PL/SQL Trigger to update another table only upon INSERT of specific record on one table

Question

Referred this answer that update triggers on every insert on tableA.

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Scenario is,

In tableA, we have WATERMARK_FLAG column of type CHAR(1) with possible values Y or N

In tableB, we have PROCESSED_FLAG column of type CHAR(1) with possible values Y or N

There is no primary key - foreign key relation between two tables

Primary key columns are same in both tables

PK(col_1, col_3, col_4)

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On INSERT of record in tableA with WATERMARK_FLAG as Y update PROCESSED_FLAG with Y in all rows of tableB

How to provide PL/SQL trigger syntax for specific INSERT?

Answer 1

Would be this one

CREATE OR REPLACE TRIGGER bi_tableA
  BEFORE INSERT ON bi_tableA
  FOR EACH ROW
  WHEN (WATERMARK_FLAG = 'Y')
DECLARE
BEGIN
  UPDATE tableB
  SET PROCESSED_FLAG = 'Y';
END bi_tableA;
/

However, the performance could decrease. But I assume this is just an exercise.