DB_FILE_MULTIBLOCK_READ_COUNT

Property	Description
Parameter type	Integer
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
Basic	No

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.

Online transaction processing (OLTP) and batch environments typically have values in the range of 4 to 16 for this parameter. DSS 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.

Note:

1. Query for the current value of the parameter

select name, value, isdefault, isses_modifiable, issys_modifiable,

isinstance_modifiable, isdeprecated, description

from v$parameter where upper(name) = ‘DB_FILE_MULTIBLOCK_READ_COUNT’;

NAME	VALUE	IS DEFAULT	ISSES_ MODIFIABLE	ISSYS_ MODIFIABLE	ISINSTANCE_ MODIFIABLE	IS DEPRECATED	DESCRIPTION
db_file_multiblock_read_count	16	FALSE	TRUE	IMMEDIATE	TRUE	FALSE	db block to be read each IO

2. This parameter is dynamic from 7.3 onwards so individual sessions can use 'ALTER SESSION SET ...' to set a larger size for batch type work. The value to choose is a matter of tuning.

Oracle initializatoin parameters