| Property | Description |
|---|---|
| Parameter type | String |
| Syntax | DB_BLOCK_CHECKING = { FALSE | OFF | LOW | MEDIUM | TRUE | FULL } |
| Default value | FALSE |
| Modifiable | ALTER SESSION, ALTER SYSTEM |
| Basic | No |
DB_BLOCK_CHECKING specifies whether or not Oracle performs block checking for database blocks.
Values:
OFF or FALSE
No block checking is performed for blocks in user tablespaces. However, semantic block checking for SYSTEM tablespace blocks is always turned on.
LOW
Basic block header checks are performed after block contents change in memory (for example, after UPDATE or INSERT statements, on-disk reads, or inter-instance block transfers in Oracle RAC).
MEDIUM
All LOW checks are performed, as well as semantic block checking for all non-index-organized table blocks.
FULL or TRUE
All LOW and MEDIUM checks are performed, as well as semantic checks for index blocks (that is, blocks of subordinate objects that can actually be dropped and reconstructed when faced with corruption).
Oracle checks a block by going through the data in the block, making sure it is logically self-consistent. Block checking can often prevent memory and data corruption. Block checking typically causes 1% to 10% overhead, depending on workload and the parameter value. The more updates or inserts in a workload, the more expensive it is to turn on block checking. You should set DB_BLOCK_CHECKING to FULL if the performance overhead is acceptable.
For backward compatibility the use of FALSE (implying OFF) and TRUE (implying FULL) is preserved.