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Oracle Technology Network
Cost Control: Inside the Oracle Optimizer

By Donald K. Burleson
OTN Member since 2001

Designing new applications for the Oracle Cost-Based Optimizer? Here's the latest information about how it works.

This article has the following sections:


External Costing with the Optimizer

Starting in Oracle9i Database and continuing in Oracle Database 10g, the optimizer has been enhanced to consider external influences when determining the best execution plan. Because the Oracle Database does not run in a vacuum, the optimizer must be able to factor-in the costs of external disk I/O and the cost of CPU cycles for each SQL operation. This process is especially critical for queries running all_rows   optimization, where minimizing server resources is a primary goal.

  • CPU_COST — The optimizer can now estimate the number of machine cycles required for an operation, and factors this cost into the execution plan calculation. The CPU costs associated with  servicing an Oracle query depends upon the current server configuration (which Oracle cannot see). In Oracle Database 10g, CPU costing is the default behavior because of the importance of considering the CPU costs associated with each SQL execution phase—thus, the savvy Oracle professional should turn on CPU costing with dbms_stats.get_system_stats. CPU costs are generally not important unless the entire Oracle instance is using excessive CPU resources.
  • IO_COST — The optimizer had been enhanced to estimate the number of physical block reads required for an operation. The I/O cost is proportional to the number of physical data blocks read by the operation. However, the optimizer has no a priori knowledge of the data buffer contents and cannot distinguish between a logical read (in-buffer) and a physical read. Because of this shortcoming, the optimizer cannot know if the data blocks are already in the RAM data buffers.

According to the Oracle documentation, the I/O and CPU costs are evaluated as follows:


Cost =  (#SRds * sreadtim + 
         #MRds * mreadtim + 
         #CPUCycles / cpuspeed ) / sreadtim


#SRDs - number of single block reads
#MRDs - number of multi block reads
#CPUCycles - number of CPU Cycles *)

sreadtim  - single block read time
mreadtim - multi block read time
cpuspeed  -  CPU cycles per second

Note that the costs are a function of the number of reads and the relative read times, plus the CPU cost estimate for the query. Also note the external costing does not consider the number of data blocks that reside in the RAM data buffers, but a future release of the optimizer is likely to consider this factor.

Here we see that Oracle uses the both the CPU and I/O cost estimations in evaluating the execution plans. This equation becomes even more complex when we factor-in parallel query where many concurrent processes are servicing the query.

The best benefit for using CPU costing is for all_rows execution plans where costs is more important than with first_rows optimization.

The all_rows optimizer mode is designed to minimize computing resources and it favors full-table scans.  Index access (first_rows) adds additional I/O overhead, but they return rows faster, back to the originating query:

Oracle full-table scan Illustration

Oracle Index access illustration

Next, let's look at how the optimizer is influenced by statistics. In order to make an intelligent decision about the best execution plan, the optimizer must use information about all of the data objects that are involved in the query. Because you control how the statistics are collected, this aspect of optimizer tuning is a critical.

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Donald K. Burleson [] is one of the world's most widely-read Oracle database experts. He has written 19 books, published more than 100 articles in national magazines, and serves as editor-in-chief of Oracle Internals, a leading Oracle database journal. Burleson's latest book is Creating a Self-Tuning Database by Rampant TechPress. Don's Web sites are , and .




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