前言
不管是Oracle还是MySQL，新版本推出的新特性，一方面给产品带来功能、性能、用户体验等方面的提升，另一方面也可能会带来一些问题，如代码bug、客户使用方法不正确引发问题等等。
案例分享
MySQL 5.7下的场景
（1）首先，创建两张表，并插入数据
mysql> select version();
+------------+
| version() |
+------------+
| 5.7.30-log |
+------------+
1 row in set (0.00 sec)
mysql> show create table test\G
*************************** 1. row ***************************
Table: test
Create Table: CREATE TABLE `test` (
`id` int(10) unsigned NOT NULL AUTO_INCREMENT,
`k` int(10) unsigned NOT NULL DEFAULT '0',
`c` char(120) NOT NULL DEFAULT '',
`pad` char(60) NOT NULL DEFAULT '',
PRIMARY KEY (`id`),
KEY `k_1` (`k`)
) ENGINE=InnoDB AUTO_INCREMENT=101 DEFAULT CHARSET=utf8mb4 MAX_ROWS=1000000
1 row in set (0.00 sec)
mysql> show create table sbtest1\G
*************************** 1. row ***************************
Table: sbtest1
Create Table: CREATE TABLE `sbtest1` (
`id` int(10) unsigned NOT NULL AUTO_INCREMENT,
`k` int(10) unsigned NOT NULL DEFAULT '0',
`c` char(120) NOT NULL DEFAULT '',
`pad` char(60) NOT NULL DEFAULT '',
PRIMARY KEY (`id`),
KEY `k_1` (`k`)
) ENGINE=InnoDB AUTO_INCREMENT=1000001 DEFAULT CHARSET=utf8mb4 MAX_ROWS=1000000
1 row in set (0.00 sec)
mysql> select count(*) from test;
+----------+
| count(*) |
+----------+
| 100 |
+----------+
1 row in set (0.00 sec)
mysql> select count(*) from sbtest1;
+----------+
| count(*) |
+----------+
| 1000000 |
+----------+
1 row in set (0.14 sec)
（2）查看两张表的统计信息，均比较准确
mysql> select table_schema,table_name,table_rows from tables where table_name='test'; +--------------+------------+------------+ | table_schema | table_name | table_rows | +--------------+------------+------------+ | test | test | 100 | +--------------+------------+------------+ 1 row in set (0.00 sec) mysql> select table_schema,table_name,table_rows from tables where table_name='sbtest1'; +--------------+------------+------------+ | table_schema | table_name | table_rows | +--------------+------------+------------+ | test | sbtest1 | 947263 | +--------------+------------+------------+ 1 row in set (0.00 sec)
（3）我们持续往test表插入1000w条记录，并再次查看统计信息，还是相对准确的，因为在默认情况下，数据变化量超过10%，就会触发统计信息更新
mysql> select count(*) from test; +----------+ | count(*) | +----------+ | 10000100 | +----------+ 1 row in set (1.50 sec) mysql> select table_schema,table_name,table_rows from tables where table_name='test'; +--------------+------------+------------+ | table_schema | table_name | table_rows | +--<div style="color:transparent">本文来源gaodai^.ma#com搞#代!码网</div>------------+------------+------------+ | test | test | 9749036 | +--------------+------------+------------+ 1 row in set (0.00 sec)
MySQL 8.0下的场景
（1）接下来我们看看8.0下的情况吧，同样地，我们创建两张表，并插入相同记录
mysql> select version();
+-----------+
| version() |
+-----------+
| 8.0.20 |
+-----------+
1 row in set (0.00 sec)
mysql> show create table test\G
*************************** 1. row ***************************
Table: test
Create Table: CREATE TABLE `test` (
`id` int unsigned NOT NULL AUTO_INCREMENT,
`k` int unsigned NOT NULL DEFAULT '0',
`c` char(120) NOT NULL DEFAULT '',
`pad` char(60) NOT NULL DEFAULT '',
PRIMARY KEY (`id`),
KEY `k_1` (`k`)
) ENGINE=InnoDB AUTO_INCREMENT=101 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci MAX_ROWS=1000000
1 row in set (0.00 sec)
mysql> show create table sbtest1\G
*************************** 1. row ***************************
Table: sbtest1
Create Table: CREATE TABLE `sbtest1` (
`id` int unsigned NOT NULL AUTO_INCREMENT,
`k` int unsigned NOT NULL DEFAULT '0',
`c` char(120) NOT NULL DEFAULT '',
`pad` char(60) NOT NULL DEFAULT '',
PRIMARY KEY (`id`),
KEY `k_1` (`k`)
) ENGINE=InnoDB AUTO_INCREMENT=1000001 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci MAX_ROWS=1000000
1 row in set (0.00 sec)
mysql> select count(*) from test;
+----------+
| count(*) |
+----------+
| 100 |
+----------+
1 row in set (0.00 sec)
mysql> select count(*) from sbtest1;
+----------+
| count(*) |
+----------+
| 1000000 |
+----------+
1 row in set (0.02 sec)
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转载请注明原文链接：MySQL 8.0统计信息不准确的原因
转载请注明原文链接：MySQL 8.0统计信息不准确的原因
