MySQL :: MySQL 8.4 Reference Manual :: 10.2.1.15 IS NULL Optimization (original) (raw)

10.2.1.15 IS NULL Optimization

MySQL can perform the same optimization on_colname_ IS NULL that it can use for_colname_ = constantvalue. For example, MySQL can use indexes and ranges to search forNULL 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_colname_ IS NULL condition for a column that is declared asNOT 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 aLEFT JOIN).

MySQL can also optimize the combination_`colname`_ =_`expr`_ OR_`colname`_ IS NULL, a form that is common in resolved subqueries.EXPLAIN showsref_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 andb 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 onb:

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