MySQL 5.7以后怎么查看索引使用情況?
0.在sys庫中查看沒用的索引
root@localhost [sys]>select * from schema_unused_indexes;
+-------------------+-------------+------------+
| object_schema | object_name | index_name |
+-------------------+-------------+------------+
| sysbench_testdata | sbtest1 | k_1 |
| sysbench_testdata | sbtest10 | k_10 |
| sysbench_testdata | sbtest3 | k_3 |
| sysbench_testdata | sbtest4 | k_4 |
| sysbench_testdata | sbtest5 | k_5 |
| sysbench_testdata | sbtest6 | k_6 |
| sysbench_testdata | sbtest7 | k_7 |
| sysbench_testdata | sbtest8 | k_8 |
| sysbench_testdata | sbtest9 | k_9 |
+-------------------+-------------+------------+
9 rows in set (0.00 sec)
- 通過show status like '%Handler_read%'方法查看:整體的
root@localhost [sysbench_testdata]>show status like '%Handler_read%';
+-----------------------+---------+
| Variable_name | Value |
+-----------------------+---------+
| Handler_read_first | 7 |
| Handler_read_key | 29 |
| Handler_read_last | 0 |
| Handler_read_next | 8446377 |
| Handler_read_prev | 0 |
| Handler_read_rnd | 20 |
| Handler_read_rnd_next | 8344612 |
+-----------------------+---------+
7 rows in set (0.00 sec)
-
Handler_read_key這個值代表了一個行將索引值讀的次數，很低的值表明增加索引得到的性能改善不高，因為索引並不經常使用。
-
Handler_read_rnd_next 的值高則查詢低效，並且應該建立索引補救。這個值是指在數據文件中讀下一行的請求數。如果正進行大量的表掃描，Handler_read_rnd_next的值較高，則通常說明表索引不正確或查詢沒有利用索引
2.查看具體某一個sql的索引使用情況 :
root@localhost [sysbench_testdata]>explain select k from sbtest2 where k=432 limit 2;
+----+-------------+---------+------------+------+---------------+------+---------+-------+--------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+------+---------------+------+---------+-------+--------+----------+-------------+
| 1 | SIMPLE | sbtest2 | NULL | ref | k_2 | k_2 | 4 | const | 110944 | 100.00 | Using index |
+----+-------------+---------+------------+------+---------------+------+---------+-------+--------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
字段說明:
Type：告訴我們對表所使用的訪問方式，主要包含如下集中類型；
◇ all：全表掃描
◇ const：讀常量，且最多只會有一條記錄匹配，由於是常量，所以實際上只需要讀一次；
◇ eq_ref：最多只會有一條匹配結果，一般是通過主鍵或者唯一鍵索引來訪問；
◇ fulltext：
◇ index：全索引掃描；
◇ index_merge：查詢中同時使用兩個（或更多）索引，然后對索引結果進行merge 之后再讀取表數據；
◇ index_subquery：子查詢中的返回結果字段組合是一個索引（或索引組合），但不是一個主鍵或者唯一索引；
◇ rang：索引范圍掃描；
◇ ref：Join 語句中被驅動表索引引用查詢；
◇ ref_or_null：與ref 的唯一區別就是在使用索引引用查詢之外再增加一個空值的查詢；
◇ system：系統表，表中只有一行數據；
◇ unique_subquery：子查詢中的返回結果字段組合是主鍵或者唯一約束；
possible_keys：可能可以利用的索引的名字。這里的索引名字是創建索引時指定的索引昵稱；如果索引沒有昵稱，則默認顯示的是索引中第一個列的名字（在本例中，它是“firstname”）。默認索引名字的含義往往不是很明顯。
key：它顯示了MySQL實際使用的索引的名字。如果它為空（或NULL），則MySQL不使用索引。
key_len：索引中被使用部分的長度，以字節計
ref：列出是通過常量（const），還是某個表的某個字段（如果是join）來過濾（通過key）
的；
rows：MySQL所認為的它在找到正確的結果之前必須掃描的記錄數。顯然，這里最理想的數字就是1。
3.通過performance_schema可以查詢到.查看sbtest2表索引情況的查詢語句:
root@localhost [sysbench_testdata]>select object_type,object_schema,object_name,index_name,count_star,count_read,COUNT_FETCH from performance_schema.table_io_waits_summary_by_index_usage where object_name='sbtest2';
- 具體查看過程:
root@localhost [sysbench_testdata]>select object_type,object_schema,object_name,index_name,count_star,count_read,COUNT_FETCH from performance_schema.table_io_waits_summary_by_index_usage where object_name='sbtest2';
ERROR 2006 (HY000): MySQL server has gone away
No connection. Trying to reconnect...
Connection id: 1697669
Current database: sysbench_testdata
+-------------+-------------------+-------------+------------+------------+------------+-------------+
| object_type | object_schema | object_name | index_name | count_star | count_read | COUNT_FETCH |
+-------------+-------------------+-------------+------------+------------+------------+-------------+
| TABLE | sysbench_testdata | sbtest2 | PRIMARY | 0 | 0 | 0 |
| TABLE | sysbench_testdata | sbtest2 | k_2 | 76287298 | 76287298 | 76287298 |
| TABLE | sysbench_testdata | sbtest2 | NULL | 8344631 | 8344631 | 8344631 |
+-------------+-------------------+-------------+------------+------------+------------+-------------+
3 rows in set (0.00 sec)
root@localhost [sysbench_testdata]>select k from sbtest2 where k=432 limit 2;
+-----+
| k |
+-----+
| 432 |
| 432 |
+-----+
2 rows in set (0.00 sec)
root@localhost [sysbench_testdata]>select object_type,object_schema,object_name,index_name,count_star,count_read,COUNT_FETCH from performance_schema.table_io_waits_summary_by_index_usage where object_name='sbtest2';
+-------------+-------------------+-------------+------------+------------+------------+-------------+
| object_type | object_schema | object_name | index_name | count_star | count_read | COUNT_FETCH |
+-------------+-------------------+-------------+------------+------------+------------+-------------+
| TABLE | sysbench_testdata | sbtest2 | PRIMARY | 0 | 0 | 0 |
| TABLE | sysbench_testdata | sbtest2 | k_2 | 76287300 | 76287300 | 76287300 |
| TABLE | sysbench_testdata | sbtest2 | NULL | 8344631 | 8344631 | 8344631 |
+-------------+-------------------+-------------+------------+------------+------------+-------------+
3 rows in set (0.01 sec)
root@localhost [sysbench_testdata]>select k from sbtest2 where id=432 limit 2;
+-------+
| k |
+-------+
| 49866 |
+-------+
1 row in set (0.00 sec)
root@localhost [sysbench_testdata]>select object_type,object_schema,object_name,index_name,count_star,count_read,COUNT_FETCH from performance_schema.table_io_waits_summary_by_index_usage where object_name='sbtest2';
+-------------+-------------------+-------------+------------+------------+------------+-------------+
| object_type | object_schema | object_name | index_name | count_star | count_read | COUNT_FETCH |
+-------------+-------------------+-------------+------------+------------+------------+-------------+
| TABLE | sysbench_testdata | sbtest2 | PRIMARY | 1 | 1 | 1 |
| TABLE | sysbench_testdata | sbtest2 | k_2 | 76287300 | 76287300 | 76287300 |
| TABLE | sysbench_testdata | sbtest2 | NULL | 8344631 | 8344631 | 8344631 |
+-------------+-------------------+-------------+------------+------------+------------+-------------+
3 rows in set (0.00 sec)
root@localhost [sysbench_testdata]>