MySQL can perform the same optimization on col_name IS NULL  that it can use for col_name = constant_value . For example, MySQL can use indexes and ranges to search for NULL  with IS NULL . 

## Examples: 

SELECT * FROM tbl_name  WHERE key_col  IS NULL; SELECT * FROM tbl_name  WHERE key_col  <=> NULL; SELECT * FROM tbl_name WHERE key_col = const1  OR key_col = const2  OR key_col  IS NULL; 

If a WHERE  clause includes a col_name IS NULL  condition for a column that is declared as NOT NULL , that expression is optimized away. This optimization does not occur in cases when the column might produce NULL  anyway (for example, if it comes from a table on the right side of a LEFT JOIN ). 

MySQL can also optimize the combination col_name  = expr  OR col_name  IS NULL , a form that is common in resolved subqueries. EXPLAIN  shows ref_or_null  when this optimization is used. 

This optimization can handle one IS NULL  for any key part. 

Some examples of queries that are optimized, assuming that there is an index on columns a  and b  of table t2 : 

SELECT * FROM t1 WHERE t1.a= expr  OR t1.a IS NULL; SELECT * FROM t1, t2 WHERE t1.a=t2.a OR t2.a IS NULL; SELECT * FROM t1, t2 WHERE (t1.a=t2.a OR t2.a IS NULL) AND t2.b=t1.b; SELECT * FROM t1, t2 WHERE t1.a=t2.a AND (t2.b=t1.b OR t2.b IS NULL); SELECT * FROM t1, t2 WHERE (t1.a=t2.a AND t2.a IS NULL AND ...) OR (t1.a=t2.a AND t2.a IS NULL AND ...); 

ref_or_null  works by first doing a read on the reference key, and then a separate search for rows with a NULL  key value. 

The optimization can handle only one IS NULL  level. In the following query, MySQL uses key lookups only on the expression (t1.a=t2.a AND t2.a IS NULL)  and is not able to use the key part on b : 

## 8.2.1.16 ORDER BY Optimization 

SELECT * FROM t1, t2 WHERE (t1.a=t2.a AND t2.a IS NULL) OR (t1.b=t2.b AND t2.b IS NULL); 

This section describes when MySQL can use an index to satisfy an ORDER BY  clause, the filesort operation used when an index cannot be used, and execution plan information available from the optimizer about ORDER BY . 

An ORDER BY  with and without LIMIT  may return rows in different orders, as discussed in Section 8.2.1.19, “LIMIT Query Optimization” . 

•Use of Indexes to Satisfy ORDER BY 

•Use of filesort to Satisfy ORDER BY 

