Oracle Database Discussions

This is a Strange Behaviour,

I've a SQL that selects into 2 tables and LOAD into another, that's why the response Time is Critical.
So
I've improved one SQL creating one Index but then I decided to further improve the Response Time by adding 4 process

SO I change Table Parallel Degree from 1 to 4 and I found this strange situation.

The Plan as changed from Index to Full Scann

ALTER TABLE SIEBELDWH.W_PRODUCT_D PARALLEL (DEGREE 4);

Plan
SELECT STATEMENT CHOOSECost: 8 Bytes: 74 Cardinality: 1
6 SORT GROUP BY Cost: 8 Bytes: 74 Cardinality: 1
5 SORT GROUP BY Cost: 8 Bytes: 74 Cardinality: 1
4 TABLE ACCESS BY INDEX ROWID SIEBELDWH.WC_SALES_PIPELINE_A Cost: 1 Bytes: 17 Cardinality: 1
3 NESTED LOOPS Cost: 4 Bytes: 74 Cardinality: 1
FULL SCAN >>>> 1 TABLE ACCESS FULL SIEBELDWH.W_PRODUCT_D Cost: 3 Bytes: 57 Cardinality: 1
2 INDEX RANGE SCAN NON-UNIQUE SIEBELDWH.WC_SALES_PIPELINE_A_X1


ALTER TABLE SIEBELDWH.W_PRODUCT_D PARALLEL (DEGREE 1);

Plan
SELECT STATEMENT CHOOSECost: 10 Bytes: 74 Cardinality: 1
6 SORT GROUP BY Cost: 10 Bytes: 74 Cardinality: 1
5 TABLE ACCESS BY INDEX ROWID SIEBELDWH.WC_SALES_PIPELINE_A Cost: 1 Bytes: 17 Cardinality: 1
4 NESTED LOOPS Cost: 5 Bytes: 74 Cardinality: 1
2 TABLE ACCESS BY INDEX ROWID SIEBELDWH.W_PRODUCT_D Cost: 4 Bytes: 57 Cardinality: 1 <<<<< INDEX ROWID
1 INDEX RANGE SCAN NON-UNIQUE SIEBELDWH.W_PRODUCT_D_X2 Cost: 3 Cardinality: 1
3 INDEX RANGE SCAN NON-UNIQUE SIEBELDWH.WC_SALES_PIPELINE_A_X1 Cardinality: 1

Do u have any Clues?

Thxs
Rgds
Carlos