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use_concat hint tips

Oracle Database Tips by Burleson Consulting

December 20, 2015

The use_concat Hint

The use_concat hint requests that a union all execution plan be used for all OR conditions in the query, rewriting the query into multiple queries. The use_concat hint is commonly invoked when a SQL query has OR conditions in the where clause.

In other words, use of the use_contact hint forces combined OR conditions and IN processing in the where clause to be transformed into a compound query using the union all set operator.

For example, consider the following query where a B-tree index exists on job, deptno and sal. It is interesting to note that if the indexes were bitmap indexes, the execution would not perform a full-table scan. This is because Oracle automatically uses bitmap indexes where a query has multiple OR conditions on bitmap index columns.

select
    ename
from
    emp
where
    deptno = 10
or
    sal < 5000
or
    job = 'CLERK';

Here we have two choices. Because all of the index columns are low cardinality, we could create three bitmap indexes on deptno, sal and clerk, causing a bitmap merge execution plan. Our other choice is to invoke use_concat to break the query into three separate B-tree index scans whose result sets will be combined with the union operator.

Here is the execution plan for this query with B-tree indexes. Note that we must perform a full-table scan to satisfy the multiple OR conditions in the where clause:

OPERATION
----------------------------------------------------------------------
OPTIONS                        OBJECT_NAME                    POSITION
------------------------------ ---------------------------- ----------  
SELECT STATEMENT                                                     1

   TABLE ACCESS
FULL                            EMP                                  1

If our indexes had been bitmap indexes, we would have seen a far faster execution plan:

 OPERATION
---------------------------------------------------------------------- OPTIONS                        OBJECT_NAME                    POSITION
------------------------------ ---------------------------- ----------  
SELECT  STATEMENT                                                    4
   TABLE ACCESS
BY INDEX ROWID                 EMP                                   1
   BITMAP CONVERSION
TO ROWIDS                                                            1
       BITMAP OR
                                                                     1
         BITMAP INDEX
SINGLE VALUE                   EMP_DEPTNO_BIT                        1

         BITMAP MERGE                                                2
         BITMAP INDEX
RANGE SCAN                     EMP_SAL_BIT                           1
         BITMAP INDEX
SINGLE VALUE                   EMP_JOB_BIT                           3

Now, returning to our example with three B-tree indexes, let's add the use_concat hint and see the change to the execution plan.

select /*+ use_concat */
    ename
from
    emp
where
    deptno = 10
or
    sal < 5000
or
    job = 'CLERK';

Here we see that the full-table scan has been replaced with a union of three queries, each using the B-tree index for the single columns and the CONCATENATION plan to union the result sets:

OPERATION
---------------------------------------------------------------------- OPTIONS                        OBJECT_NAME                    POSITION
------------------------------ ---------------------------- ----------  
SELECT STATEMENT
                                                                      3
   CONCATENATION
     TABLE ACCESS
BY INDEX ROWID                 EMP                                    1

       INDEX
RANGE SCAN                     EMP_JOB                                1

     TABLE ACCESS
BY INDEX ROWID                 EMP                                    2

       INDEX
RANGE SCAN                     EMP_SAL                                1

     TABLE ACCESS
BY INDEX ROWID                 EMP                                    3

       INDEX
RANGE SCAN                     EMP_DEPT                               1

There are times when it is possible for Oracle to ignore your use_concat hint.  While it is true Oracle will generally not ignore an properly referenced use_concat or other hint, the following conditions can result in failure of your use_concat or other hint to achieve the desired result:

  • Syntax errors: Misspelling the hint will cause it to be ignored.  For example, use_concat will be used but use_contac will not and will be ignored:

    select /*+ use_contac */
        ename
    from
        emp
    where
        deptno = 10
    or
        sal < 5000
    or
        job = 'CLERK';
  • Semantics errors: If a use_concat hint argument is misspelled, it will be ignored.

    select /*+ use_concat */
        enane
    from
        emp
    where
        deptno = 10
    or
        sal < 5000
    or
        job = 'CLERK';

 

   
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Note: This Oracle documentation was created as a support and Oracle training reference for use by our DBA performance tuning consulting professionals.  Feel free to ask questions on our Oracle forum.

Verify experience! Anyone considering using the services of an Oracle support expert should independently investigate their credentials and experience, and not rely on advertisements and self-proclaimed expertise. All legitimate Oracle experts publish their Oracle qualifications.

Errata?  Oracle technology is changing and we strive to update our BC Oracle support information.  If you find an error or have a suggestion for improving our content, we would appreciate your feedback.  Just  e-mail:  and include the URL for the page.


                    









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