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Don Burleson Blog 







Migrating to cost-based SQL optimization - Part 1

Oracle Tips by Burleson Consulting
January 31, 2003
Don Burleson

With a large number of Oracle shops using the rule-based optimizer (RBO), migration to cost-based optimization (CBO) will become a important task when migrating to Oracle10i, where the RBO will disappear.

As you may know, Oracle provides several parameters that can adjust the behavior of the CBO to make it more like rule-based optimization: 

  • Optimizer_index_caching

  • Optimizer_index_cost_adj

  • Optimizer_max_permutations

  • Optimizer_search_limit

Today we examine optimizer_index_caching, and we will cover the other parameters in later tips.  For more information on the CBO please refer to Oracle High-Performance SQL Tuning by Oracle Press.

The optimizer_index_caching parameter

The optimizer_index_caching parameter is a percentage parameter with valid values between zero and 100. This parameter lets you adjust the behavior of the cost-based optimizer to select nested loop joins more often or less often. The cost of executing a nested loop join where an index is used to access the inner table is highly dependent on the caching of that index in the buffer cache. The amount of index caching depends on factors, such as the load on the system and the block access patterns of different users, that the optimizer cannot predict. Of course, you may cache an index by placing the data block in the KEEP pool, thereby ensuring that the blocks are always cached.

Setting optimizer_index_caching to a higher percentage makes nested loop joins look less expensive to the optimizer, which will be more likely to pick nested loop joins over hash or sort merge joins.

The default value for the optimizer_index_caching parameter is 0, which gives the highest preference to hash joins and sort merge joins. Resetting this parameter can be very dangerous if you are not using stored outlines because it could change the execution plans for thousands of SQL statements. Also, because the cost-based optimizer will generally only invoke sort merge joins when there are no indexes on the joined tables, this parameter has the most effect on the invocation of hash joins.

According to the Oracle documentation "OPTIMIZER_INDEX_CACHING favors using selective indexes. That is, if you use a relatively low value for this parameter, the optimizer effectively models the caches of all non-leaf index blocks. In this case, the optimizer bases the cost of using this index primarily on the basis of its selectivity. Thus, by setting OPTIMIZER_INDEX_CACHING to a low value, you achieve the desired modeling of the index caching without over using possibly undesirable indexes that have poor selectivity."



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