How do NULL values affect performance in a database search?

Question

In our product we have a generic search engine, and trying to optimze the search performance. A lot of the tables used in the queries allow null values. Should we redesign o

Answer 1

If your column doesn't contain NULLs it is best to declare this column NOT NULL, the optimizer may be able to take more efficient path.

However, if you have NULLs in your column you don't have much choice (a non-null default value may create more problems than it solves).

As Quassnoi mentionned, NULLs are not indexed in Oracle, or to be more precise, a row won't be indexed if all the indexed columns are NULL, this means:

• that NULLs can potentially speed up your research because the index will have fewer rows
• you can still index the NULL rows if you add another NOT NULL column to the index or even a constant.

The following script demonstrates a way to index NULL values:

CREATE TABLE TEST AS 
SELECT CASE
          WHEN MOD(ROWNUM, 100) != 0 THEN
           object_id
          ELSE
           NULL
       END object_id
  FROM all_objects;

CREATE INDEX idx_null ON test(object_id, 1);

SET AUTOTRACE ON EXPLAIN

SELECT COUNT(*) FROM TEST WHERE object_id IS NULL;