Oracle Parameter optimizer_index_cost_adj Performance

From Oracle Documentation,

Default value 100
Modifiable ALTER SESSION, ALTER SYSTEM
Range of values 1 to 10000

OPTIMIZER_INDEX_COST_ADJ lets you tune optimizer behavior for access path selection to be more or less index friendly—that is, to make the optimizer more or less prone to selecting an index access path over a full table scan.

The default for this parameter is 100 percent, at which the optimizer evaluates index access paths at the regular cost. Any other value makes the optimizer evaluate the access path at that percentage of the regular cost. For example, a setting of 50 makes the index access path look half as expensive as normal.

The default value 100 and 0 for OPTIMIZER_INDEX_COST_ADJ and OPTIMIZER_INDEX_CACHING respectively are set for Data Warehouse System.

For OLTP system, setting 10 to OPTIMIZER_INDEX_COST_ADJ and 90 to OPTIMIZER_INDEX_CACHING will perform good result.

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Oracle Parameter db_block_checksum Performance

From Oracle Documentation,

Default value TYPICAL
Modifiable ALTER SYSTEM
Range of values OFF | TYPICAL | FULL

DB_BLOCK_CHECKSUM determines whether DBWn and the direct loader will calculate a checksum (a number calculated from all the bytes stored in the block) and store it in the cache header of every data block when writing it to disk. Checksums are verified when a block is read – only if this parameter is TYPICAL or FULL and the last write of the block stored a checksum. In FULL mode, Oracle also verifies the checksum before a change application from update/delete statements and recomputes it after the change is applied. In addition, Oracle gives every log block a checksum before writing it to the current log.

If this parameter is set to OFF, DBWn calculates checksums only for the SYSTEM tablespace, but not for user tablespaces.

Checksums allow Oracle to detect corruption caused by underlying disks, storage systems, or I/O systems. If set to FULL, DB_BLOCK_CHECKSUM also catches in-memory corruptions and stops them from making it to the disk. Turning on this feature in TYPICAL mode causes only an additional 1% to 2% overhead. In the FULL mode it causes 4% to 5% overhead. Oracle recommends that you set DB_BLOCK_CHECKSUM to TYPICAL. For backward compatibility we preserve use of TRUE (implying TYPICAL) and FALSE (implying OFF) values.

Note: If your application is I/O intensive and you are short on CPU capacity, then you might want to disable it.