SORT_MULTIBLOCK_READ_COUNT

Parameter type:	Integer
Parameter class:	Dynamic. Scope = ALTER SESSION, ALTER SYSTEM ...DEFERRED.
Default value:	2
Range of values:	1 to operating system dependent value
SORT_MULTIBLOCK_READ_COUNT specifies the number of database blocks Oracle reads each time a sort performs a read from a temporary segment. Oracle performs sort operations using an amount of memory specified by the SORT_AREA_SIZE parameter. If the data being sorted does not fit into that amount of memory, Oracle writes sections of the data to temporary segments in the form of sorted runs.
After all the data has been partially sorted to these runs, Oracle merges the runs by reading pieces of them from the temporary segment into memory to produce the final sorted output. If SORT_AREA_SIZE is not large enough to merge all the runs at once, subsets of the runs are merged in a number of merge passes.
If you increase the value of SORT_MULTIBLOCK_READ_COUNT, Oracle reads a larger section of each run into memory during a merge pass. Doing so reduces the number of runs that can be merged in one merge pass. Any increase in I/O throughput obtained by increasing SORT_MULTIBLOCK_READ_COUNT may be offset by an increase in total I/O due to an increase in the number of merge passes.
See Also: Oracle8i Designing and Tuning for Performance for more information on setting this parameter. "SORT_AREA_SIZE".