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Don Burleson Blog 







Oracle SQL Tuning Advisor

Oracle Tips by Burleson Consulting


Before Oracle10g, it was extremely difficult to track index usage and see how SQL statements behaved except when they were in the library cache.  With Oracle10g, it is now possible to track SQL behavior over time and ensure that all SQL is using an optimal execution plan, and Oracle10g provides the ability to track SQL execution metrics with new dba_hist tables, most notably dba_hist_sqlstat and dba_hist_sql_plan.

It is important to track the relationship between database objects, such as tables, indexes, and the SQL that accesses the objects.

Oracle SQL execution plans for any given statement may change if the system statistics change, dynamic sampling is used, materialized views are created, or indexes are created or dropped. 

The SQL Tuning Advisor

The SQL Tuning Advisor (STA) works with the Automatic Tuning Optimizer (ATO) to analyze historical SQL workload using data from the AWR, and it generates recommendations for new indexes and materialized views that will reduce the disk I/O associated with troublesome SQL statements.

The STA is primarily designed to replace the manual tuning of SQL statements and speed up the overall SQL tuning process. The SQL Tuning Advisor studies poorly executing SQL statements and evaluates resource consumption in terms of CPU, I/O, and temporary space.

The advisor receives one or more SQL statements as input and provides advice on how to optimize their execution plans, gives the rationale for the advice, the estimated performance benefit, and the actual command to implement the advice.

The STA can be thought of as a container for conducting and analyzing many tuning tasks. It calls the optimizer internally and performs the analysis as follows:

  • Executes the stale or missing statistics analysis and makes a recommendation to collect, if necessary.
  • Plans the tuning analysis and creates a SQL Profile. The SQL Profile is a collection of the historical information of prior runs of the SQL statement, comparison details of the actual and estimated cardinality, and predicate selectivity, etc. SQL Profile is stored persistently in the data dictionary, so it does not require any application code changes.
  • Performs the access path analysis. The Optimizer recommends new indexes that produce a significantly faster execution path.
  • Restructure the SQL statement. Optimizer identifies SQL statements that have bad plans and makes relevant suggestions to restructure them.

The plan analysis mode, which creates the SQL Profiles, is a significant stage where additional information for the query is collected by the optimizer. This analysis is not possible in the normal mode.

Such a SQL profile helps generate a better execution plan than the normal optimization. Additional tasks like checking for advanced predicate selectivity, correlation between columns, join skews, and complex predicates such as functions, help in profiling the SQL statement. Once a statement is profiled and stored, it can be used at will.

The Ion tool is also excellent for identifying SQL to tune and it can show SQL execution over time with stunning SQL graphics.

Get the Complete
Oracle SQL Tuning Information 

The landmark book "Advanced Oracle SQL Tuning  The Definitive Reference"  is filled with valuable information on Oracle SQL Tuning. This book includes scripts and tools to hypercharge Oracle 11g performance and you can buy it for 30% off directly from the publisher.




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Note: This Oracle documentation was created as a support and Oracle training reference for use by our DBA performance tuning consulting professionals.  Feel free to ask questions on our Oracle forum.

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