2012-04-17Emulating Partial Indexes in the Oracle Database

The strange way, the Oracle Database handles NULL in indexes, can be used to emulate partial indexes. For that, you just use NULL for rows that shall not be indexed.

For demonstration, we emulate the following partial index:

CREATE INDEX messages_todo
          ON messages (receiver)
       WHERE processed = 'N'

First, we need a function that returns the RECEIVER value only if the PROCESSED value is 'N'.

CREATE OR REPLACE
FUNCTION pi_processed(processed CHAR, receiver NUMBER)
RETURN NUMBER
DETERMINISTIC
AS BEGIN
   IF processed IN ('N') THEN
      RETURN receiver;
   ELSE
      RETURN NULL;
   END IF;
END;
/

The function must be deterministic, so it can be used in an index definition.

Now, we can create an index that contains only the rows having PROCESSED='N'.

CREATE INDEX messages_todo
          ON messages (pi_processed(processed, receiver));

To use the index, you must use the indexed expression in the query:

SELECT message
  FROM messages
 WHERE pi_processed(processed, receiver) = ?

----------------------------------------------------------
|Id | Operation                   | Name          | Cost |
----------------------------------------------------------
| 0 | SELECT STATEMENT            |               | 5330 |
| 1 |  TABLE ACCESS BY INDEX ROWID| MESSAGES      | 5330 |
|*2 |   INDEX RANGE SCAN          | MESSAGES_TODO | 5303 |
----------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------
   2 - access("PI_PROCESSED"("PROCESSED","RECEIVER")=:X)