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MySQL5.7SYS系統SCHEMA

編輯：MySQL綜合教程

MySQL5.7SYS系統SCHEMA

在說明系統數據庫之前，先來看下MySQL在數據字典方面的演變歷史：
MySQL4.1 提供了information_schema 數據字典。從此可以很簡單的用SQL語句來檢索需要的系統元數據了。
MySQL5.5 提供了performance_schema 性能字典。但是這個字典比較專業，一般人可能也就看看就不了了之了。
MySQL5.7 提供了 sys系統數據庫。 sys數據庫裡面包含了一系列的存儲過程、自定義函數以及視圖來幫助我們快速的了解系統的元數據信息。

sys系統數據庫結合了information_schema和performance_schema的相關數據，讓我們更加容易的檢索元數據。現在呢，我就示范下幾種場景下如何快速的使用。

第一，
比如之前想要知道某個表是否存在與否，可以用以下兩種方法：
A，悲觀的方法，寫SQL從information_schema中拿信息：

mysql> SELECT IF(COUNT(*) = 0,'Not exists!','Exists!') AS 'result' FROM information_schema.tables WHERE table_schema = 'new_feature' AND table_name = 't1';
+-------------+
| result      |
+-------------+
| Not exists! |
+-------------+
1 row in set (0.00 sec)

B，樂觀的方法，假設表存在，寫一個存儲過程：

DELIMITER $$
USE `new_feature`$$


DROP PROCEDURE IF EXISTS `sp_table_exists`$$


CREATE DEFINER=`root`@`localhost` PROCEDURE `sp_table_exists`(
    IN db_name VARCHAR(64),
    IN tb_name VARCHAR(64),
    OUT is_exists VARCHAR(60)
    )
BEGIN
      DECLARE no_such_table CONDITION FOR 1146;
      DECLARE EXIT HANDLER FOR no_such_table
      BEGIN
        SET is_exists = 'Not exists!';
      END;
      
      SET @stmt = CONCAT('select 1 from ',db_name,'.',tb_name);
      PREPARE s1 FROM @stmt;
      EXECUTE s1;
      DEALLOCATE PREPARE s1;
      SET is_exists = 'Exists!';
    END$$

DELIMITER ;

現在來調用：

mysql> call sp_table_exists('new_feature','t1',@result);
Query OK, 0 rows affected (0.00 sec)

mysql> select @result;
+-------------+
| @result     |
+-------------+
| Not exists! |
+-------------+
1 row in set (0.00 sec)

現在我們直接用sys數據庫裡面現有的存儲過程來進行調用，

mysql> CALL table_exists('new_feature','t1',@v_is_exists);
Query OK, 0 rows affected (0.00 sec)


mysql> SELECT IF(@v_is_exists = '','Not exists!',@v_is_exists) AS 'result';
+-------------+
| result      |
+-------------+
| Not exists! |
+-------------+
1 row in set (0.00 sec)

第二，獲取沒有使用過的索引。

mysql> SELECT * FROM schema_unused_indexes;
+---------------+-------------+--------------+
| object_schema | object_name | index_name   |
+---------------+-------------+--------------+
| new_feature   | t1          | idx_log_time |
| new_feature   | t1          | idx_rank2    |
+---------------+-------------+--------------+
2 rows in set (0.00 sec)

第三，檢索指定數據庫下面的表掃描信息，過濾出執行次數大於10的查詢，

mysql> SELECT * FROM statement_analysis WHERE db='new_feature' AND full_scan = '*'  AND exec_count > 10\G
*************************** 1. row ***************************
            query: SHOW STATUS 
               db: new_feature
        full_scan: *
       exec_count: 26
        err_count: 0
       warn_count: 0
    total_latency: 74.68 ms
      max_latency: 3.86 ms
      avg_latency: 2.87 ms
     lock_latency: 4.50 ms
        rows_sent: 9594
    rows_sent_avg: 369
    rows_examined: 9594
rows_examined_avg: 369
    rows_affected: 0
rows_affected_avg: 0
       tmp_tables: 0
  tmp_disk_tables: 0
      rows_sorted: 0
sort_merge_passes: 0
           digest: 475fa3ad9d4a846cfa96441050fc9787
       first_seen: 2015-11-16 10:51:17
        last_seen: 2015-11-16 11:28:13
*************************** 2. row ***************************
            query: SELECT `state` , `round` ( SUM ... uration (summed) in sec` DESC 
               db: new_feature
        full_scan: *
       exec_count: 12
        err_count: 0
       warn_count: 12
    total_latency: 16.43 ms
      max_latency: 2.39 ms
      avg_latency: 1.37 ms
     lock_latency: 3.54 ms
        rows_sent: 140
    rows_sent_avg: 12
    rows_examined: 852
rows_examined_avg: 71
    rows_affected: 0
rows_affected_avg: 0
       tmp_tables: 24
  tmp_disk_tables: 0
      rows_sorted: 140
sort_merge_passes: 0
           digest: 538e506ee0075e040b076f810ccb5f5c
       first_seen: 2015-11-16 10:51:17
        last_seen: 2015-11-16 11:28:13
2 rows in set (0.01 sec)

第四，同樣繼續上面的，過濾出有臨時表的查詢，

mysql> SELECT * FROM statement_analysis WHERE db='new_feature' AND tmp_tables > 0 ORDER BY tmp_tables DESC LIMIT 1\G
*************************** 1. row ***************************
            query: SELECT `performance_schema` .  ... name` . `SUM_TIMER_WAIT` DESC 
               db: new_feature
        full_scan: *
       exec_count: 2
        err_count: 0
       warn_count: 0
    total_latency: 87.96 ms
      max_latency: 59.50 ms
      avg_latency: 43.98 ms
     lock_latency: 548.00 us
        rows_sent: 101
    rows_sent_avg: 51
    rows_examined: 201
rows_examined_avg: 101
    rows_affected: 0
rows_affected_avg: 0
       tmp_tables: 332
  tmp_disk_tables: 15
      rows_sorted: 0
sort_merge_passes: 0
           digest: ff9bdfb7cf3f44b2da4c52dcde7a7352
       first_seen: 2015-11-16 10:24:42
        last_seen: 2015-11-16 10:24:42
1 row in set (0.01 sec)

可以看到上面查詢詳細的詳細，再也不用執行show status 手工去過濾了。

第五，檢索執行次數排名前五的語句，

mysql> SELECT statement,total FROM user_summary_by_statement_type WHERE `user`='root' ORDER BY total DESC LIMIT 5;
+-------------------+-------+
| statement         | total |
+-------------------+-------+
| jump_if_not       | 17635 |
| freturn           |  3120 |
| show_create_table |   289 |
| Field List        |   202 |
| set_option        |   190 |
+-------------------+-------+
5 rows in set (0.01 sec)

示例我就寫這麼多了，詳細的去看使用手冊並且自己摸索去吧。