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Oracle SQL tuning transformations

Oracle11g Tips by Burleson Consulting

December 16, 2007

The Oracle cost-based optimizer tries to improve its ability to re-write and transform sub-optimal SQL query forms.  While most Oracle professionals are aware how the CBO will perform query transformations to point to materialized views, replacing an ?order by? sort with index sort retrieval, and eliminating certain subqueries (by replacing them with standard joins and NOT NULL tests), the optimizer continues to advance in its ability to transform query syntax into ever more efficient forms.

Lillian Hobbs recently gave a presentation on SQL Tuning, and noted these type of query transformations that are used in SQL Tuning:

  • CNT:  count(col) to count(*) transformation
  • CVM:   complex view merging
  • FPD ? filter push-down
  • JE:  Join Elimination
  • JF: join factorization
  • JPPD: join predicate push-down
  • OBYE:  order by elimination
  • PM:  predicate move-around
  • SPJ: select, project, join
  • SU: subquery unnesting

Under subquery un-nesting we see these common applications:

  • Transforming EXISTS subquery to a join.

  • Transform ALL/NOT EXISTS subquery into a regular anti-join.

These are the abbreviations that are used within an optimizer trace file to indicate the type of optimizer transformation.  For example, an optimizer trace file might refer to a join elimination thusly:

JE: eliminate table: DEPARTMENTS

New 11g SQL transformations

Ms. Hobbs also listed these new SQL query transformations within Oracle11g. 

11g CBQT - cost-based query transformation
11g JPPD
- join predicate push-down
11g CNT - count(col) to count(*) transformation 
11g JE - Join Elimination 

In addition, she listed these types of CBO transformations from earlier release of Oracle:

OJPPD - old-style (non-cost-based) JPPD
FPD - filter push-down
PM - predicate move-around
CVM - complex view merging
SPJ - select-project-join
SJC - set join conversion
SU - subquery unnesting
OBYE - order by elimination
ST - star transformation


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