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Oracle De-supports Rule-based SQL Optimizer

Oracle Tips by Burleson Consulting

Oracle has announced that with Oracle 11g, the rule-based optimizer (RBO) will no longer be supported, and you need to move quickly to migrate to the cost-based optimizer (CBO).

OPTIMIZER_MODE=RULE is not supported in Oracle Database 11g; the code has only been left in Oracle to enable easy migrations from RBO to CBO. We do not recommended you use OPTIMIZER_MODE=RULE as a long term strategy in Oracle Database 11g as the code can be removed at any time now that it is de-supported.

Migrating to the cost-based optimizer is tricky and dangerous, and you need to hire experts who have experience doing an RBO to CBO migration. 

Don't risk an unplanned outage.

For those experienced DBA's who wish to migrate away from rule-based optimization, here are some tips.

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 Oracle11g, 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. 

Important Note:  Prior to Oracle 10g, adjusting these optimizer parameters was the only way to compensate for sample size issues with dbms_stats.  As of 10g, the use of dbms_stats.gather_system_stats and improved sampling within dbms_stats had made adjustments to these parameters far less important.  Ceteris Parabus, always adjust CBO statistics before adjusting optimizer parms.  For more details on optimizer parameters, see my latest book "Oracle Tuning: The Definitive Reference". 


This is the most important parameter of all, and the default setting of 100 is incorrect for most Oracle systems.  For some OLTP systems, re-setting this parameter to a smaller value (between 10- to 30) may result in huge performance gains!

If you are having slow performance because the CBO first_rows optimizer mode is favoring too many full-table scans, you can reduce the value of the optimizer_index_cost_adj parameter to immediately tune all of the SQL in your database to favor index scans over full-table scans. This is sometimes a "silver bullet" that can improve the performance of an entire database in cases where the database is OLTP and you have verified that the full-table scan costing is too low.

Even in Oracle, the CBO sometimes falsely determines that the cost of full-table scan is less than the cost of an index access. The optimizer_index_cost_adj parameter is a great approach to whole-system SQL tuning, but you will need to evaluate the overall effect by slowly resetting the value down from 100 and observing the percentage of full-tale scans. You can also slowly bump down the value of optimizer_index_cost_adj when you bounce the database and then either use the access.sql or plan.sql scripts or reexamine SQL from the STATSPACK stats$sql_summary table to see the net effect of index scans on the whole database.

The plan.sql script (see code depot from book below) uses the v$sql_plan view and a quickly the reduction in sub-optimal, large-table full-table scans:

                          Full table scans and counts
          Note that "K" indicates in the table is in the KEEP pool.

OWNER          NAME                      NUM_ROWS  C K   BLOCKS  NBR_FTS
-------------- ------------------------  --------- - - -------- --------
SYS            DUAL                                N          2   97,237
SYSTEM         SQLPLUS_PRODUCT_PROFILE             N K        2   16,178
DONALD         PAGE                      3,450,209 N    932,120    9,999
DONALD         RWU_PAGE                        434 N          8    7,355
DONALD         PAGE_IMAGE                   18,067 N      1,104    5,368
DONALD         SUBSCRIPTION                    476 N K      192    2,087
DONALD         PRINT_PAGE_RANGE                 10 N K       32      874


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 Oracle, selective indexes are favored by optimizier_index_caching.  The result of using lower values for this parameter will be the optimizer effectively modeling the caches of non-leaf index blocks.

For this situation, the cost of using this index will be based mostly on its selectivity.  Using a lower valuse of optimizer_index_caching will result in an index caching model that is less likely to overuse potentially lesser desirable indexes with poorer selectivity.

Even though Oracle has deprecated the rule-based optimizer, Oracle continues to use the rule hint in Oracle 11g, as shown by this Data Pump internal SQL:

 Module: Data Pump Worker
'TABLE_T', '7')), KU$.OBJ_NUM ,KU$.ANC_OBJ.NAME  . . .  

More information is available on the following pages:

Get the Complete
Oracle SQL Tuning Information 

The landmark book "Advanced Oracle SQL Tuning  The Definitive Reference"  is filled with valuable information on Oracle SQL Tuning. This book includes scripts and tools to hypercharge Oracle 11g performance and you can buy it for 30% off directly from the publisher.



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