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Union all View (or common table expression of a UNION ALL) lead to HSJOIN
Worked with developer and provided following explanation: When there is a join between a UNION ALL view (or a common table expression of a UNION ALL) to another input (let's named it Z), to be able to utilize index access on the tables under the UNION ALL from the join columns (in this case, ACNT_ID from the tables SMB.MBA_TRDG_YEAR_xxxx), it is required that the input Z and the join predicates are rewritten to join with the tables directly. The most common use cases is when Z is a single table. In this reported query, Z is derived from a GROUP BY operation, hence it is not rewritten to join with the tables under UNION ALL view directly. Consequently, the join columns are not used to probe into the indexes, although index access method is chosen. There are reasons Db2 does not rewrite when Z is a GROUP BY or any other operations. One is because those operations are need to re-computed for the join of each table under the UNION ALL, which could end up more costly than the original form.
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