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Oracle optimizer costing equations

Oracle Tips by Burleson Consulting
July 2, 2009

The Oracle cost-based optimizer displays cost figures, but these are very misleading because a low cost value does not always indicate the "real" lowest cost that is select by the optimizer.  In other words, Oracle does not always choose the lowest "cost" value, and Oracle does not disclose the exact behavior of their optimizer, a prized competitive edge over other competing databases.

While Oracle does not publish the internal machinations of their proprietary optimizer costing model, many Oracle experts have reverse engineered the software and developed equations for estimating the cost.

Warning about the "cost" display:  It’s important to note that the cost figures that are displayed in an execution plan are not described in any Oracle documentation, and it’s been demonstrated that the plan with the lowest “cost” number is not always the plan chosen by the optimizer.  Further, the “cost” figures do not always indicate the “best:” execution plan for a query, given the divergent optimizer goals of first_rows (optimizer for response time) and all_rows (optimize for minimizing computing resources).

Given  my warnings, there are many experts who have attempted to reverse engineer the costing algorithms, with mixed success.

 Vivek Sharma has this note where he described tuning a difficult query by adjusting the optimizer_index_cost_adj parameter for a specific user.  Sharma notes that the “best” action (changing the query with a hint) was impossible because of code access limitations.  This limitation of "you cannot changed the SQL" is common with seeded ad-hoc SQL or with vendor packages. 

In this case, he used a login trigger to set optimizer_index_cost_adj to a higher value, thereby increasing the probability that the Oracle optimizer would choose a full scan operation.

This Oracle 8i database, Mr. Sharma applies costing equations for various execution plans.  Here he notes the costs of calculating index scan costs and table scan costs:

The Cost Calculation of an Index Scan and Table Scan via Index is:

- Index Cost = Blevel+ceil(leaf_blocks*ix_selectivity)

- Table Scan cost = ceil(clustering_factor*table_selectivity)

- Nested loops Cost = Cost of an Outer Table + (Cost of Inner Table * Cardinality of Outer Table)

- Nested Loops Table Scan Cost = (Current Cost of Inner Table Scan * Outer Table Card)*OICA/100

Again, beware that Oracle does not publishing their costing internals, and that the "cost" values in an execution plan are misleading because they do not always reflect the "real" best access plan for an SQL statement.


 

 

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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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