE: The ORDER BY in the next line is what saves us from deadlocks.
FOR r IN ( SELECT id, field_a, field_b FROM table1_updates_gtt ORDER BY id ) LOOP
-- TODO: replace with BULK processing for better performance, if DMLs can affect a lot of rows
UPDATE search_flat sf
SET sf.field_a = r.field_a,
sf.field_b = r.field_b
WHERE sf.id = r.id
AND ( sf.field_a <> r.field_a
OR (sf.field_a IS NULL AND r.field_a IS NOT NULL)
OR (sf.field_a IS NOT NULL AND r.field_a IS NULL)
OR sf.field_b <> r.field_b
OR (sf.field_b IS NULL AND r.field_b IS NOT NULL)
OR (sf.field_b IS NOT NULL AND r.field_b IS NULL)
);
END LOOP;
END AFTER STATEMENT;
END table1_search_maint_trg;
Also, as numerous commenters have pointed out, it's probably better to use a materialized view for this. If you are on 12.2 or later, real-time materialized views (aka "ENABLE ON QUERY COMPUTATION") offer a lot of promise for this sort of thing. No COMMIT overhead to your application and real-time search results. It's just that search time degrades slightly if there are a lot of recent updates to the underlying tables.