Using the db_file_multiblock_read_count init.ora Parameter
In addition, note the relationship between db_block_size and the multi_block_read_count parameter.
At the physical level in UNIX, Oracle always reads in a minimum of 64K blocks. Therefore, the values of
- multi_block_read_count and
should be set such that their product is 64K. For example:
8K blocks db_block_size=8192 db_file_multiblock_read_count=8
16K blocks db_block_size=16384 db_file_multiblock_read_count=4
If you specify an invalid value for db_file_multiblock_read_count in your init.ora file, Oracle will re-adjust it on your behalf to equal 64K.
|Default value ||The default value corresponds to the maximum I/O size that can be efficiently performed and is platform-dependent|
|Modifiable ||ALTER SESSION, ALTER SYSTEM|
|Range of values ||Operating system-dependent|
DB_FILE_MULTIBLOCK_READ_COUNT is one of the parameters you can use to minimize I/O during table scans. It specifies the
maximum number of blocks read in one I/O operation during a sequential scan.
The total number of I/Os needed to perform a full table scan depends on such factors as the size of the table, the multiblock read count, and whether parallel execution is being utilized for the operation.
As of Oracle Database 10g release 2, the default value of this parameter is a value that corresponds to the maximum I/O size that can be performed efficiently.
This value is platform-dependent and is 1MB for most platforms.
Because the parameter is expressed in blocks, it will be set to a value that is equal to the maximum I/O size that can be performed efficiently divided by the standard block size.
Note that if the number of sessions is extremely large the multiblock read count value is decreased to avoid the buffer cache getting flooded with too many table scan buffers.
Even though the default value may be a large value, the optimizer will not favor large plans if you do not set this parameter. It would do so only if you explicitly set this parameter to a large value.
(OLTP) Online transaction processing and batch environments typically have values in the range of 4 to 16 for this parameter. (DSS) Decision support systems and data warehouse environments tend to benefit most from maximizing the value of this parameter.
The optimizer is more likely to choose a full table scan over an index if the value of this parameter is high.
The maximum value is the operating system's maximum I/O size expressed as Oracle blocks ((max I/O size)/DB_BLOCK_SIZE). If you set this parameter to a value greater than the maximum, Oracle uses the maximum.