MySQL Compatibility

TiDB is highly compatible with the MySQL 5.7 protocol and the common features and syntax of MySQL 5.7. The ecosystem tools for MySQL 5.7 (PHPMyAdmin, Navicat, MySQL Workbench, mysqldump, and Mydumper/myloader) and the MySQL client can be used for TiDB.

However, some features of MySQL are not supported. This could be because there is now a better way to solve the problem (such as XML functions superseded by JSON), or a lack of current demand versus effort required (such as stored procedures and functions). Some features might also be difficult to implement as a distributed system.

• In addition, TiDB does not support the MySQL replication protocol, but provides specific tools to replicate data with MySQL.
  • Replicate data from MySQL: TiDB Data Migration (DM) is a tool that supports the full data migration and the incremental data replication from MySQL/MariaDB into TiDB.
  • Replicate data to MySQL: TiCDC is a tool for replicating the incremental data of TiDB by pulling TiKV change logs. TiCDC uses the MySQL sink to replicate the incremental data of TiDB to MySQL.

Unsupported features

• Stored procedures and functions
• Triggers
• Events
• User-defined functions
• FOREIGN KEY constraints #18209
• Temporary tables #1248
• FULLTEXT syntax and indexes #1793
• SPATIAL (also known as GIS/GEOMETRY) functions, data types and indexes #6347
• Character sets other than utf8, utf8mb4, ascii, latin1 and binary
• SYS schema
• Optimizer trace
• XML Functions
• X-Protocol #1109
• Savepoints #6840
• Column-level privileges #9766
• XA syntax (TiDB uses a two-phase commit internally, but this is not exposed via an SQL interface)
• CREATE TABLE tblName AS SELECT stmt syntax #4754
• CHECK TABLE syntax #4673
• CHECKSUM TABLE syntax #1895
• GET_LOCK and RELEASE_LOCK functions #14994
• LOAD DATA with the REPLACE keyword #24515

Features that are different from MySQL

Auto-increment ID

• In TiDB, auto-increment columns are only guaranteed to be unique and incremental on a single TiDB server, but they are not guaranteed to be incremental among multiple TiDB servers or allocated sequentially. It is recommended that you do not mix default values and custom values. Otherwise, you might encounter the Duplicated Error error message.

• You can use the tidb_allow_remove_auto_inc system variable to allow or forbid removing the AUTO_INCREMENT column attribute. The syntax of removing the column attribute is ALTER TABLE MODIFY or ALTER TABLE CHANGE.

• TiDB does not support adding the AUTO_INCREMENT column attribute, and this attribute cannot be recovered once it is removed.

• See AUTO_INCREMENT for more details.

mysql> CREATE TABLE t(id INT UNIQUE KEY AUTO_INCREMENT);
Query OK, 0 rows affected (0.05 sec)

mysql> INSERT INTO t VALUES(),(),();
Query OK, 3 rows affected (0.00 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> SELECT _tidb_rowid, id FROM t;
+-------------+------+
| _tidb_rowid | id   |
+-------------+------+
|           4 |    1 |
|           5 |    2 |
|           6 |    3 |
+-------------+------+
3 rows in set (0.01 sec)

Performance schema

TiDB uses a combination of Prometheus and Grafana to store and query the performance monitoring metrics. Performance schema tables return empty results in TiDB.

Query Execution Plan

The output format, output content, and the privilege setting of Query Execution Plan (EXPLAIN/EXPLAIN FOR) in TiDB is greatly different from those in MySQL.

The MySQL system variable optimizer_switch is read-only in TiDB and has no effect on query plans. You can also use optimizer hints in similar syntax to MySQL, but the available hints and implementation might differ.

See Understand the Query Execution Plan for more details.

Built-in functions

TiDB supports most of the MySQL built-in functions, but not all. The statement SHOW BUILTINS provides a list of functions that are available.

See also: TiDB SQL Grammar.

DDL

In TiDB, all supported DDL changes are performed online. Compared with DDL operations in MySQL, the DDL operations in TiDB have the following major restrictions:

• Multiple operations cannot be completed in a single ALTER TABLE statement. For example, it is not possible to add multiple columns or indexes in a single statement. Otherwise, the Unsupported multi schema change error might be output.
• Different types of indexes (HASH|BTREE|RTREE|FULLTEXT) are not supported, and will be parsed and ignored when specified.
• Adding/Dropping the primary key of the CLUSTERED type is unsupported. For more details about the primary key of the CLUSTERED type, refer to clustered index.
• Changing the field type to its superset is unsupported. For example, TiDB does not support changing the field type from INTEGER to VARCHAR, or from TIMESTAMP to DATETIME. Otherwise, the error information Unsupported modify column: type %d not match origin %d might be output.
• Change/Modify data type does not currently support "lossy changes", such as changing from BIGINT to INT.
• Change/Modify decimal columns does not support changing the precision.
• Change/Modify integer columns does not permit changing the UNSIGNED attribute.
• The ALGORITHM={INSTANT,INPLACE,COPY} syntax functions only as an assertion in TiDB, and does not modify the ALTER algorithm. See ALTER TABLE for further details.
• TiDB supports HASH, RANGE, and LIST partitioning types. For an unsupported partition type, TiDB returns Warning: Unsupported partition type, treat as normal table.
• Table partitioning supports ADD, DROP, and TRUNCATE operations. Other partition operations are ignored. The following table partition syntaxes are not supported:
  • PARTITION BY KEY
  • SUBPARTITION
  • {CHECK|OPTIMIZE|REPAIR|IMPORT|DISCARD|REBUILD|REORGANIZE|COALESCE} PARTITION

Analyze table

Statistics Collection works differently in TiDB than in MySQL, in that it is a relatively lightweight and short-lived operation in MySQL/InnoDB, while in TiDB it completely rebuilds the statistics for a table and can take much longer to complete.

These differences are documented further in ANALYZE TABLE.

Limitations of SELECT syntax

• The syntax SELECT ... INTO @variable is not supported.
• The syntax SELECT ... GROUP BY ... WITH ROLLUP is not supported.
• The syntax SELECT .. GROUP BY expr does not imply GROUP BY expr ORDER BY expr as it does in MySQL 5.7.

For details, see the SELECT statement reference.

UPDATE statement

See the UPDATE statement reference.

Views

Views in TiDB are not updatable. They do not support write operations such as UPDATE, INSERT, and DELETE.

Storage engines

For compatibility reasons, TiDB supports the syntax to create tables with alternative storage engines. In implementation, TiDB describes the metadata as the InnoDB storage engine.

TiDB supports storage engine abstraction similar to MySQL, but you need to specify the storage engine using the --store option when you start the TiDB server.

SQL modes

TiDB supports most SQL modes:

• The compatibility modes, such as ORACLE and POSTGRESQL are parsed but ignored. Compatibility modes are deprecated in MySQL 5.7 and removed in MySQL 8.0.
• The ONLY_FULL_GROUP_BY mode has minor semantic differences from MySQL 5.7.
• The NO_DIR_IN_CREATE and NO_ENGINE_SUBSTITUTION SQL modes in MySQL are accepted for compatibility, but are not applicable to TiDB.

Default differences

• Default character set:
  • The default value in TiDB is utf8mb4.
  • The default value in MySQL 5.7 is latin1.
  • The default value in MySQL 8.0 is utf8mb4.
• Default collation:
  • The default collation of utf8mb4 in TiDB is utf8mb4_bin.
  • The default collation of utf8mb4 in MySQL 5.7 is utf8mb4_general_ci.
  • The default collation of utf8mb4 in MySQL 8.0 is utf8mb4_0900_ai_ci.
• Default value of foreign_key_checks:
  • The default value in TiDB is OFF and currently TiDB only supports OFF.
  • The default value in MySQL 5.7 is ON.
• Default SQL mode:
  • The default SQL mode in TiDB includes these modes: ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION.
  • The default SQL mode in MySQL:
    • The default SQL mode in MySQL 5.7 is the same as TiDB.
    • The default SQL mode in MySQL 8.0 includes these modes: ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_ENGINE_SUBSTITUTION.
• Default value of lower_case_table_names:
  • The default value in TiDB is 2 and currently TiDB only supports 2.
  • The default value in MySQL:
    • On Linux: 0
    • On Windows: 1
    • On macOS: 2
• Default value of explicit_defaults_for_timestamp:
  • The default value in TiDB is ON and currently TiDB only supports ON.
  • The default value in MySQL:
    • For MySQL 5.7: OFF.
    • For MySQL 8.0: ON.

Date and Time

Named timezone

• TiDB uses all time zone rules currently installed in the system for calculation (usually the tzdata package). You can use all time zone names without importing the time zone table data. You cannot modify the calculation rules by importing the time zone table data.
• MySQL uses the local time zone by default and relies on the current time zone rules built into the system (such as when to start daylight saving time) for calculation; and the time zone cannot be specified by the time zone name without importing the time zone table data.

Type system differences

The following column types are supported by MySQL, but NOT by TiDB:

• FLOAT4/FLOAT8
• SQL_TSI_* (including SQL_TSI_MONTH, SQL_TSI_WEEK, SQL_TSI_DAY, SQL_TSI_HOUR, SQL_TSI_MINUTE and SQL_TSI_SECOND, excluding SQL_TSI_YEAR)

Incompatibility caused by deprecated features

TiDB does not implement certain features that have been marked as deprecated in MySQL, including:

• Specifying precision for floating point types. MySQL 8.0 deprecates this feature, and it is recommended to use the DECIMAL type instead.
• The ZEROFILL attribute. MySQL 8.0 deprecates this feature, and it is recommended to instead pad numeric values in your application.