PARALLEL_INDEX HINT

The PARALLEL_INDEX hint instructs the optimizer to use the specified number of concurrent servers to parallelize index range scans, full scans, and fast full scans for partitioned indexes.

The parallel_index hint specifies the desired number of concurrent servers that can be used to parallelize index range scans for partitioned indexes.

When using the parallel_index hint, specifying DEFAULT or no DEGREE value tells the query coordinator check the initialization parameter default degree of parallelism (parallel_automatic_tuning).

create table t1
 as
 with generator as (
  select
  rownum id
  from dual
  connect by
  level <= 50000
 )
 select
  rownum id,
  mod(rownum,5000) modded,
  lpad(rownum,10,'0') v1,
  lpad('x',30,'x') padding
 from
  generator v1,
  generator v2
 where
  rownum <= 50000
 ;
 -----------------------------------------------------------
 create index t1_i1 on t1(modded);

Check the execution Plan for the below query.

Select /*+ parallel_index(t1 t1_i1 20) */ count(modded) from t1;
 ----------------------------------------------------------------------------------
 | Id | Operation | Name | Rows | Bytes | Cost | Time |
 ----------------------------------------------------------------------------------
 | 0 | SELECT STATEMENT | | 1 | 4 | 31 | 00:00:01 |
 | 1 | SORT AGGREGATE | | 1 | 4 | | |
 | 2 | PX COORDINATOR | | | | | |
 | 3 | PX SEND QC (RANDOM) | :TQ10000 | 1 | 4 | | |
 | 4 | SORT AGGREGATE | | 1 | 4 | | |
 | 5 | PX BLOCK ITERATOR | | 50000 | 200000 | 31 | 00:00:01 |
 | 6 | INDEX FAST FULL SCAN | T1_I1 | 50000 | 200000 | 31 | 00:00:01 |
 ----------------------------------------------------------------------------------

Lets do a comparison of the above plan with other index hints

select /*+ index_ffs(t1 t1_i1) */ count(modded) from t1;
  Plan Hash Value : 2608623722
 
 ---------------------------------------------------------------------------
 | Id | Operation | Name | Rows | Bytes | Cost | Time |
 ---------------------------------------------------------------------------
 | 0 | SELECT STATEMENT | | 1 | 4 | 31 | 00:00:01 |
 | 1 | SORT AGGREGATE | | 1 | 4 | | |
 | 2 | INDEX FAST FULL SCAN | T1_I1 | 50000 | 200000 | 31 | 00:00:01 |
 ---------------------------------------------------------------------------
 
 

select /*+ parallel_index(t1 t1_i1 20) parallel(t1 10) */ count(modded) from t1;
 ----------------------------------------------------------------------------------
 | Id | Operation | Name | Rows | Bytes | Cost | Time |
 ----------------------------------------------------------------------------------
 | 0 | SELECT STATEMENT | | 1 | 4 | 2 | 00:00:01 |
 | 1 | SORT AGGREGATE | | 1 | 4 | | |
 | 2 | PX COORDINATOR | | | | | |
 | 3 | PX SEND QC (RANDOM) | :TQ10000 | 1 | 4 | | |
 | 4 | SORT AGGREGATE | | 1 | 4 | | |
 | 5 | PX BLOCK ITERATOR | | 50000 | 200000 | 2 | 00:00:01 |
 | 6 | INDEX FAST FULL SCAN | T1_I1 | 50000 | 200000 | 2 | 00:00:01 |

Also go through FISRT_ROWS hint in oracle.