I've downloaded the employees database and executed some queries for benchmarking purposes.
Then I noticed that one query didn't use a covering index, although there was a corresponding index that I created earlier. Only when I added a FORCE INDEX clause to the query, it used a covering index.
I've uploaded two files, one is the executed SQL queries and the other is the results.
Can you tell why the query uses a covering-index only when a FORCE INDEX clause is added? The EXPLAIN shows that in both cases, the index dept_no_from_date_idx is being used anyway.
To adapt myself to the standards of SO, I'm also writing the content of the two files here:
The SQL queries:
USE employees;
/* Creating an index for an index-covered query */
CREATE INDEX dept_no_from_date_idx ON dept_emp (dept_no, from_date);
/* Show `dept_emp` table structure, indexes and generic data */
SHOW TABLE STATUS LIKE "dept_emp";
DESCRIBE dept_emp;
SHOW KEYS IN dept_emp;
/* The EXPLAIN shows that the subquery doesn't use a covering-index */
EXPLAIN SELECT SQL_NO_CACHE * FROM dept_emp INNER JOIN (
/* The subquery should use a covering index, but isn't */
SELECT SQL_NO_CACHE emp_no, dept_no FROM dept_emp WHERE dept_no="d001" ORDER BY from_date DESC LIMIT 20000,50
) AS `der` USING (`emp_no`, `dept_no`);
/* The EXPLAIN shows that the subquery DOES use a covering-index,
thanks to the FORCE INDEX clause */
EXPLAIN SELECT SQL_NO_CACHE * FROM dept_emp INNER JOIN (
/* The subquery use a covering index */
SELECT SQL_NO_CACHE emp_no, dept_no FROM dept_emp FORCE INDEX(dept_no_from_date_idx) WHERE dept_no="d001" ORDER BY from_date DESC LIMIT 20000,50
) AS `der` USING (`emp_no`, `dept_no`);
The results:
--------------
/* Creating an index for an index-covered query */
CREATE INDEX dept_no_from_date_idx ON dept_emp (dept_no, from_date)
--------------
Query OK, 331603 rows affected (33.95 sec)
Records: 331603 Duplicates: 0 Warnings: 0
--------------
/* Show `dept_emp` table structure, indexes and generic data */
SHOW TABLE STATUS LIKE "dept_emp"
--------------
+----------+--------+---------+------------+--------+----------------+-------------+-----------------+--------------+-----------+----------------+---------------------+-------------+------------+-----------------+----------+----------------+---------+
| Name | Engine | Version | Row_format | Rows | Avg_row_length | Data_length | Max_data_length | Index_length | Data_free | Auto_increment | Create_time | Update_time | Check_time | Collation | Checksum | Create_options | Comment |
+----------+--------+---------+------------+--------+----------------+-------------+-----------------+--------------+-----------+----------------+---------------------+-------------+------------+-----------------+----------+----------------+---------+
| dept_emp | InnoDB | 10 | Compact | 331883 | 36 | 12075008 | 0 | 21544960 | 29360128 | NULL | 2010-05-04 13:07:49 | NULL | NULL | utf8_general_ci | NULL | | |
+----------+--------+---------+------------+--------+----------------+-------------+-----------------+--------------+-----------+----------------+---------------------+-------------+------------+-----------------+----------+----------------+---------+
1 row in set (0.47 sec)
--------------
DESCRIBE dept_emp
--------------
+-----------+---------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-----------+---------+------+-----+---------+-------+
| emp_no | int(11) | NO | PRI | NULL | |
| dept_no | char(4) | NO | PRI | NULL | |
| from_date | date | NO | | NULL | |
| to_date | date | NO | | NULL | |
+-----------+---------+------+-----+---------+-------+
4 rows in set (0.05 sec)
--------------
SHOW KEYS IN dept_emp
--------------
+----------+------------+-----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+----------+------------+-----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| dept_emp | 0 | PRIMARY | 1 | emp_no | A | 331883 | NULL | NULL | | BTREE | |
| dept_emp | 0 | PRIMARY | 2 | dept_no | A | 331883 | NULL | NULL | | BTREE | |
| dept_emp | 1 | emp_no | 1 | emp_no | A | 331883 | NULL | NULL | | BTREE | |
| dept_emp | 1 | dept_no | 1 | dept_no | A | 7 | NULL | NULL | | BTREE | |
| dept_emp | 1 | dept_no_from_date_idx | 1 | dept_no | A | 13 | NULL | NULL | | BTREE | |
| dept_emp | 1 | dept_no_from_date_idx | 2 | from_date | A | 165941 | NULL | NULL | | BTREE | |
+----------+------------+-----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
6 rows in set (0.23 sec)
--------------
/* The EXPLAIN shows that the subquery doesn't use a covering-index */
EXPLAIN SELECT SQL_NO_CACHE * FROM dept_emp INNER JOIN (
/* The subquery should use a covering index, but isn't */
SELECT SQL_NO_CACHE emp_no, dept_no FROM dept_emp WHERE dept_no="d001" ORDER BY from_date DESC LIMIT 20000,50
) AS `der` USING (`emp_no`, `dept_no`)
--------------
+----+-------------+------------+--------+----------------------------------------------+-----------------------+---------+------------------------+-------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------+--------+----------------------------------------------+-----------------------+---------+------------------------+-------+-------------+
| 1 | PRIMARY | <derived2> | ALL | NULL | NULL | NULL | NULL | 50 | |
| 1 | PRIMARY | dept_emp | eq_ref | PRIMARY,emp_no,dept_no,dept_no_from_date_idx | PRIMARY | 16 | der.emp_no,der.dept_no | 1 | |
| 2 | DERIVED | dept_emp | ref | dept_no,dept_no_from_date_idx | dept_no_from_date_idx | 12 | | 21402 | Using where |
+----+-------------+------------+--------+----------------------------------------------+-----------------------+---------+------------------------+-------+-------------+
3 rows in set (0.09 sec)
--------------
/* The EXPLAIN shows that the subquery DOES use a covering-index,
thanks to the FORCE INDEX clause */
EXPLAIN SELECT SQL_NO_CACHE * FROM dept_emp INNER JOIN (
/* The subquery use a covering index */
SELECT SQL_NO_CACHE emp_no, dept_no FROM dept_emp FORCE INDEX(dept_no_from_date_idx) WHERE dept_no="d001" ORDER BY from_date DESC LIMIT 20000,50
) AS `der` USING (`emp_no`, `dept_no`)
--------------
+----+-------------+------------+--------+----------------------------------------------+-----------------------+---------+------------------------+-------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------+--------+----------------------------------------------+-----------------------+---------+------------------------+-------+--------------------------+
| 1 | PRIMARY | <derived2> | ALL | NULL | NULL | NULL | NULL | 50 | |
| 1 | PRIMARY | dept_emp | eq_ref | PRIMARY,emp_no,dept_no,dept_no_from_date_idx | PRIMARY | 16 | der.emp_no,der.dept_no | 1 | |
| 2 | DERIVED | dept_emp | ref | dept_no_from_date_idx | dept_no_from_date_idx | 12 | | 37468 | Using where; Using index |
+----+-------------+------------+--------+----------------------------------------------+-----------------------+---------+------------------------+-------+--------------------------+
3 rows in set (0.05 sec)
Bye
