Why would an indexed column return results slowly when querying for `IS NULL`?

Question

I have a table with 25 million rows, indexed appropriately.

But adding the clause AND status IS NULL turns a super fast query into a crazy slow query.

Please help me speed it up.

Query:

SELECT 
    student_id,
    grade,
    status
FROM 
    grades
WHERE 
        class_id = 1
    AND status IS NULL       -- This line delays results from <200ms to 40-70s!
    AND grade BETWEEN 0 AND 0.7
LIMIT 25;

Table:

CREATE TABLE IF NOT EXISTS `grades` (
  `student_id` BIGINT(20) NOT NULL,
  `class_id` INT(11) NOT NULL,
  `grade` FLOAT(10,6) DEFAULT NULL,
  `status` INT(11) DEFAULT NULL,
  UNIQUE KEY `unique_key` (`student_id`,`class_id`),
  KEY `class_id` (`class_id`),
  KEY `status` (`status`),
  KEY `grade` (`grade`)
) ENGINE=INNODB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;

Local development shows results instantly (<200ms). Production server is huge slowdown (40-70 seconds!).

Can you point me in the right direction to debug?

Explain:

+----+-------------+--------+-------------+-----------------------+-----------------+---------+------+-------+--------------------------------------------------------+
| id | select_type | table  | type        | possible_keys         | key             | key_len | ref  | rows  | Extra                                                  |
+----+-------------+--------+-------------+-----------------------+-----------------+---------+------+-------+--------------------------------------------------------+
|  1 | SIMPLE      | grades | index_merge | class_id,status,grade | status,class_id | 5,4     | NULL | 26811 | Using intersect(status,class_id); Using where          |
+----+-------------+--------+-------------+-----------------------+-----------------+---------+------+-------+--------------------------------------------------------+

Answer 1

A SELECT statement can only use one index per table.

Presumably the query before just did a scan using the sole index class_id for your condition class_id=1 . Which will probably filter your result set nicely before checking the other conditions.

The optimiser is 'incorrectly' choosing an index merge on class_id and status for the second query and checking 26811 rows which is probably not optimal. You could hint at the class_id index by adding USING INDEX (class_id) to the end of the FROM clause. You may get some joy with a composite index on (class_id,status,grade) which may run the query faster as it can match the first two and then range scan the grade . I'm not sure how this works with null though.

I'm guessing the ORDER BY pushed the optimiser to choose the class_id index again and returned your query to it's original speed.