Explain Plan - Parallel Processing Degree of 2 and CPU_Cost

Question

Explain Plan - Parallel Processing Degree of 2 and CPU_Cost
When I use a hint to use parallel processing with a degree of 2
the I/O cost seems consistently divided by (1.8) but the cpu cost
adjustment is inconsistent(between 2.17 and 2.62).
Any ideas on why the cpu cost varies with each table ?
Is there a formula to adjust the cpu_cost ?
Thanks,
Summary:
The i/o cost reduction is consistent (divide by 1.8)
Table 1: 763/424 = 1.8
Table 2: 18774/10430 = 1.8
Table 3(not shown): 5/1.8 = 3
But the cpu cost reduction varies:(between 2.17 and 2.62)
Table 1: 275812018/122353500 = 2.25
Table 2: 7924072407/3640755000 = 2.17
Table 3(not shown): 791890/301467 = 2.62
Example:
Oracle Database 10.2.0.4.0
Table 1:
1.) Full table scan on Table 1 without parallel processing.
EXPLAIN PLAN FOR select/*+ CPU_COSTING
PARALLEL(table_1,1)*/
* from table_1
SQL> select cost,io_cost,cpu_cost from plan_table;
IO_COST CPU_COST
763 275812018
2.) Process Table 1 in parallel with a degree of 2.
EXPLAIN PLAN FOR select/*+ CPU_COSTING
PARALLEL(table_1,2)*/
* from table_1
IO_COST CPU_COST
424 122353500
Table 2:
3.) Full table scan on Table 2 without parallel processing.
EXPLAIN PLAN FOR select/*+ CPU_COSTING
PARALLEL(table_2,1)*/
* from table_2
IO_COST CPU_COST
18774 7924072407
4.) Process Table 2 in parallel with a degree of 2.
EXPLAIN PLAN FOR select/*+ CPU_COSTING
PARALLEL(table_2,2)*/
* from table_2
IO_COST CPU_COST
10430 3640755000

Answer 1

The COST value is for the benefit of the CBO; not YOU.
What should be more important to you is the elapsed run time for the SQL