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Observation: Correlated Subquery uses EXISTS or NOT EXISTS
You coded a subquery using EXISTS or NOT EXISTS predicates. If the subquery is a correlated subquery, the Db2 optimizer will not consider access through an available index.
The following example includes a correlated subquery using EXISTS:
SELECT * A.empno, A.lastname, A.firstname
FROM T1 A
WHERE EXISTS
(SELECT * FROM T2 B
WHERE B.empno = A.empno);
Consider recoding the correlated subquery as a JOIN and referencing any indexed columns:
SELECT A.empno, A.lastname, A.firstname FROM T1 A, T2 B
WHERE A.empno = B.empno;