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Don Burleson Blog 







index_fss Hint

Expert Oracle Database Tips by Donald BurlesonApril 13, 2015

The index_fss Hint

The index fast full scan is used in cases where a query can be resolved without accessing any table rows. Remember not to confuse the index fast full scan with a full-index scan.

When the index_ffs is invoked, the optimizer will scan all of the blocks in the index using multiblock reads and access the index in non-sequence order. You can also make an index fast full scan even faster by combining the index_fss hint with a parallel hint.

The index_ffs hint can also be used to trick the SQL optimizer when you need to select on the values for the second column in a concatenated index. This is because the entire index is accessed, and, depending on the amount of parallelism, an index fast full scan may be faster than an index range scan. Using the index_ffs hint is especially useful for shops where the tables are huge and creating a new index would require gigabytes of extra disk space. In the case of very large tables where no high-level index key exists for the required search column, the fast full-index scan will always be faster than a full-table scan.

For example, consider the following concatenated index on two non-unique columns.

create index
    (deptno, job);

Now, consider the following SQL, and assume that there is no index on the job column.

    job = 'SALESMAN'

Here is the execution plan. As we expect, we see a full-table scan on the emp table:

---------------------------------------------------------------------- OPTIONS                        OBJECT_NAME                    POSITION
------------------------------ ---------------------------- ----------
FULL                           EMP                                   1

Now, we take the same query and add the index_ffs hint, making sure to specify the table name and the index name.

select  /*+ index_ffs(emp, dept_job_idx) */
    job = 'SALESMAN'

Here we see that the full-table scan is replaced by the faster fast full-index scan:

---------------------------------------------------------------------- OPTIONS                        OBJECT_NAME                    POSITION
------------------------------ ---------------------------- ----------
BY INDEX ROWID                 EMP                                   1

FULL SCAN                      DEPT_JOB_IDX                          1

Note: The index fast full scan execution plan is the mechanism behind fast index create and recreate.

In simple terms, the cost-based optimizer will make the decision about whether to invoke the fast full-index scan in accordance with the table and index statistics. 

You can use the index_ffs hint to force a fast-full index scan by specifying the index_ffs in the code.  The following is one more example of how the index_ffs hint can be used:

 select distinct /*+ index_ffs(c,pk_auto) /*
group by

The index_ffs hint is commonly combined with the parallel_index hint to improve performance. For example, the following query forces the use of a fast full-index scan with parallelism by using both the index_ffs and parallel_index hints:

select /*+ index_ffs(car pk_auto) parallel_index(car pk_auto)*/
   distinct color,
group by

Whether or not performance will benefit from using index_ffs to force a fast-full index is not intuitive.  It is going to be up to the developer and/or DBA to run test cases with and without the index_ffs hint and review the execution plans to verify if there is any improvement in run time.


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