Gathering SQL execution details

In addition to TKPROF (SQL*Trace) and the SQL*Plus "set autotrace on" command, we see new tools for getting additional SQL execution information.

Staring with Oracle9i, we see several exciting new execution plan columns and you can ruin specialized queries to see estimates of resource usage associated with specific steps of Oracle SQL execution: 

• CPU_COST - The CPU cost of the operation as estimated by the optimizer's cost-based approach. For statements that use the rule-based approach, this column is null. The value of this column is proportional to the number of machine cycles required for the operation. 

• IO_COST - The I/O cost of the operation as estimated by the optimizer's cost-based approach. For statements that use the rule-based approach, this column is null. The value of this column is proportional to the number of data blocks read by the operation  

• TEMP_SPACE - The temporary space, in bytes, used by the operation as estimated by the optimizer's cost-based approach. For statements that use the rule-based approach, or for operations that don't use any temporary space, this column is null.

In Oracle 10g, also see gather_plan_statistics hint tips where you can gather estimated rows vs. actual rows.  Oracle claims to use this information to make more intelligent choices of execution plans, but there is a serious problem with this approach. 

1 – The CBO has no a-priori knowledge of the data buffer contents – Because of this shortcoming, the CBO cannot know if the data blocks are already in the RAM data buffers.

2 – CPU costs depend upon system load – The CPU costs associated with servicing an Oracle query depend upon the server load, and CPU costs are generally not important unless the entire Oracle instance is using excessive CPU resources.

 3 – Temporary segment space is transient – Even though the CBO estimates the TEMP_SPACE costs at optimization time, this execution plan will become permanent for the SQL query until the executable is flushed from the library cache.  Hence, obsolete execution plans may be used to execute a query.

Oracle also has two new views to show active work area space, the v$sql_workarea and the v$sql_workarea_active views.  The v$sql_workarea_active view will display all of the work areas that are currently executing in the instance.  Note that small sorts (under 65,535 bytes) are excluded from the view. The v$sql_workarea_active view can be used to quickly monitor the size of all large active work areas.