在说明系统数据库之前，先来看下mysql在数据字典方面的演变历史：
mysql4.1 提供了information_schema 数据字典。从此可以很简单的用sql语句来检索需要的系统元数据了。
mysql5.5 提供了performance_schema 性能字典。 但是这个字典比较专业，一般人可能也就看看就不了了之了。
mysql5.7 提供了 sys系统数据库。 sys数据库里面包含了一系列的存储过程、自定义函数以及视图来帮助我们快速的了解系统的元数据信息。
sys系统数据库结合了information_schema和performance_schema的相关数据，让我们更加容易的检索元数据。 现在呢，我就示范下几种场景下如何快速的使用。
第一，
比如之前想要知道某个表是否存在与否，可以用以下两种方法：
a， 悲观的方法，写sql从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，乐观的方法，假设表存在，写一个存储过程：
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 ;
现在来调用：
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)
现在我们直接用sys数据库里面现有的存储过程来进行调用，
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，获取没有使用过的索引。
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， 检索指定数据库下面的表扫描信息，过滤出执行次数大于10的查询，
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， 同样继续上面的，过滤出有临时表的查询，
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)
可以看到上面查询详细的详细，再也不用执行show status 手工去过滤了。
第五， 检索执行次数排名前五的语句，
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)
示例我就写这么多了，详细的去看使用手册并且自己摸索去吧。
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