tidb: add description about TIDB_CHECK_CONSTRAINTS

TOC.md

@@ -1000,6 +1000,7 @@
         - [`TABLES`](/information-schema/information-schema-tables.md)
         - [`TABLE_CONSTRAINTS`](/information-schema/information-schema-table-constraints.md)
         - [`TABLE_STORAGE_STATS`](/information-schema/information-schema-table-storage-stats.md)
+        - [`TIDB_CHECK_CONSTRAINTS`](/information-schema/information-schema-tidb-check-constraints.md)
         - [`TIDB_HOT_REGIONS`](/information-schema/information-schema-tidb-hot-regions.md)
         - [`TIDB_HOT_REGIONS_HISTORY`](/information-schema/information-schema-tidb-hot-regions-history.md)
         - [`TIDB_INDEXES`](/information-schema/information-schema-tidb-indexes.md)

information-schema/information-schema-check-constraints.md

@@ -29,6 +29,7 @@ DESC CHECK_CONSTRAINTS;
 下述示例使用 `CREATE TABLE` 语句添加 `CHECK` 约束：
 
 ```sql
+SET GLOBAL tidb_enable_check_constraint = ON;
 CREATE TABLE test.t1 (id INT PRIMARY KEY, CHECK (id%2 = 0));
 SELECT * FROM CHECK_CONSTRAINTS\G
 ```

information-schema/information-schema-tidb-check-constraints.md

@@ -0,0 +1,59 @@
+---
+title: TIDB_CHECK_CONSTRAINTS
+summary: 了解 INFORMATION_SCHEMA 表 `TIDB_CHECK_CONSTRAINTS`。
+---
+
+# TIDB\_CHECK\_CONSTRAINTS
+
+`TIDB_CHECK_CONSTRAINTS` 表提供关于表上 [`CHECK` 约束](/constraints.md#check-约束)的信息。除了包含 [`CHECK_CONSTRAINTS`](/information-schema/information-schema-check-constraints.md) 表中的信息之外，`TIDB_CHECK_CONSTRAINTS` 还提供了定义 `CHECK` 约束的表名和表 ID。
+
+```sql
+USE INFORMATION_SCHEMA;
+DESC TIDB_CHECK_CONSTRAINTS;
+```
+
+命令输出如下：
+
+```sql
++--------------------+-------------+------+------+---------+-------+
+| Field              | Type        | Null | Key  | Default | Extra |
++--------------------+-------------+------+------+---------+-------+
+| CONSTRAINT_CATALOG | varchar(64) | NO   |      | NULL    |       |
+| CONSTRAINT_SCHEMA  | varchar(64) | NO   |      | NULL    |       |
+| CONSTRAINT_NAME    | varchar(64) | NO   |      | NULL    |       |
+| CHECK_CLAUSE       | longtext    | NO   |      | NULL    |       |
+| TABLE_NAME         | varchar(64) | YES  |      | NULL    |       |
+| TABLE_ID           | bigint(21)  | YES  |      | NULL    |       |
++--------------------+-------------+------+------+---------+-------+
+6 rows in set (0.00 sec)
+```
+
+下述示例使用 `CREATE TABLE` 语句添加 `CHECK` 约束：
+
+```sql
+SET GLOBAL tidb_enable_check_constraint = ON;
+CREATE TABLE test.t1 (id INT PRIMARY KEY, CHECK (id%2 = 0));
+SELECT * FROM TIDB_CHECK_CONSTRAINTS\G
+```
+
+命令输出如下：
+
+```sql
+*************************** 1. row ***************************
+CONSTRAINT_CATALOG: def
+ CONSTRAINT_SCHEMA: test
+   CONSTRAINT_NAME: t1_chk_1
+      CHECK_CLAUSE: (`id` % 2 = 0)
+        TABLE_NAME: t1
+          TABLE_ID: 107
+1 row in set (0.02 sec)
+```
+
+`TIDB_CHECK_CONSTRAINTS` 表的字段描述如下：
+
+* `CONSTRAINT_CATALOG`：约束的目录，始终为 `def`。
+* `CONSTRAINT_SCHEMA`：约束的库名。
+* `CONSTRAINT_NAME`：约束的名字。
+* `CHECK_CLAUSE`：检查约束的子句。
+* `TABLE_NAME`：约束所在表的名字。
+* `TABLE_ID`：约束所在表的 ID。