数据库内核月报 - 2019 / 06

MySQL · 引擎特性 · CHECK CONSTRAINT

即使MySQL8.0已经GA了,官方仍然在向其中增加新的功能,比如在最新的MySQL8.0.16版本中,增加了一个众望所归的功能:CHECK CONSTRAINT,也就是说可以自动对写入的数据进行约束检查。这个特性的worklog号码为929,已经是十几年前的需求了,终于在8.0实现了。(实际上这也是标准SQL功能,像PostgreSQL, Oracle等都有这个功能)

本文简单阐述下其使用方式以及相关实现

如何使用

其实在之前的版本中,已经实现了标准语法 CHECK(expr), 但是实际上是被忽略掉的,在新版本中,可以在列或者表上做一些约束条件,语法如下:

1.如下是表级约束

[CONSTRAINT [symbol]] CHECK (expr) [[NOT] ENFORCED]

其中symbol用来命名约束条件的唯一名字,如果没有指定的话,Mysql也会自动生成约束名,但要注意,在同一个库下面,约束名字不能重复,必须具有唯一性 expr是一个表达式,结果为bool类型 enforced是默认选定的,你也可以手动选定,表示必须满足约束条件才允许写入, 但如果选择NOT ENFORCED的话,则表示约束条件虽然创建了,但并不强制

2.如下是列级别约束, 可以在创建列的时候同时指定约束条件

CHECK (expr)

3.示例:

我们举个简单的例子:

  root@test 10:23:42>CREATE TABLE t1
  -> (
      ->   CHECK (c1 <> c2),
      ->   c1 INT CHECK (c1 > 10),
      ->   c2 INT CONSTRAINT c2_positive CHECK (c2 > 0),
      ->   c3 INT CHECK (c3 < 100),
      ->   CONSTRAINT c1_nonzero CHECK (c1 <> 0),
      ->   CHECK (c1 > c3)
      -> );
  Query OK, 0 rows affected (0.01 sec)

  root@test 10:23:58>SHOW CREATE TABLE t1\G
  *************************** 1. row ***************************
  Table: t1
  Create Table: CREATE TABLE `t1` (
      `c1` int(11) DEFAULT NULL,
      `c2` int(11) DEFAULT NULL,
      `c3` int(11) DEFAULT NULL,
      CONSTRAINT `c1_nonzero` CHECK ((`c1` <> 0)),
      CONSTRAINT `c2_positive` CHECK ((`c2` > 0)),
      CONSTRAINT `t1_chk_1` CHECK ((`c1` <> `c2`)),
      CONSTRAINT `t1_chk_2` CHECK ((`c1` > 10)),
      CONSTRAINT `t1_chk_3` CHECK ((`c3` < 100)),
      CONSTRAINT `t1_chk_4` CHECK ((`c1` > `c3`))
      ) ENGINE=InnoDB DEFAULT CHARSET=latin1
  1 row in set (0.01 sec)


  # 违反了约束条件t1_chk_1, 即c1 != c2
  root@test 10:24:07>INSERT INTO t1 VALUES (1,1,1);
  ERROR 3819 (HY000): Check constraint 't1_chk_1' is violated.

既然约束名必须唯一,那如果我们把t1 rename成t2, 再新建一个t1会怎么样呢 ?

  root@test 10:25:35>rename table t1 to t2;
  Query OK, 0 rows affected (0.01 sec)

  root@test 10:25:37>show create table t2\G
  *************************** 1. row ***************************
  Table: t2
  Create Table: CREATE TABLE `t2` (
      `c1` int(11) DEFAULT NULL,
      `c2` int(11) DEFAULT NULL,
      `c3` int(11) DEFAULT NULL,
      CONSTRAINT `c1_nonzero` CHECK ((`c1` <> 0)),
      CONSTRAINT `c2_positive` CHECK ((`c2` > 0)),
      CONSTRAINT `t2_chk_1` CHECK ((`c1` <> `c2`)),
      CONSTRAINT `t2_chk_2` CHECK ((`c1` > 10)),
      CONSTRAINT `t2_chk_3` CHECK ((`c3` < 100)),
      CONSTRAINT `t2_chk_4` CHECK ((`c1` > `c3`))
      ) ENGINE=InnoDB DEFAULT CHARSET=latin1
  1 row in set (0.00 sec)

  root@test 10:25:40>create table t1 like t2;
  Query OK, 0 rows affected (0.00 sec)

  root@test 10:25:51>show create table t1;
  +-------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
  | Table | Create Table                                                                                                                                                                                                                                                                                                                                                                                                                        |
  +-------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
  | t1    | CREATE TABLE `t1` (
      `c1` int(11) DEFAULT NULL,
      `c2` int(11) DEFAULT NULL,
      `c3` int(11) DEFAULT NULL,
      CONSTRAINT `t1_chk_1` CHECK ((`c1` <> 0)),
      CONSTRAINT `t1_chk_2` CHECK ((`c2` > 0)),
      CONSTRAINT `t1_chk_3` CHECK ((`c1` <> `c2`)),
      CONSTRAINT `t1_chk_4` CHECK ((`c1` > 10)),
      CONSTRAINT `t1_chk_5` CHECK ((`c3` < 100)),
      CONSTRAINT `t1_chk_6` CHECK ((`c1` > `c3`))
      ) ENGINE=InnoDB DEFAULT CHARSET=latin1 |

可以看到t1 rename成t2后,其中自动生成的约束名被更改成t2_前缀的。而创建的t2 like t1表, 依然完整的继承了原来t1表的约束条件,但命名全部改成了t1_前缀的。

你也可以通过alter table的方式增加或删除约束:

  root@test 10:45:08>ALTER TABLE t1 ADD CONSTRAINT check ((c1 + c2 + c3) > 1000);
  Query OK, 0 rows affected (0.02 sec)
  Records: 0  Duplicates: 0  Warnings: 0

  root@test 10:45:39>INSERT INTO t1 values (1999, 50, 90);
  Query OK, 1 row affected (0.00 sec)

  root@test 10:46:23>INSERT INTO t1 values (100, 50, 90);
  ERROR 3819 (HY000): Check constraint 't1_chk_7' is violated.

  root@test 10:46:53>ALTER TABLE t1 DROP CHECK t1_chk_7;
  Query OK, 0 rows affected (0.01 sec)
  Records: 0  Duplicates: 0  Warnings: 0

  root@test 10:47:17>INSERT INTO t1 values (100, 50, 90);
  Query OK, 1 row affected (0.00 sec)

InnoDB新增了一个data dictionary表check_constrains, 你可以从information_schema表下面查询:

  root@(none) 10:54:05>SELECT * FROM INFORMATION_SCHEMA.CHECK_CONSTRAINTS;
  +--------------------+-------------------+-----------------+----------------+
  | CONSTRAINT_CATALOG | CONSTRAINT_SCHEMA | CONSTRAINT_NAME | CHECK_CLAUSE   |
  +--------------------+-------------------+-----------------+----------------+
  | def                | test              | t2_chk_1        | (`c1` <> `c2`) |
  | def                | test              | t2_chk_2        | (`c1` > 10)    |
  | def                | test              | c2_positive     | (`c2` > 0)     |
  | def                | test              | t2_chk_3        | (`c3` < 100)   |
  | def                | test              | c1_nonzero      | (`c1` <> 0)    |
  | def                | test              | t2_chk_4        | (`c1` > `c3`)  |
  | def                | test              | t1_chk_1        | (`c1` <> 0)    |
  | def                | test              | t1_chk_2        | (`c2` > 0)     |
  | def                | test              | t1_chk_3        | (`c1` <> `c2`) |
  | def                | test              | t1_chk_4        | (`c1` > 10)    |
  | def                | test              | t1_chk_5        | (`c3` < 100)   |
  | def                | test              | t1_chk_6        | (`c1` > `c3`)  |
  +--------------------+-------------------+-----------------+----------------+
12 rows in set (0.00 sec)

相关实现

1.新增代码文件

sql/sql_check_constraint.cc

sql/dd/impl/system_views/check_constraints.cc

sql/dd/impl/types/check_constraint_impl.cc

2.表达式定义及存储

InnoDB新增了一个数据词典表mysql.check_constraints用来存储所有的约束条件,表的定义在文件sql/dd/impl/tables/check_constraints.cc中, 相关堆栈

  mysql_execute_command
  |-> Sql_cmd_create_table::execute 
  |-> mysql_create_table
  |-> prepare_check_constraints_for_create
  |--> generate_check_constraint_name  //自动生成constraint名字
  |-> mysql_create_table_no_lock -> create_table_impl -> rea_create_base_table 
  |-> dd::cache::Dictionary_client::store
  .....
  |-> dd::Collection<dd::Check_constraint*>::store_items
  |-> d::Check_constraint_impl::store   // 存储到数据词典表中

3.载入内存及显示

先存储到dd::Table中,当打开table share时,拷贝到TABLE_SHARE::check_constraint_share_list

  open_table
  |-> get_table_share_with_discover
  |-> get_table_share
  |-> dd::cache::Dictionary_client::acquire   //去dd获取uncached的表上的定义,存储到dd:Table中
  |-> open_table_def   // 构建table share表定义
  |--> fill_check_constraints_from_dd  // 将约束条件拷贝到table share中

在每次实例化线程可操作的TABLE对象时,再从table share中读取,存储到TABLE::table_check_constraint_list中 参考函数:open_table_from_share

4.检查约束

每次插入或修改数据,都需要检查对应的约束条件 参考函数: invoke_table_check_constraints

参考文档

MySQL 8.0.16 Introducing CHECK constraint

WL#929: CHECK constraints

官方文档

相关代码