mysql收集统计信息

一、手动  
执行Analyze table 
innodb和myisam存储引擎都可以通过执行“Analyze table tablename”来收集表的统计信息，除非执行计划不准确，否则不要轻易执行该操作，如果是很大的表该操作会影响表的性能。

二、自动触发
以下行为会自动触发统计信息的收集

1.第一次打开表的时候
2.表修改的行超过1/6或者20亿条时
3.当有新的记录插入时
4.执行show index from tablename或者执行show table stauts、查询information_schema.tablesstatistics 时


三、开启参数innodb_stats_on_metadata
当开启参数innodb_stats_on_metadata后访问以下表也会触发统计信息的收集
在访问以下表时，innodb表的统计信息可自动收集
information_schema.TABLES
information_schema.STATISTICS
information_schema.PARTITIONS
information_schema.KEY_COLUMN_USAGE
information_schema.TABLE_CONSTRAINTS
information_schema.REFERENTIAL_CONSTRAINTS
information_schema.table_constraints


参数说明：
Innodb_stats_sample_pages：每次收集统计信息时采样的页数，默认为20
innodb_stats_persistent：默认on，将analyze table产生的统计信息保存于磁盘，直至下次analyze table为止，此举避免了统计信息动态更新，保证了执行计划的稳定，对于大表也节省了收集统计信息的所需资源;

查询表的统计信息

mysql> select  TABLE_SCHEMA,table_name,column_name,CARDINALITY from information_schema.STATISTICS  b where  b.table_name='salaries';
+--------------+------------+-------------+-------------+
| TABLE_SCHEMA | table_name | column_name | CARDINALITY |
+--------------+------------+-------------+-------------+
| employees    | salaries   | emp_no      |     2843953 |
| employees    | salaries   | from_date   |     2843953 |
+--------------+------------+-------------+-------------+
2 rows in set (0.00 sec)
mysql> show index from employees.salaries;
+----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table    | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| salaries |          0 | PRIMARY  |            1 | emp_no      | A         |     2843953 |     NULL | NULL   |      | BTREE      |         |               |
| salaries |          0 | PRIMARY  |            2 | from_date   | A         |     2843953 |     NULL | NULL   |      | BTREE      |         |               |
+----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
2 rows in set (0.00 sec)

mysql> select table_rows from information_schema.tables where table_name='salaries';
+------------+
| table_rows |
+------------+
|    2843953 |
+------------+
1 row in set (0.00 sec)
mysql> select count(1)  from  employees.salaries;
+----------+
| count(1) |
+----------+
|  2844047 |
+----------+
1 row in set (0.52 sec)
表的真实数据为 2844047
手动分析下表在看统计信息
mysql> analyze table employees.salaries;
+--------------------+---------+----------+----------+
| Table              | Op      | Msg_type | Msg_text |
+--------------------+---------+----------+----------+
| employees.salaries | analyze | status   | OK       |
+--------------------+---------+----------+----------+
1 row in set (0.50 sec)

mysql> select table_rows from information_schema.tables where table_name='salaries';
+------------+
| table_rows |
+------------+
|    2838489 |
+------------+
1 row in set (0.01 sec)

mysql> select  TABLE_SCHEMA,table_name,column_name,CARDINALITY from information_schema.STATISTICS  b where  b.table_name='salaries';
+--------------+------------+-------------+-------------+
| TABLE_SCHEMA | table_name | column_name | CARDINALITY |
+--------------+------------+-------------+-------------+
| employees    | salaries   | emp_no      |     2838489 |
| employees    | salaries   | from_date   |     2838489 |
+--------------+------------+-------------+-------------+
2 rows in set (0.00 sec)

mysql> show index from employees.salaries;
+----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table    | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| salaries |          0 | PRIMARY  |            1 | emp_no      | A         |     2838489 |     NULL | NULL   |      | BTREE      |         |               |
| salaries |          0 | PRIMARY  |            2 | from_date   | A         |     2838489 |     NULL | NULL   |      | BTREE      |         |               |
+----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
2 rows in set (0.00 sec)
统计信息和真实数据一致