Important Initialization parameters that affect the CBO

Important Initialization parameters that affect the CBO
Apart from generating statistics, the setup of the below mentioned parameters play a very crucial role in dictating how your system will work. Setting these will depend a lot on what kind of environment you want to create, Online, Batch processing, Data warehousing or a combination of more than one.
1) OPTIMIZER_MODE
This will decide the mode in which the optimizer engine should run in. Valid values are RULE, CHOOSE, ALL_ROWS, FIRST_ROWS (_n).
Optionally, the OPTIMIZER_MODE can be set to CHOOSE. This is kind of an intermediate option between RBO and CBO. In fact, it tries to run the query in either CBO or RBO depending on the availability or unavailability of statistics. Therefore, if the tables present in the query have statistics generated on them, CBO (ALL_ROWS only) is preferred or RBO is taken up.
e.g.: optimizer_mode = first_rowsFor an OLTP system, FIRST_ROWS would be the ideal option for fast response time. ALL_ROWS is meant for batch-oriented applications. Note, a plan that produces the first n rows with the fastest response time might not be an optimal plan if requirement is to obtain the entire result, so decide as per the need of the application

2) OPTIMIZER_INDEX_COST_ADJOptimizer_Index_Cost_Ad takes a value between 1 and 10000 (default 100). Setting this value makes the index more or less friendly to the optimizer. For example setting it to 50 tells the optimizer that an execution plan that uses index(es) is half as expensive as other execution plans that are not using them. The lower the value (less than 100), the less full table scan executions will take place in the system.
Use of indexes in OLTP is a crucial factor for deciding the response time, but sometimes, full table scans provide better throughput. Nested loop joins are influenced by this parameter value. Set this parameter to a value that is optimal for your application. I recommend set this parameter between 10 - 20 for OLTP and 50 for DSS Systems..
If you are having slow performance because the CBO first_rows optimizer mode is favoring too many full-table scans, you can reset 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 a "silver bullet" that can improve the performance of an entire database in cases where the database is OTLP and you have verified that the full-table scan costing is too low.

3) OPTIMIZER_INDEX_CACHING
This tells optimizer to favor nested loops and IN-list iterators over sort-merge and hash joins. The default value is 100 and makes nested loops and IN-list operator look less expensive and more appealing to Optimizer. The value basically indicates the percentage of index blocks that the optimizer should assume are present in cache. I recommend set this parameter to 85.

4) DB_FILE_MULTIBLOCK_READ_COUNT
This parameter determines the number of database blocks read in one input/output operation during a full table scan. The value set is used in computing the cost of full table scans. OLTP systems will not benefit by setting a high value for this parameter as the computed cost for full table scan execution plans would reduced. The maximum size is Operating system dependent.
e.g.: db_file_multiblock_read_count = 1 (may be set to a higher value)
5) SORT_AREA_SIZE
This parameter defines the maximum memory space allocated per user process that requires sorting of data and for insert and updates to bitmap indexes. Optimizer uses the set value for determining the cost of sorting in a query. This defaults to 64K. Normally a size of 64K to 1M is appropriate for OLTP systems.
Sort-merge joins are influenced by this parameter value. The bigger the size the more appealing will be sort-merge joins over nested-loop and hash joins. The lower the size the more sorting will take place in temporary segments. So assign a value that has been evaluated for your setup.
Oracle recommends the use of PGA_AGGREGATE_TARGET instead of this parameter from Oracle 9i.
6) SORT_MULTIBLOCK_READ_COUNT
This specifies the number of database blocks to be read each time a sort performs a read from a temporary segment. The default value is 2. General recommendation is to keep the default value. However, if you intend to hike up the SORT_AREA_SIZE considerably, you may evaluate the improvement by changing this parameter.
e.g.: sort_multiblock_read_count = 2
7) HASH_JOIN_ENABLED
Hash joins are available only in CBO. In hash joins, a hash table is created on the join key of the smallest sized table. It then joins the other tables to find the match. Hash joins may prove to be faster than other type of joins in some conditions, especially when the index is missing or search criteria is not very selective. Hash joins require a large amount of memory as the hash tables are retained; this may sometimes result in memory swapping.
Nested-loop joins return the first row faster than sort-merge and hash joins and are preferred for OLTP, but other types of joins cannot be ignored for running other aspects of the applications.
e.g.: hash_join_enabled = true
8) HASH_AREA_SIZE
This specifies the maximum amount of memory in bytes to be used for a hash join per process. It is defaulted to 2 times SORT_AREA_SIZE.
Oracle recommends the use of PGA_AGGREGATE_TARGET instead of this parameter from Oracle 9i.
e.g.: hash_area_size = 2097152
Setting this to a very low number may sometimes result in the following error.
ORA-6580: Hash Join ran out of memory while keeping large rows in memory.
9) HASH_MULTIBLOCK_IO_COUNT
This specifies how many sequential blocks a hash join reads and writes in one Input/Output activity. Oracle recommends not changing or assigning a value to this parameter; this will let oracle decide on the appropriate value for each individual query. In such casees, the value of the parameter will appear as 0 in the V$PARAMETER view.
This parameter is renamed to _HASH_MULTIBLOCK_IO_COUNT as of Oracle 9i.
e.g.: hash_multi_block_io_count = 0
10) BITMAP_MERGE_AREA_SIZE
This parameter is relevant for systems using bitmap indexes. It specifies the amount of memory Oracle uses to merge bitmaps retrieved from a range scan of a bitmap index. The default value is 1 MB, which is considered sufficient for most setups.
Oracle recommends use of PGA_AGGREGATE_TARGET instead of this parameter from Oracle 9i.
e.g.: bitmap_merge_area_size = 1048576

11) QUERY_REWRITE_ENABLED
This parameter is relevant for systems using Materialized views, Function based indexes and stored outlines. Setting this parameter enables query rewrite for the database. The materialized views should also be query rewrite enabled to allow the optimizer to redirect a query to it rather than the actual tables listed in the FROM clause. Query rewriting is internally done by the optimizer based on what results are expected and whether these are already present in materialized view form.
e.g.: query_rewrite_enabled = true

12) QUERY_REWRITE_INTEGRITY
This parameter determines the extent to which the optimizer must enforce query rewriting to Materialized views. This determines the accuracy of the query rewrite. It can be set to ENFORCED, TRUSTED or STALE_TOLERATED. ENFORCED option uses Materialized views if they contain fresh data, thus guarantying the integrity and consistency of data. TRUSTED uses relationships that have been defined in the system for rewriting the query. STALE_TOLERATED allows the use of relationships that are not enforced and may use stale data. For OLTP systems, ENFORCED would be the ideal setup, as reports need to be up-to-date.
e.g.: query_rewrite_integrity = enforced

13) ALWAYS_ANTI_JOIN
This parameter specifies the join method for anti-joins, for example when a NOT IN operator is present in your query. It can be set to NESTED_LOOPS, MERGE or HASH. It is defaulted to NESTED_LOOPS in Oracle 8i and CHOOSE in Oracle 9i.
This parameter is renamed to _ALWAYS_ANTI_JOIN as of Oracle 9i.
e.g.: always_anti_join = nested_loops
14) ALWAYS_SEMI_JOIN
This parameter specifies the join method for semi-joins. These types of joins are carried out by Optimizer after transforming a query. In such joins, duplicate values from the inner table are removed and then the type of join specified in the parameter is used to perform a semi-join. It can be set to NESTED_LOOPS, MERGE or HASH. In Oracle 8i, it is defaulted to STANDARD and in Oracle 9i it is defaulted to CHOOSE, to pick up an appropriate join.
This parameter is renamed to _ALWAYS_SEMI_JOIN as of Oracle 9i.
e.g.: always_semi_join = nested_loops
15) STAR_TRANSFORMATION_ENABLED
This specifies whether query transformation will be applied to star queries. It can be set to TRUE, FALSE or TEMP_DISABLE (transformation will take place but will not use temporary tables). I presently set it to FALSE due to some known issues of sub-optimal queries being generated. If you intend to use this, please upgrade your version to 8.1.7.4 and above.
e.g.: star_transformation_enabled = false
16) PARALLEL_BROADCAST_ENABLED
This parameter refers to parallel executions in cluster databases. It is meant for improving hash and sort-merge join operations where a very large result set is joined with a very small result set. When this option is enabled, the optimizer broadcasts a copy of all rows in the smaller result set to all cluster databases that are processing some rows of the larger result set.
It is obsolete in release 9.2.0.
e.g.: parallel_broadcast_enabled = false

17) OPTIMIZER_DYNAMIC_SAMPLING
This parameter is introduced in release 9i. It is meant for situations where tables are not analyzed. As CBO depends heavily on statistics, the parameter tells the optimizer to sample the unanalyzed tables that are being used in a query. A level of 0 to 10 can be specified, the higher the value the more time optimizer spends in sampling.
e.g.: optimizer_dynamic_sampling = 1

18) PARTITION_VIEW_ENABLED
This parameter is meant for backward compatibility to support partition views. Oracle recommends use of partition tables rather than partition views. If you are migrating to CBO, chances are that you may not be using partition views.
e.g.: partition_view_enabled = false

19) CURSOR_SHARING
This parameter determines what kind of SQL statements can share the same cursor. It can be set to FORCE, SIMILAR or EXACT. FORCE will try to squeeze statements that may differ in some literals to share the same cursor. SIMILAR is somewhat the same but will try to maintain the plan optimization for identical statements. EXACT allows statements with exact identical text to share a cursor.
Using FORCE may sometimes result in unexpected results.
e.g.: cursor_sharing = exact

20) PGA_AGGREGATE_TARGET
Introduced in Oracle 9i, this parameter specifies the aggregate PGA memory available to all server processes attached to an instance. This parameter can be set for automatic sizing of SQL working areas. It replaces other existing parameters like SORT_AREA_SIZE, BITMAP_MERGE_AREA_SIZE and HASH_AREA_SIZE.
It can be set to a value between 10 MB to 4000 GB, depending on the setup requirement. 1- Make a first estimate for PGA_AGGREGATE_TARGET  based on the following rule:
    PGA_AGGREGATE_TARGET  = (Total Physical Memory * 0.16)    - For OLTP systems  (16% of Server Physical Mem)
    PGA_AGGREGATE_TARGET  = (Total Physical Memory * 0 .4  )   - For DSS systems   (40% of Server Physical Mem)