Sets the assumption about the effective size of the disk cache that is available to a single query for the legacy query optimizer (planner). This is factored into estimates of the cost of using an index; a higher value makes it more likely index scans will be used, a lower value makes it more likely sequential scans will be used. This parameter has no effect on the size of shared memory allocated by a Greenplum server instance, nor does it reserve kernel disk cache; it is used only for estimation purposes.