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AWR and SQL Tuning


Oracle Tips by Burleson Consulting

AWR and SQL Tuning

Just as the v$sql_plan  view revolutionized Oracle9i tuning, the new Oracle10g AWR tables have revolutionized SQL tuning.  Most Oracle experts note that SQL optimization is one of the most important factors in database tuning, yet the transient nature of SQL execution has made it difficult to see the impact of SQL execution over time.  All of this changed with Oracle10g.

The AWR tables contain useful information about the time-series execution plans for SQL statements, and this repository can be used to display details about the frequency of usage for table and indexes. 

The following basic relationships between database objects and SQL statements are important:

       Each SQL statement may generate many access plans: From the information on dynamic sampling and dbms_stats, the execution plans for SQL statements will change over time to accommodate changes in the data they access.  It is important to understand how and when a frequently executed SQL statement changes its access plan.

       Each object is access by many access plans: In most OLTP systems, tables and indexes show repeating patterns of usage and clear patterns can be detected when averaging object access of day-of-the-week and hour-of-the-day.

 

Figure 15.18: Time-series relationships between SQL and database objects

 

Figure 15.18 shows that there is a many-to-many relationship between any given SQL statement and the tables they access.  Once this fundamental relationship is clear, the AWR tables can be used to perform time-based SQL tuning.  We have the following AWR tables for SQL tuning as shown in 15.19:

       dba_hist_sqlstat

       dba_hist_sql_summary

       dba_hist_sql_workarea

       dba_hist_sql_plan

       dba_hist_sql_workarea_histogram

 

Figure 15.19: The dba_hist views for SQL tuning

These simple tables represent a revolution in Oracle SQL tuning, and time-series techniques can be employed to optimize SQL with better results than ever before.  The following section provides a closer look at these AWR tables and the amazing details that they can provide about SQL execution over time.

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

 
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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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