问题
I create a new table in Mysql, add some rows, yet the Auto_increment field of show tables still returns NULL.
The mysql manual says: this field should return: "The next Auto_increment value"
https://dev.mysql.com/doc/refman/8.0/en/show-table-status.html
What am I doing wrong?
How can I find the next auto_increment value correctly?
Steps to reproduce:
create table `test` (
`id` int(5) not null auto_increment,
`name` varchar(256),
PRIMARY KEY(`id`)
);
Then I run :
show table status where name like 'test';
Result:
Name, Engine, Version, ..., Auto_increment, ...
'test', 'InnoDB', '10', ..., NULL, ...
Then I run:
insert into test values(null,'name1');
insert into test values(null,'name2');
insert into test values(null,'name3');
Edit: -other insert syntax-
insert into test (name) values('name4');
insert into test (name) values('name5');
insert into test (name) values('name6');
Fetch the status of the table
show table status where name like 'test';
Result
Name, Engine, Version, ..., Auto_increment, ...
'test', 'InnoDB', '10', ..., NULL, ...
Data in the table
select * from test;
Result:
1 name1
2 name2
3 name3
For your information:
SHOW VARIABLES LIKE "%version%";
Result:
'innodb_version', '8.0.12'
'protocol_version', '10'
'slave_type_conversions', ''
'tls_version', 'TLSv1,TLSv1.1,TLSv1.2'
'version', '8.0.12'
'version_comment', 'MySQL Community Server - GPL'
'version_compile_machine', 'x86_64'
'version_compile_os', 'Win64'
'version_compile_zlib', '1.2.11'
edit: autocommit:
SHOW VARIABLES LIKE "autocommit";
result:
'autocommit', 'ON'
Edit:
After a while it automagically starts working. No clear reason how to make it start working.
回答1:
It's a feature.. not a bug.
The table statistics are cached. To disable the cache and always have the latest version you should change the server variable that indicates the duration of the cache-clear to 0:
SET PERSIST information_schema_stats_expiry = 0
The default value of this property has changed to 86400 (24 hours) in Mysql 8.x
Example:
SET PERSIST information_schema_stats_expiry = 86400
-- 86400 is the default value of mysql 8.x if you have never changed this you don't need to set this
show variables like 'information_schema_stats_expiry';
+---------------------------------+-------+
| Variable_name | Value |
+---------------------------------+-------+
| information_schema_stats_expiry | 86400 |
+---------------------------------+-------+
create schema mytest;
create table `test` (
`id` int(5) not null auto_increment,
`name` varchar(256),
PRIMARY KEY(`id`)
);
insert into test values(null,'name1')
insert into test values(null,'name2')
insert into test values(null,'name3')
show table status where name like 'test';
+------+--------+---------+------------+------+----------------+-------------+-----------------+--------------+-----------+----------------+---------------------+---------------------+------------+--------------------+----------+----------------+---------+
| Name | Engine | Version | Row_format | Rows | Avg_row_length | Data_length | Max_data_length | Index_length | Data_free | Auto_increment | Create_time | Update_time | Check_time | Collation | Checksum | Create_options | Comment |
+------+--------+---------+------------+------+----------------+-------------+-----------------+--------------+-----------+----------------+---------------------+---------------------+------------+--------------------+----------+----------------+---------+
| test | InnoDB | 10 | Dynamic | 3 | 5461 | 16384 | 0 | 0 | 0 | 4 | 2018-10-09 15:32:15 | 2018-10-09 15:32:16 | NULL | utf8mb4_0900_ai_ci | NULL | | |
+------+--------+---------+------------+------+----------------+-------------+-----------------+--------------+-----------+----------------+---------------------+---------------------+------------+--------------------+----------+----------------+---------+
-- The Auto_increment field is correctly set to 4.. but is now cached.
insert into test values(null,'name3');
show table status where name like 'test';
+------+--------+---------+------------+------+----------------+-------------+-----------------+--------------+-----------+----------------+---------------------+---------------------+------------+--------------------+----------+----------------+---------+
| Name | Engine | Version | Row_format | Rows | Avg_row_length | Data_length | Max_data_length | Index_length | Data_free | Auto_increment | Create_time | Update_time | Check_time | Collation | Checksum | Create_options | Comment |
+------+--------+---------+------------+------+----------------+-------------+-----------------+--------------+-----------+----------------+---------------------+---------------------+------------+--------------------+----------+----------------+---------+
| test | InnoDB | 10 | Dynamic | 3 | 5461 | 16384 | 0 | 0 | 0 | 4 | 2018-10-09 15:32:15 | 2018-10-09 15:32:16 | NULL | utf8mb4_0900_ai_ci | NULL | | |
+------+--------+---------+------------+------+----------------+-------------+-----------------+--------------+-----------+----------------+---------------------+---------------------+------------+--------------------+----------+----------------+---------+
-- The Auto_increment is still 4 (it was cached).
drop schema mytest
Now we change the configuration:
SET PERSIST information_schema_stats_expiry = 0
and we run the same test:
show variables like 'information_schema_stats_expiry'
+---------------------------------+-------+
| Variable_name | Value |
+---------------------------------+-------+
| information_schema_stats_expiry | 0 |
+---------------------------------+-------+
create schema mytest;
create table `test` (
`id` int(5) not null auto_increment,
`name` varchar(256),
PRIMARY KEY(`id`)
);
insert into test values(null,'name1');
insert into test values(null,'name2');
insert into test values(null,'name3');
show table status where name like 'test';
+------+--------+---------+------------+------+----------------+-------------+-----------------+--------------+-----------+----------------+---------------------+---------------------+------------+--------------------+----------+----------------+---------+
| Name | Engine | Version | Row_format | Rows | Avg_row_length | Data_length | Max_data_length | Index_length | Data_free | Auto_increment | Create_time | Update_time | Check_time | Collation | Checksum | Create_options | Comment |
+------+--------+---------+------------+------+----------------+-------------+-----------------+--------------+-----------+----------------+---------------------+---------------------+------------+--------------------+----------+----------------+---------+
| test | InnoDB | 10 | Dynamic | 3 | 5461 | 16384 | 0 | 0 | 0 | 4 | 2018-10-09 15:32:49 | 2018-10-09 15:32:49 | NULL | utf8mb4_0900_ai_ci | NULL | | |
+------+--------+---------+------------+------+----------------+-------------+-----------------+--------------+-----------+----------------+---------------------+---------------------+------------+--------------------+----------+----------------+---------+
-- Auto_increment is 4, but the result is not cached!
insert into test values(null,'name3');
show table status where name like 'test';
+------+--------+---------+------------+------+----------------+-------------+-----------------+--------------+-----------+----------------+---------------------+---------------------+------------+--------------------+----------+----------------+---------+
| Name | Engine | Version | Row_format | Rows | Avg_row_length | Data_length | Max_data_length | Index_length | Data_free | Auto_increment | Create_time | Update_time | Check_time | Collation | Checksum | Create_options | Comment |
+------+--------+---------+------------+------+----------------+-------------+-----------------+--------------+-----------+----------------+---------------------+---------------------+------------+--------------------+----------+----------------+---------+
| test | InnoDB | 10 | Dynamic | 4 | 4096 | 16384 | 0 | 0 | 0 | 5 | 2018-10-09 15:32:49 | 2018-10-09 15:32:49 | NULL | utf8mb4_0900_ai_ci | NULL | | |
+------+--------+---------+------------+------+----------------+-------------+-----------------+--------------+-----------+----------------+---------------------+---------------------+------------+--------------------+----------+----------------+---------+
-- The Auto_increment field is now 5 (a correct, not cached value)
drop schema mytest;
来源：https://stackoverflow.com/questions/52705477/mysql-show-table-status-auto-increment-is-not-correct