show create table a; +-------+..."/>
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MySQLl數(shù)據(jù)量不一樣，導(dǎo)致走不同的索引

發(fā)布時(shí)間：2020-06-11 13:18:32 來(lái)源：網(wǎng)絡(luò) 閱讀：1893 作者：corasql 欄目：MySQL數(shù)據(jù)庫(kù)

1、測(cè)試環(huán)境：MySQL 5.7.17

2、測(cè)試表結(jié)構(gòu)

mysql> show create table a;
+-------+--------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                         |
+-------+--------------------------------------------------------------------------------------------------------------------------------------+
| a     | CREATE TABLE `a` (
  `id` int(11) NOT NULL,
  `name` char(20) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=gbk |
+-------+--------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
mysql> show create table b;
+-------+------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                       |
+-------+------------------------------------------------------------------------------------------------------------------------------------+
| b     | CREATE TABLE `b` (
  `id` int(11) NOT NULL,
  `tx` char(20) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=gbk |
+-------+------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)


3、兩張表的數(shù)據(jù)量

mysql> select count(*) from a;
+----------+
| count(*) |
+----------+
|        7 |
+----------+
1 row in set (0.00 sec)
mysql> select count(*) from b;
+----------+
| count(*) |
+----------+
|       10 |
+----------+
1 row in set (0.00 sec)


4、查看執(zhí)行計(jì)劃

mysql> explain select name from a,b where a.id=b.id;
+----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+
| id | select_type | table | partitions | type   | possible_keys | key     | key_len | ref       | rows | filtered | Extra       |
+----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+
|  1 | SIMPLE      | a     | NULL       | ALL    | PRIMARY       | NULL    | NULL    | NULL      |    7 |   100.00 | NULL        |
|  1 | SIMPLE      | b     | NULL       | eq_ref | PRIMARY       | PRIMARY | 4       | apex.a.id |    1 |   100.00 | Using index |
+----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)
mysql> explain select name from a,b where b.id=a.id;
+----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+
| id | select_type | table | partitions | type   | possible_keys | key     | key_len | ref       | rows | filtered | Extra       |
+----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+
|  1 | SIMPLE      | a     | NULL       | ALL    | PRIMARY       | NULL    | NULL    | NULL      |    7 |   100.00 | NULL        |
|  1 | SIMPLE      | b     | NULL       | eq_ref | PRIMARY       | PRIMARY | 4       | apex.a.id |    1 |   100.00 | Using index |
+----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)
mysql> explain select name from b,a where b.id=a.id;
+----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+
| id | select_type | table | partitions | type   | possible_keys | key     | key_len | ref       | rows | filtered | Extra       |
+----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+
|  1 | SIMPLE      | a     | NULL       | ALL    | PRIMARY       | NULL    | NULL    | NULL      |    7 |   100.00 | NULL        |
|  1 | SIMPLE      | b     | NULL       | eq_ref | PRIMARY       | PRIMARY | 4       | apex.a.id |    1 |   100.00 | Using index |
+----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)



5、向a表插入3數(shù)據(jù)，使兩表數(shù)據(jù)量一樣，查看執(zhí)行計(jì)劃，發(fā)現(xiàn)第三條語(yǔ)句的執(zhí)行計(jì)劃發(fā)生了變化

mysql> insert into a values(8,'test');
Query OK, 1 row affected (0.00 sec)
mysql> insert into a values(9,'test');
Query OK, 1 row affected (0.00 sec)
mysql> insert into a values(10,'test');
Query OK, 1 row affected (0.01 sec)
mysql> select count(*) from a;
+----------+
| count(*) |
+----------+
|       10 |
+----------+
1 row in set (0.00 sec)
mysql> explain select name from a,b where a.id=b.id;
+----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+
| id | select_type | table | partitions | type   | possible_keys | key     | key_len | ref       | rows | filtered | Extra       |
+----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+
|  1 | SIMPLE      | a     | NULL       | ALL    | PRIMARY       | NULL    | NULL    | NULL      |   10 |   100.00 | NULL        |
|  1 | SIMPLE      | b     | NULL       | eq_ref | PRIMARY       | PRIMARY | 4       | apex.a.id |    1 |   100.00 | Using index |
+----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)
mysql> explain select name from a,b where b.id=a.id;
+----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+
| id | select_type | table | partitions | type   | possible_keys | key     | key_len | ref       | rows | filtered | Extra       |
+----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+
|  1 | SIMPLE      | a     | NULL       | ALL    | PRIMARY       | NULL    | NULL    | NULL      |   10 |   100.00 | NULL        |
|  1 | SIMPLE      | b     | NULL       | eq_ref | PRIMARY       | PRIMARY | 4       | apex.a.id |    1 |   100.00 | Using index |
+----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)
mysql> explain select name from b,a where b.id=a.id;
+----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+
| id | select_type | table | partitions | type   | possible_keys | key     | key_len | ref       | rows | filtered | Extra       |
+----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+
|  1 | SIMPLE      | b     | NULL       | index  | PRIMARY       | PRIMARY | 4       | NULL      |   10 |   100.00 | Using index |
|  1 | SIMPLE      | a     | NULL       | eq_ref | PRIMARY       | PRIMARY | 4       | apex.b.id |    1 |   100.00 | NULL        |
+----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)

6、向a表插入1條數(shù)據(jù)，使a表數(shù)據(jù)量大于b表，查看執(zhí)行計(jì)劃，三條語(yǔ)句執(zhí)行計(jì)劃都發(fā)現(xiàn)了變化

mysql> insert into a values(11,'test');
Query OK, 1 row affected (0.01 sec)
mysql> select count(*) from a;
+----------+
| count(*) |
+----------+
|       11 |
+----------+
1 row in set (0.00 sec)
mysql> explain select name from a,b where a.id=b.id;
+----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+
| id | select_type | table | partitions | type   | possible_keys | key     | key_len | ref       | rows | filtered | Extra       |
+----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+
|  1 | SIMPLE      | b     | NULL       | index  | PRIMARY       | PRIMARY | 4       | NULL      |   10 |   100.00 | Using index |
|  1 | SIMPLE      | a     | NULL       | eq_ref | PRIMARY       | PRIMARY | 4       | apex.b.id |    1 |   100.00 | NULL        |
+----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)
mysql> explain select name from a,b where b.id=a.id;
+----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+
| id | select_type | table | partitions | type   | possible_keys | key     | key_len | ref       | rows | filtered | Extra       |
+----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+
|  1 | SIMPLE      | b     | NULL       | index  | PRIMARY       | PRIMARY | 4       | NULL      |   10 |   100.00 | Using index |
|  1 | SIMPLE      | a     | NULL       | eq_ref | PRIMARY       | PRIMARY | 4       | apex.b.id |    1 |   100.00 | NULL        |
+----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)
mysql> explain select name from b,a where b.id=a.id;
+----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+
| id | select_type | table | partitions | type   | possible_keys | key     | key_len | ref       | rows | filtered | Extra       |
+----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+
|  1 | SIMPLE      | b     | NULL       | index  | PRIMARY       | PRIMARY | 4       | NULL      |   10 |   100.00 | Using index |
|  1 | SIMPLE      | a     | NULL       | eq_ref | PRIMARY       | PRIMARY | 4       | apex.b.id |    1 |   100.00 | NULL        |
+----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+
2 rows in set, 1 warning (0.01 sec)






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