157.The DB_BLOCK_CHECKING initialization parameter is set to OFF. Which block checking would be performed?

A. The Oracle database will perform block checking for the index blocks only

B. The Oracle database will not perform block checking for any of the data blocks

C. The Oracle database will perform block checking for the default permanent tablespace only

D. The Oracle database will perform block checking for the data blocks in all user tablespaces

E. The Oracle database will perform block checking for the data blocks in the SYSTEM tablespace only

Answer: E

答案解析：

参考：http://docs.oracle.com/cd/E11882_01/server.112/e40402/initparams048.htm#REFRN10029


  • 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 and full semantic checks are performed for all objects except indexes (whose contents can be reconstructed by a drop+rebuild on encountering a corruption).

    • FULL or TRUE

      All LOW and MEDIUM checks and full semantic checks are performed for all objects.

    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. Specific DML overhead may be higher. 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.