FOREIGN KEY Constraints

Foreign keys allow cross-table references of related data, while foreign key constraints ensure the consistency of related data. Starting from v6.6.0, TiDB supports foreign keys and foreign key constraints. Starting from v8.5.0, this feature becomes generally available.

Warning

The foreign key feature is typically employed to enforce referential integrity constraint checks. It might cause performance degradation, so it is recommended to conduct thorough testing before using it in performance-sensitive scenarios.

The foreign key is defined in the child table. The syntax is as follows:

ForeignKeyDef
ReferenceOption

ForeignKeyDef
         ::= ( 'CONSTRAINT' Identifier )? 'FOREIGN' 'KEY'
             Identifier? '(' ColumnName ( ',' ColumnName )* ')'
             'REFERENCES' TableName '(' ColumnName ( ',' ColumnName )* ')'
             ( 'ON' 'DELETE' ReferenceOption )?
             ( 'ON' 'UPDATE' ReferenceOption )?

ReferenceOption
         ::= 'RESTRICT'
           | 'CASCADE'
           | 'SET' 'NULL'
           | 'SET' 'DEFAULT'
           | 'NO' 'ACTION'

Naming

The naming of a foreign key follows the following rules:

If a name is specified in CONSTRAINT identifier, the specified name is used.
If no name is specified in CONSTRAINT identifier, but a name is specified in FOREIGN KEY identifier, the name specified in the FOREIGN KEY identifier is used.
If neither CONSTRAINT identifier nor FOREIGN KEY identifier specifies a name, a name is automatically generated, such as fk_1, fk_2, and fk_3.
The foreign key name must be unique in the current table. Otherwise, an error ERROR 1826: Duplicate foreign key constraint name 'fk' is reported when the foreign key is created.

Restrictions

When creating a foreign key, the following conditions must be met:

Neither the parent table nor the child table is a temporary table.
The user has the REFERENCES privilege on the parent table.
The columns referenced by the foreign key in the parent table and the child table are of the same data type and have the same size, precision, length, character set, and collation.
The columns in the foreign key cannot reference themselves.
The columns in the foreign key and the columns in the referenced parent table have the same index, and the order of the columns in the index matches that in the foreign key. This is to use the index to avoid full table scans when performing foreign key constraint checks.
- If there is no corresponding foreign key index in the parent table, an error ERROR 1822: Failed to add the foreign key constraint. Missing index for constraint 'fk' in the referenced table 't' is reported.
- If there is no corresponding foreign key index in the child table, an index is automatically created with the same name as the foreign key.
It is not supported to create a foreign key on a column of the BLOB or TEXT type.
It is not supported to create a foreign key on a partitioned table.
It is not supported to create a foreign key on a virtual generated column.

Reference operations

If an UPDATE or DELETE operation affects a foreign key value in the parent table, the corresponding foreign key value in the child table is determined by the reference operation defined by the ON UPDATE or ON DELETE clause in the foreign key definition. The reference operations include the following:

CASCADE: automatically updates or deletes the matching rows in the child table when the UPDATE or DELETE operation affects the parent table. The cascade operation is performed in a depth-first manner.
SET NULL: automatically sets the matching foreign key columns in the child table to NULL when the UPDATE or DELETE operation affects the parent table.
RESTRICT: denies the UPDATE or DELETE operation if the child table contains matching rows.
NO ACTION: the same as RESTRICT.
SET DEFAULT: the same as RESTRICT.

If there is no matching foreign key value in the parent table, the INSERT or UPDATE operation on the child table is denied.

If the foreign key definition does not specify ON DELETE or ON UPDATE, the default behavior is NO ACTION.

If the foreign key is defined on a STORED GENERATED COLUMN, the CASCADE, SET NULL, and SET DEFAULT references are not supported.

Usage examples of foreign keys

The following example uses a single-column foreign key to associate the parent table and the child table:

CREATE TABLE parent (
    id INT KEY
);

CREATE TABLE child (
    id INT,
    pid INT,
    INDEX idx_pid (pid),
    FOREIGN KEY (pid) REFERENCES parent(id) ON DELETE CASCADE
);

The following is a more complex example where the product_order table has two foreign keys that reference the other two tables. One foreign key references two indexes on the product table, and the other references a single index on the customer table:

CREATE TABLE product (
    category INT NOT NULL,
    id INT NOT NULL,
    price DECIMAL(20,10),
    PRIMARY KEY(category, id)
);

CREATE TABLE customer (
    id INT KEY
);

CREATE TABLE product_order (
    id INT NOT NULL AUTO_INCREMENT,
    product_category INT NOT NULL,
    product_id INT NOT NULL,
    customer_id INT NOT NULL,

    PRIMARY KEY(id),
    INDEX (product_category, product_id),
    INDEX (customer_id),

    FOREIGN KEY (product_category, product_id)
      REFERENCES product(category, id)
      ON UPDATE CASCADE ON DELETE RESTRICT,

    FOREIGN KEY (customer_id)
      REFERENCES customer(id)
);

Create a foreign key constraint

To create a foreign key constraint, you can use the following ALTER TABLE statement:

ALTER TABLE table_name
    ADD [CONSTRAINT [identifier]] FOREIGN KEY
    [identifier] (col_name, ...)
    REFERENCES tbl_name (col_name,...)
    [ON DELETE reference_option]
    [ON UPDATE reference_option]

The foreign key can be self-referencing, that is, referencing the same table. When you add a foreign key constraint to a table using ALTER TABLE, you need to first create an index on the parent table column that the foreign key references.

Delete a foreign key constraint

To delete a foreign key constraint, you can use the following ALTER TABLE statement:

ALTER TABLE table_name DROP FOREIGN KEY fk_identifier;

If the foreign key constraint is named when it is created, you can reference the name to delete the foreign key constraint. Otherwise, you have to use the constraint name automatically generated to delete the constraint. You can use SHOW CREATE TABLE to view the foreign key name:

mysql> SHOW CREATE TABLE child\G
*************************** 1. row ***************************
       Table: child
Create Table: CREATE TABLE `child` (
  `id` int DEFAULT NULL,
  `pid` int DEFAULT NULL,
  KEY `idx_pid` (`pid`),
  CONSTRAINT `fk_1` FOREIGN KEY (`pid`) REFERENCES `test`.`parent` (`id`) ON DELETE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin

Foreign key constraint check

TiDB supports foreign key constraint check, which is controlled by the system variable foreign_key_checks. By default, this variable is set to ON, meaning that the foreign key constraint check is enabled. This variable has two scopes: GLOBAL and SESSION. Keeping this variable enabled can ensure the integrity of foreign key reference relationships.

The effect of disabling foreign key constraint check is as follows:

When you delete a parent table referenced by a foreign key, the deletion can succeed only when the foreign key constraint check is disabled.
When you import data to a database, the order of creating tables might be different from the foreign key dependency order, which might cause the creation of tables to fail. Only when the foreign key constraint check is disabled can the tables be created successfully. In addition, disabling the foreign key constraint check can speed up data import.
When you import data to a database, if the data of the child table is imported first, an error will be reported. Only when the foreign key constraint check is disabled can the data of the child table be imported successfully.
If an ALTER TABLE operation to be executed involves a change of the foreign key, this operation succeeds only when the foreign key constraint check is disabled.

When the foreign key constraint check is disabled, the foreign key constraint check and reference operation are not executed, except for the following scenarios:

If the execution of ALTER TABLE might result in wrong definition of the foreign key, an error is still reported during the execution.
When deleting the index required by the foreign key, you should delete the foreign key first. Otherwise, an error is reported.
When you create a foreign key but it does not meet related conditions or restrictions, an error is reported.

Locking

When INSERT or UPDATE a child table, the foreign key constraint checks whether the corresponding foreign key value exists in the parent table, and locks the row in the parent table to avoid the foreign key value being deleted by other operations violating the foreign key constraint. The locking behavior is equivalent to performing a SELECT FOR UPDATE operation on the row where the foreign key value is located in the parent table.

Because TiDB currently does not support LOCK IN SHARE MODE, if a child table accepts a large number of concurrent writes and most of the referenced foreign key values are the same, there might be serious locking conflicts. It is recommended to disable foreign_key_checks when writing a large number of child table data.

Definition and metadata of foreign keys

To view the definition of a foreign key constraint, execute the SHOW CREATE TABLE statement:

mysql> SHOW CREATE TABLE child\G
*************************** 1. row ***************************
       Table: child
Create Table: CREATE TABLE `child` (
  `id` int DEFAULT NULL,
  `pid` int DEFAULT NULL,
  KEY `idx_pid` (`pid`),
  CONSTRAINT `fk_1` FOREIGN KEY (`pid`) REFERENCES `test`.`parent` (`id`) ON DELETE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin

You can also get information about foreign keys using either of the following system tables:

The following provides examples:

Get information about foreign keys from the INFORMATION_SCHEMA.KEY_COLUMN_USAGE system table:

mysql> SELECT TABLE_SCHEMA, TABLE_NAME, COLUMN_NAME, CONSTRAINT_NAME FROM INFORMATION_SCHEMA.KEY_COLUMN_USAGE WHERE REFERENCED_TABLE_SCHEMA IS NOT NULL;
+--------------+---------------+------------------+-----------------+
| TABLE_SCHEMA | TABLE_NAME    | COLUMN_NAME      | CONSTRAINT_NAME |
+--------------+---------------+------------------+-----------------+
| test         | child         | pid              | fk_1            |
| test         | product_order | product_category | fk_1            |
| test         | product_order | product_id       | fk_1            |
| test         | product_order | customer_id      | fk_2            |
+--------------+---------------+------------------+-----------------+

Get information about foreign keys from the INFORMATION_SCHEMA.TABLE_CONSTRAINTS system table:

mysql> SELECT * FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS WHERE CONSTRAINT_TYPE='FOREIGN KEY'\G
***************************[ 1. row ]***************************
CONSTRAINT_CATALOG | def
CONSTRAINT_SCHEMA  | test
CONSTRAINT_NAME    | fk_1
TABLE_SCHEMA       | test
TABLE_NAME         | child
CONSTRAINT_TYPE    | FOREIGN KEY

Get information about foreign keys from the INFORMATION_SCHEMA.REFERENTIAL_CONSTRAINTS system table:

mysql> SELECT * FROM INFORMATION_SCHEMA.REFERENTIAL_CONSTRAINTS\G
***************************[ 1. row ]***************************
CONSTRAINT_CATALOG        | def
CONSTRAINT_SCHEMA         | test
CONSTRAINT_NAME           | fk_1
UNIQUE_CONSTRAINT_CATALOG | def
UNIQUE_CONSTRAINT_SCHEMA  | test
UNIQUE_CONSTRAINT_NAME    | PRIMARY
MATCH_OPTION              | NONE
UPDATE_RULE               | NO ACTION
DELETE_RULE               | CASCADE
TABLE_NAME                | child
REFERENCED_TABLE_NAME     | parent

View execution plans with foreign keys

You can use the EXPLAIN statement to view execution plans. The Foreign_Key_Check operator performs the foreign key constraint check on DML statements that are executed.

mysql> explain insert into child values (1,1);
+-----------------------+---------+------+---------------+-------------------------------+
| id                    | estRows | task | access object | operator info                 |
+-----------------------+---------+------+---------------+-------------------------------+
| Insert_1              | N/A     | root |               | N/A                           |
| └─Foreign_Key_Check_3 | 0.00    | root | table:parent  | foreign_key:fk_1, check_exist |
+-----------------------+---------+------+---------------+-------------------------------+

You can use the EXPLAIN ANALYZE statement to view the execution of the foreign key reference behavior. The Foreign_Key_Cascade operator performs foreign key referencing for DML statements that are executed.

mysql> explain analyze delete from parent where id = 1;
+----------------------------------+---------+---------+-----------+---------------------------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+---------------------------------------------+-----------+------+
| id                               | estRows | actRows | task      | access object                   | execution info                                                                                                                                                                               | operator info                               | memory    | disk |
+----------------------------------+---------+---------+-----------+---------------------------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+---------------------------------------------+-----------+------+
| Delete_2                         | N/A     | 0       | root      |                                 | time:117.3µs, loops:1                                                                                                                                                                        | N/A                                         | 380 Bytes | N/A  |
| ├─Point_Get_1                    | 1.00    | 1       | root      | table:parent                    | time:63.6µs, loops:2, Get:{num_rpc:1, total_time:29.9µs}                                                                                                                                     | handle:1                                    | N/A       | N/A  |
| └─Foreign_Key_Cascade_3          | 0.00    | 0       | root      | table:child, index:idx_pid      | total:1.28ms, foreign_keys:1                                                                                                                                                                 | foreign_key:fk_1, on_delete:CASCADE         | N/A       | N/A  |
|   └─Delete_7                     | N/A     | 0       | root      |                                 | time:904.8µs, loops:1                                                                                                                                                                        | N/A                                         | 1.11 KB   | N/A  |
|     └─IndexLookUp_11             | 10.00   | 1       | root      |                                 | time:869.5µs, loops:2, index_task: {total_time: 371.1µs, fetch_handle: 357.3µs, build: 1.25µs, wait: 12.5µs}, table_task: {total_time: 382.6µs, num: 1, concurrency: 5}                      |                                             | 9.13 KB   | N/A  |
|       ├─IndexRangeScan_9(Build)  | 10.00   | 1       | cop[tikv] | table:child, index:idx_pid(pid) | time:351.2µs, loops:3, cop_task: {num: 1, max: 282.3µs, proc_keys: 0, rpc_num: 1, rpc_time: 263µs, copr_cache_hit_ratio: 0.00, distsql_concurrency: 15}, tikv_task:{time:220.2µs, loops:0}   | range:[1,1], keep order:false, stats:pseudo | N/A       | N/A  |
|       └─TableRowIDScan_10(Probe) | 10.00   | 1       | cop[tikv] | table:child                     | time:223.9µs, loops:2, cop_task: {num: 1, max: 168.8µs, proc_keys: 0, rpc_num: 1, rpc_time: 154.5µs, copr_cache_hit_ratio: 0.00, distsql_concurrency: 15}, tikv_task:{time:145.6µs, loops:0} | keep order:false, stats:pseudo              | N/A       | N/A  |
+----------------------------------+---------+---------+-----------+---------------------------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+---------------------------------------------+-----------+------+

Compatibility

Compatibility between TiDB versions

Before v6.6.0, TiDB supports the syntax of creating foreign keys, but the created foreign keys are ineffective. If you upgrade a TiDB cluster created before v6.6.0 to v6.6.0 or later, the foreign keys created before the upgrade are still ineffective. Only the foreign keys created in v6.6.0 or later versions are effective. You can delete the invalid foreign key and create a new one to make the foreign key constraints effective. You can use the SHOW CREATE TABLE statement to check whether the foreign keys are effective. The invalid foreign key has a /* FOREIGN KEY INVALID */ comment.

mysql> SHOW CREATE TABLE child\G
***************************[ 1. row ]***************************
Table        | child
Create Table | CREATE TABLE `child` (
  `id` int DEFAULT NULL,
  `pid` int DEFAULT NULL,
  KEY `idx_pid` (`pid`),
  CONSTRAINT `fk_1` FOREIGN KEY (`pid`) REFERENCES `test`.`parent` (`id`) ON DELETE CASCADE /* FOREIGN KEY INVALID */
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin

Compatibility with TiDB tools

DM does not support foreign keys. DM disables the foreign_key_checks of the downstream TiDB when replicating data to TiDB. Therefore, the cascading operations caused by foreign keys are not replicated from the upstream to the downstream, which might cause data inconsistency.
TiCDC v6.6.0 is compatible with foreign keys. The previous versions of TiCDC might report an error when replicating tables with foreign keys. It is recommended to disable the foreign_key_checks of the downstream TiDB cluster when using a TiCDC version earlier than v6.6.0.
BR v6.6.0 is compatible with foreign keys. The previous versions of BR might report an error when restoring tables with foreign keys to a v6.6.0 or later cluster. It is recommended to disable the foreign_key_checks of the downstream TiDB cluster before restoring the cluster when using a BR earlier than v6.6.0.
When you use TiDB Lightning, if the target table uses a foreign key, it is recommended to disable the foreign_key_checks of the downstream TiDB cluster before importing data. For versions earlier than v6.6.0, disabling this system variable does not take effect, and you need to grant the REFERENCES privilege for the downstream database user, or manually create the target table in the downstream database in advance to ensure smooth data import.

Dumpling is compatible with foreign keys.

When you use sync-diff-inspector to compare data between the upstream and downstream databases, if the database versions are different and there is an invalid foreign key in the downstream TiDB, sync-diff-inspector might report a table schema inconsistent error. This is because TiDB v6.6.0 adds a /* FOREIGN KEY INVALID */ comment for the invalid foreign key.

Compatibility with MySQL

When you create a foreign key without specifying a name, the name generated by TiDB is different from that generated by MySQL. For example, the foreign key name generated by TiDB is fk_1, fk_2, and fk_3, while the foreign key name generated by MySQL is table_name_ibfk_1, table_name_ibfk_2, and table_name_ibfk_3.

Both MySQL and TiDB parse but ignore "inline REFERENCES specifications". Only REFERENCES specifications that are part of FOREIGN KEY definitions are checked and enforced. The following example creates a foreign key constraint using a REFERENCES clause:

CREATE TABLE parent (
    id INT KEY
);

CREATE TABLE child (
    id INT,
    pid INT REFERENCES parent(id)
);

SHOW CREATE TABLE child;

The output shows that the child table does not contain any foreign keys:

+-------+-------------------------------------------------------------+
| Table | Create Table                                                |
+-------+-------------------------------------------------------------+
| child | CREATE TABLE `child` (                                      |
|       |   `id` int DEFAULT NULL,                                |
|       |   `pid` int DEFAULT NULL                                |
|       | ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin |
+-------+-------------------------------------------------------------+