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Status Not under consideration
Workspace Db2
Created by Guest
Created on Feb 10, 2015

new optimizer guideline to break CSE for a specific query or as a global guideline(RIMS)

Customer encountered scenario where use of CSE resulted in degradation in query performance (details below). This requirement is to implement a new optimizer guideline that will allow customer to break CSE for a specific query or as a global guideline. Wenbin: 'This behavior appears to be due to Common Sub Expression (CSE) functionality. The two Union branch queries both reference the view FDW.V_BSW_DERIVATIVES, and they both call F. F itself is a UNION view with two branches {F1, F2}. F1 is complex join under NLJOIN 12 (called F1), while F2 is simply a select from a single table FDW.V_BSW_DERIVATIVES_P3_2012. Now when compiling the UNION query, the optimizer makes FDW.V_BSW_DERIVATIVES a Common Cub Expression as TEMP 10. Our Query rewrite doesn't break that CSE as it is a UNION, this decision is because materialisation is generally good for performance in most cases. But when compiling query 2 alone, it's local predicates on F can eliminate branch F1 and leaves F2 to join with the outer two base tables.