Constraints

Overview

TiDB supports the same basic constraints supported in MySQL with the following exceptions:

PRIMARY KEY and UNIQUE constraints are checked lazily by default. By batching checks until when the transaction commits, TiDB is able to reduce network communication. This behavior can be changed by setting tidb_constraint_check_in_place to TRUE.
FOREIGN KEY constraints are not currently enforced by DML.

Foreign Key

TiDB currently only supports FOREIGN KEY creation in DDL commands. For example:

CREATE TABLE users (
 id INT NOT NULL PRIMARY KEY AUTO_INCREMENT,
 doc JSON
);

CREATE TABLE orders (
 id INT NOT NULL PRIMARY KEY AUTO_INCREMENT,
 user_id INT NOT NULL,
 doc JSON,
 FOREIGN KEY fk_user_id (user_id) REFERENCES users(id)
);

mysql> SELECT table_name, column_name, constraint_name, referenced_table_name, referenced_column_name
FROM information_schema.key_column_usage WHERE table_name IN ('users', 'orders');
+------------+-------------+-----------------+-----------------------+------------------------+
| table_name | column_name | constraint_name | referenced_table_name | referenced_column_name |
+------------+-------------+-----------------+-----------------------+------------------------+
| users      | id          | PRIMARY         | NULL                  | NULL                   |
| orders     | id          | PRIMARY         | NULL                  | NULL                   |
| orders     | user_id     | fk_user_id      | users                 | id                     |
+------------+-------------+-----------------+-----------------------+------------------------+
3 rows in set (0.00 sec)

TiDB also supports the syntax to DROP FOREIGN KEY and ADD FOREIGN KEY via the ALTER TABLE command:

ALTER TABLE orders DROP FOREIGN KEY fk_user_id;
ALTER TABLE orders ADD FOREIGN KEY fk_user_id (user_id) REFERENCES users(id);

Notes

TiDB supports foreign keys so that no errors are reported for this syntax when migrating data from other databases. Currently, foreign keys are not enforced as part of DML operations. For example, even though there is no such record as id=123 in the users table, the following transaction commits successfully in TiDB:
```
    START TRANSACTION;
    INSERT INTO orders (user_id, doc) VALUES (123, NULL);
    COMMIT;
```
In TiDB, foreign key information is not displayed in the execution result of the SHOW CREATE TABLE statement.

Not Null

TiDB supports the NOT NULL constraint with identical semantics to MySQL. For example:

CREATE TABLE users (
 id INT NOT NULL PRIMARY KEY AUTO_INCREMENT,
 age INT NOT NULL,
 last_login TIMESTAMP
);

mysql> INSERT INTO users (id,age,last_login) VALUES (NULL,123,NOW());
Query OK, 1 row affected (0.02 sec)

mysql> INSERT INTO users (id,age,last_login) VALUES (NULL,NULL,NOW());
ERROR 1048 (23000): Column 'age' cannot be null

mysql> INSERT INTO users (id,age,last_login) VALUES (NULL,123,NULL);
Query OK, 1 row affected (0.03 sec)

The first INSERT statement succeeded because NULL is permitted as a special value for columns defined as AUTO_INCREMENT. This results in the next auto-value being allocated.
The second INSERT statement fails because the age column was defined as NOT NULL.
The third INSERT statement succeeds because last_login did not explicitly specify the column as NOT NULL. The default behavior is to permit NULL values.

Primary Key

In TiDB, PRIMARY KEY constraints are checked lazily by default. By batching checks until when the transaction commits, TiDB is able to reduce network communication. For example:

mysql> CREATE TABLE t1 (id INT NOT NULL PRIMARY KEY);
Query OK, 0 rows affected (0.01 sec)

mysql> INSERT INTO t1 VALUES (1);
Query OK, 1 row affected (0.00 sec)

mysql> START TRANSACTION;
Query OK, 0 rows affected (0.00 sec)

mysql> INSERT INTO t1 VALUES (1); -- does not error
Query OK, 1 row affected (0.00 sec)

mysql> INSERT INTO t1 VALUES (2);
Query OK, 1 row affected (0.00 sec)

mysql> COMMIT; -- triggers an error
ERROR 1062 (23000): Duplicate entry '1' for key 'PRIMARY'

PRIMARY KEY constraints otherwise have similar behavior and restrictions to MySQL:

mysql> CREATE TABLE t1 (a INT NOT NULL PRIMARY KEY);
Query OK, 0 rows affected (0.12 sec)

mysql> CREATE TABLE t2 (a INT NULL PRIMARY KEY);
ERROR 1171 (42000): All parts of a PRIMARY KEY must be NOT NULL; if you need NULL in a key, use UNIQUE instead

mysql> CREATE TABLE t3 (a INT NOT NULL PRIMARY KEY, b INT NOT NULL PRIMARY KEY);
ERROR 1068 (42000): Multiple primary key defined

mysql> CREATE TABLE t4 (a INT NOT NULL, b INT NOT NULL, PRIMARY KEY (a,b));
Query OK, 0 rows affected (0.10 sec)

Table t2 failed to be created because the column a permitted NULL values.
Table t3 failed because there can only be one PRIMARY KEY on a table.
Table t4 was successful, because even though there can only be one primary key, it may be defined as a composite of multiple columns.

In addition to these semantics, TiDB also imposes the restriction that once a table is created, the PRIMARY KEY can not be changed.

Unique

In TiDB, UNIQUE constraints are checked lazily by default. By batching checks until when the transaction commits, TiDB is able to reduce network communication. For example:

DROP TABLE IF EXISTS users;
CREATE TABLE users (
 id INT NOT NULL PRIMARY KEY AUTO_INCREMENT,
 username VARCHAR(60) NOT NULL,
 UNIQUE KEY (username)
);
INSERT INTO users (username) VALUES ('dave'), ('sarah'), ('bill');

mysql> START TRANSACTION;
Query OK, 0 rows affected (0.00 sec)

mysql> INSERT INTO users (username) VALUES ('jane'), ('chris'), ('bill');
Query OK, 3 rows affected (0.00 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> INSERT INTO users (username) VALUES ('steve'),('elizabeth');
Query OK, 2 rows affected (0.00 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> COMMIT;
ERROR 1062 (23000): Duplicate entry 'bill' for key 'username'

The first INSERT statement does not cause a duplicate key error, as it would in MySQL. This check is deferred until the COMMIT statement is executed.

By changing tidb_constraint_check_in_place to TRUE, UNIQUE constraints will be checked as statements are executed. For example:

DROP TABLE IF EXISTS users;
CREATE TABLE users (
 id INT NOT NULL PRIMARY KEY AUTO_INCREMENT,
 username VARCHAR(60) NOT NULL,
 UNIQUE KEY (username)
);
INSERT INTO users (username) VALUES ('dave'), ('sarah'), ('bill');

mysql> SET tidb_constraint_check_in_place = TRUE;
Query OK, 0 rows affected (0.00 sec)

mysql> START TRANSACTION;
Query OK, 0 rows affected (0.00 sec)

mysql> INSERT INTO users (username) VALUES ('jane'), ('chris'), ('bill');
ERROR 1062 (23000): Duplicate entry 'bill' for key 'username'

..

The first INSERT statement causes a duplicate key error. This results in additional network communication, and will likely decrease insert throughput.