Je dirige un EXPLAIN
:
mysql> explain select last_name from employees order by last_name;
+----+-------------+-----------+------+---------------+------+---------+------+-------+----------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-----------+------+---------------+------+---------+------+-------+----------------+
| 1 | SIMPLE | employees | ALL | NULL | NULL | NULL | NULL | 10031 | Using filesort |
+----+-------------+-----------+------+---------------+------+---------+------+-------+----------------+
1 row in set (0.00 sec)
Les index de ma table:
mysql> show index from employees;
+-----------+------------+---------------+--------------+---------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-----------+------------+---------------+--------------+---------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| employees | 0 | PRIMARY | 1 | subsidiary_id | A | 6 | NULL | NULL | | BTREE | | |
| employees | 0 | PRIMARY | 2 | employee_id | A | 10031 | NULL | NULL | | BTREE | | |
| employees | 1 | idx_last_name | 1 | last_name | A | 10031 | 700 | NULL | | BTREE | | |
| employees | 1 | date_of_birth | 1 | date_of_birth | A | 10031 | NULL | NULL | YES | BTREE | | |
| employees | 1 | date_of_birth | 2 | subsidiary_id | A | 10031 | NULL | NULL | | BTREE | | |
+-----------+------------+---------------+--------------+---------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
5 rows in set (0.02 sec)
Il existe un index sur last_name mais l'optimiseur ne l'utilise pas.
Moi aussi:
mysql> explain select last_name from employees force index(idx_last_name) order by last_name;
+----+-------------+-----------+------+---------------+------+---------+------+-------+----------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-----------+------+---------------+------+---------+------+-------+----------------+
| 1 | SIMPLE | employees | ALL | NULL | NULL | NULL | NULL | 10031 | Using filesort |
+----+-------------+-----------+------+---------------+------+---------+------+-------+----------------+
1 row in set (0.00 sec)
Mais encore l'indice est pas utilisé! Qu'est-ce que je fais mal ici?
Cela a-t-il à voir avec le fait que l'indice est NON_UNIQUE
? BTW le nom estVARCHAR(1000)
Mise à jour demandée par @RolandoMySQLDBA
mysql> SELECT COUNT(DISTINCT last_name) DistinctCount FROM employees;
+---------------+
| DistinctCount |
+---------------+
| 10000 |
+---------------+
1 row in set (0.05 sec)
mysql> SELECT COUNT(1) FROM (SELECT COUNT(1) Count500,last_name FROM employees GROUP BY last_name HAVING COUNT(1) > 500) A;
+----------+
| COUNT(1) |
+----------+
| 0 |
+----------+
1 row in set (0.15 sec)
SELECT COUNT(1) FullTableCount FROM employees;
et 2) SELECT * FROM (SELECT COUNT(1) Count500,last_name FROM employees GROUP BY last_name HAVING COUNT(1) > 500) A LIMIT 10;
.
SELECT COUNT(DISTINCT last_name) DistinctCount FROM employees;
2)SELECT COUNT(1) FROM (SELECT COUNT(1) Count500,last_name FROM employees GROUP BY last_name HAVING COUNT(1) > 500) A;
. Quel est le résultat de chaque comptage?