Oracle. Select all if parameter is null else return specific item issue

Question

I need to return all records if input parameter is null.

I\'ve written a simple query

declare  
   l_sql varchar2(100);
   i number := 1;
 begin
   l_sq         


        

Answer 1

IS NULL suppresses the index usage. Because NULL values are not indexed.

There are two ways to make the use of index with IS NULL :

1.BITMAP index. However, more applicable in OLTP systems.

2.My favourite way, and nice to demonstrate. We could make the leaves of the b-tree index a constant. Thus, making use of index while querying for NULL. Basically, the NULLs are all together, at the top/bottom of the index. Oracle can use the index forwards or backwards, so doesn't really matter. And it does a full scan of the index.

I have answered a similar question here http://www.orafaq.com/forum/mv/msg/194746/625371/#msg_625371

The first scenario won't use the index due to the OR is null condition :

SQL> SELECT * FROM PROD_NEW;

PROFILE_TYPE
---------------

Prod
Prodparallel

Prod

SQL> CREATE INDEX PROD_NEW_I1 ON PROD_NEW
  2    (PROFILE_TYPE
  3    );

Index created.

SQL> EXPLAIN PLAN FOR SELECT * FROM PROD_NEW WHERE PROFILE_TYPE = 'Prod' OR PROFILE_TYPE IS NULL;

Explained.

SQL> SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY);

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
Plan hash value: 2121244107

------------------------------------------------------------------------------
| Id  | Operation         | Name     | Rows  | Bytes | Cost (%CPU)| Time     |
------------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |          |     3 |    15 |     3   (0)| 00:00:01 |
|*  1 |  TABLE ACCESS FULL| PROD_NEW |     3 |    15 |     3   (0)| 00:00:01 |
------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------

   1 - filter("PROFILE_TYPE" IS NULL OR "PROFILE_TYPE"='Prod')

13 rows selected

Let's make the leaves constant :

SQL> DROP INDEX PROD_NEW_I1;

Index dropped.

SQL> CREATE INDEX PROD_NEW_I1 ON PROD_NEW
  2    (PROFILE_TYPE,1
  3    );

Index created.

SQL> EXPLAIN PLAN FOR SELECT * FROM PROD_NEW WHERE PROFILE_TYPE = 'Prod' OR PROFILE_TYPE IS NULL;

Explained.

SQL> SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY);

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
Plan hash value: 1272076902

--------------------------------------------------------------------------------
| Id  | Operation        | Name        | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------
|   0 | SELECT STATEMENT |             |     3 |    15 |     1   (0)| 00:00:01 |
|*  1 |  INDEX FULL SCAN | PROD_NEW_I1 |     3 |    15 |     1   (0)| 00:00:01 |
--------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------

   1 - filter("PROFILE_TYPE" IS NULL OR "PROFILE_TYPE"='Prod')

13 rows selected.

SQL>