8.2.1.13 IS NULL Optimization
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
, a form that is common in resolved subqueries. col_name
= expr
OR col_name
IS NULLEXPLAIN
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
:
SELECT * FROM t1, t2
WHERE (t1.a=t2.a AND t2.a IS NULL)
OR (t1.b=t2.b AND t2.b IS NULL);