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Query Transformations

Learn about the power of query transformations in Oracle and how they can significantly improve query performance. Discover different types of transformations and their impact on query execution.

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Query Transformations

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  1. They Don’t Call it the Optimizer for Nothing Steve Catmull UTOUG Fall Symposium 2012 Query Transformations

  2. Agenda • What is a Query Transformation • The Case for Query Transformations • Example Transformations • Summary

  3. About the Examples • Transformations are typically cost sensitive. • I will explain the idea behind the transformation • I will show you what the pattern looks like in the explain plan. • Version: 11.2.0.3

  4. What is a Transformation • They are designed to not effect your query results. • The Optimizer thinks there is a better way to execute a query by rewriting it. • Sometimes the rewrite is significant • It is often very clever • Mathematically sound • Adds/Removes/Changes operations

  5. The Case for Transformation • Oracle presented transformation techniques at VLDB in 2006. Their tests showed significant improvement.

  6. Types of Transformation • Cost-Based • The proposed transformation is added to the set of candidate plans it considers. • Those plans are costed and one is selected. • Rule-Based • When certain conditions exist, the optimization is always desirable. • ORDER BY Elimination • Join Elimination Source: Cost-Based Query Transformation in Oracle, VLDB 2006, Oracle USA Presentation

  7. Rule Based Transformations

  8. ORDER BY Elimination

  9. Join Elimination • Why join two tables if you don’t have to?

  10. Join Elimination • Change query to return only customer columns. • Why join to COUNTRIES?

  11. Join Elimination DDL • Primary / foreign key relationship between the tables • ENABALED / VALIDATE • DISABLED / RELY • Single column key Source: http://optimizermagic.blogspot.com/2008/06/why-are-some-of-tables-in-my-query.html

  12. Cost-Based Transformations

  13. SubqueryUnnesting

  14. Subquery Pseudo Code • For every row in CUSTOMERS • Probe into SALES for sales by that customer ID who had >=2 quantity in the transaction.

  15. Un-Nesting Requirements • No HAVING in subquery • Subquery must be during or before WHERE clause in order of operations. • ….

  16. OR Expansion Source: https://blogs.oracle.com/optimizer/entry/or_expansion_transformation

  17. OR Expansion Notes • Sensitive to cardinalities / statistics • Enables two different NORMAL indexes on the same table to be accessed in the same SQL statement. Fancy trick. • Sightings in the wild depend a lot of your data and query patterns.

  18. Complex View Merging Source: http://jonathanlewis.wordpress.com/2007/03/08/transformation-and-optimisation/

  19. Complex View Merging Fail

  20. Predicate Pushdown

  21. Join Predicate Pushdown • Opens up joining to a view using nested loop index methods. • “Normally, a view cannot be joined with an index-based nested loop join” Source: https://blogs.oracle.com/optimizer/entry/basics_of_join_predicate_pushdown_in_oracle

  22. View Join Plan • Executes view independent of results of CUSTOMERS table.

  23. View Join Plan (Pushdown) • Each row in CUSTOMERS, send the CUST_ID to step 6 and execute view.

  24. Pushdown Takeaways • Tight correlation with nested loops. • Let it happen naturally. • Errant cardinality estimates may cause this to be used. Ways to correct • Use hint • Simplify query • More clever than it looks. • Shows up frequently in DSS systems.

  25. Coalescing Subqueries (ST)

  26. Coalescing Subqueries Plan • Transformed 2 exists into a single OR. • Only 1 hit against CUSTOMERS. • Un-nested query into a hash join • Added a predicate from constraints

  27. Materialized View Rewrite • You have users often group on common columns on a large table.

  28. MV Rewrite (baseline) selectprod_id, promo_id, count(distinct cust_id) AScust_id_cnt, sum(amount_sold) ASsold_amt fromsh.sales group byprod_id, promo_id

  29. Building an MV • If we build a materialized view to help speed up this query…

  30. MV Rewrite (MVScript) CREATEMATERIALIZEDVIEW SH.SALES_SMRY1_MV BUILDIMMEDIATE REFRESHCOMPLETE ENABLEQUERYREWRITE AS select prod_id,promo_id, count(distinctcust_id)AScust_id_cnt, sum(amount_sold)ASsold_amt from sh.sales groupby prod_id,promo_id

  31. MV Rewrite – Another Query `

  32. MV Rewrite Requirements

  33. Temp Transformation

  34. A Future Glimpse (uncorrelated subsumed subquery) • At 2009 VLDB, Oracle submitted a technique called uncorrelated subsumed subquery.

  35. Known Transformation List

  36. Summary • Oracle is adding more and more transformation rules • For complex DSS environments, transformers are you friend. It doesn’t mean you don’t fight. • Constraints can enable some transformations even when disabled and not validated.

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