MySQL 5.7以後怎麼檢視索引使用情況?

1. 通過show status like '%Handler_read%'方法檢視:整體的

[email protected] [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效能改善不高，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。這個值是指在資料檔案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.檢視具體某一個sql的索引使用情況 :

[email protected] [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)

 

欄位說明:
Type：告訴我們對錶所使用的訪問方式，主要包含如下集中型別；
◇ all：全表掃描
◇ const：讀常量，且最多隻會有一條記錄匹配，由於是常量，所以實際上只需要讀一次；
◇ eq_ref：最多隻會有一條匹配結果，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，然後對索引結果進行merge 之後再讀取表資料；
◇ index_subquery：子查詢中的返回結果欄位組合是一個索引（或索引組合），但不是一個主鍵或者唯一索引；
◇ rang：索引範圍掃描；
◇ 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，表中只有一行資料；
◇ unique_subquery：子查詢中的返回結果欄位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。這裡的索引名字是建立索引時指定的索引暱稱；如果索引沒有暱稱，則預設顯示的是索引中第一個列的名字（在本例中，它是“firstname”）。預設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，以位元組計

ref：列出是通過常量（const），還是某個表的某個欄位（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，這裡最理想的數字就是1。

3.通過performance_schema可以查詢到.檢視sbtest2表索引情況的查詢語句:

[email protected] [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';

• 具體檢視過程:

[email protected] [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)

[email protected] [sysbench_testdata]>select k from sbtest2 where k=432 limit 2;                                                                                                       
+-----+
| k   |
+-----+
| 432 |
| 432 |
+-----+
2 rows in set (0.00 sec)

[email protected] [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)

[email protected] [sysbench_testdata]>select k from sbtest2 where id=432 limit 2;                                                                                                               
+-------+
| k     |
+-------+
| 49866 |
+-------+
1 row in set (0.00 sec)

[email protected] [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)

[email protected] [sysbench_testdata]>