मैं एक चलाता हूं 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)
मेरी तालिका में अनुक्रमणिकाएँ:
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)
Last_name पर एक इंडेक्स है लेकिन ऑप्टिमाइज़र इसका उपयोग नहीं करता है।
इसलिए मैं करता हूँ:
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)
लेकिन फिर भी सूचकांक का उपयोग नहीं किया जाता है! मुझसे यहां क्या गलत हो रहा है?
क्या इस तथ्य के साथ यह करना है कि सूचकांक क्या है NON_UNIQUE
? BTW last_name हैVARCHAR(1000)
@RolandoMySQLDBA द्वारा अनुरोधित अपडेट
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;
और २) 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;
। प्रत्येक गणना का परिणाम क्या है?