2012-04-17Indexing NULL in the Oracle Database

The Oracle database does not include rows in an index if all indexed columns are NULL. That means that every index is a partial index—like having a where clause:

CREATE INDEX idx
          ON tbl (A, B, C, ...)
       WHERE A IS NOT NULL
         AND B IS NOT NULL
         AND C IS NOT NULL
             ...;         

Consider the EMP_DOP index. It has only one column: the DATE_OF_BIRTH. A row that does not have a DATE_OF_BIRTH value is not added to this index.

INSERT INTO employees ( subsidiary_id, employee_id
                      , first_name   , last_name
                      , phone_number)
               VALUES ( ?, ?, ?, ?, ? );   

The insert statement does not set the DATE_OF_BIRTH so it defaults to NULL—hence, the record is not added to the EMP_DOB index. As a consequence, the index cannot support a query for records where DATE_OF_BIRTH IS NULL:

SELECT first_name, last_name
  FROM employees
 WHERE date_of_birth IS NULL;

----------------------------------------------------
| Id | Operation         | Name      | Rows | Cost |
----------------------------------------------------
|  0 | SELECT STATEMENT  |           |    1 |  477 |
|* 1 |  TABLE ACCESS FULL| EMPLOYEES |    1 |  477 |
----------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------
   1 - filter("DATE_OF_BIRTH" IS NULL)

Nevertheless, the record is inserted into a concatenated index if at least one index column is not NULL:

CREATE INDEX demo_null
          ON employees (subsidiary_id, date_of_birth);

The above created row is added to the index because the SUBSIDIARY_ID is not NULL. This index can thus support a query for all employees of a specific subsidiary that have no DATE_OF_BIRTH value:

SELECT first_name, last_name
  FROM employees
 WHERE subsidiary_id = ?
   AND date_of_birth IS NULL;

--------------------------------------------------------------
| Id | Operation                   | Name      | Rows | Cost |
--------------------------------------------------------------
|  0 | SELECT STATEMENT            |           |    1 |    2 |
|  1 |  TABLE ACCESS BY INDEX ROWID| EMPLOYEES |    1 |    2 |
|* 2 |   INDEX RANGE SCAN          | DEMO_NULL |    1 |    1 |
--------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------
   2 - access("SUBSIDIARY_ID"=TO_NUMBER(?) 
          AND "DATE_OF_BIRTH" IS NULL)

Please note that the index covers the entire where clause; all filters are used as access predicates during the INDEX RANGE SCAN.

We can extend this concept for the original query to find all records where DATE_OF_BITH IS NULL. For that, the DATE_OF_BIRTH column has to be the leftmost column in the index so that it can be used as access predicate. Although we do not need a second index column for the query itself, we add another column that can never be NULL to make sure the index has all rows. We can use any column that has a NOT NULL constraint, like SUBSIDIARY_ID, for that purpose.

Alternatively, we can use a constant expression that can never be NULL. That makes sure the index has all rows—even if DATE_OF_BIRTH is NULL.

DROP   INDEX emp_dob;
CREATE INDEX emp_dob ON employees (date_of_birth, '1');

Technically, this index is a function-based index. This example also dis­proves the myth that the Oracle database cannot index NULL.