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MySQL Compatibility

TiDB supports both the MySQL wire protocol and the majority of its syntax. This means that you can use your existing MySQL connectors and clients, and your existing applications can often be migrated to TiDB without changing any application code.

Currently TiDB Server advertises itself as MySQL 5.7 and works with most MySQL database tools such as PHPMyAdmin, Navicat, MySQL Workbench, mysqldump, and Mydumper/myloader.

However, TiDB does not support some of MySQL features or behaves differently from MySQL because these features cannot be easily implemented in a distributed system. For some MySQL syntax, TiDB can parse but does not process it. For example, the ENGINE table option and the PARTITION BY clause in the CREATE TABLE statement can be parsed but are ignored.

Note:

This page refers to general differences between MySQL and TiDB. Refer to Security Compatibility with MySQL for more details.

Unsupported features

  • Stored procedures and functions
  • Views
  • Triggers
  • Events
  • User-defined functions
  • FOREIGN KEY constraints #18209
  • Temporary tables #1248
  • FULLTEXT/SPATIAL functions and indexes #1793
  • Character sets other than utf8, utf8mb4, ascii, latin1 and binary
  • Collations other than BINARY
  • Add primary key
  • Drop primary key
  • SYS schema
  • Optimizer trace
  • XML Functions
  • X-Protocol
  • Savepoints
  • Column-level privileges
  • CREATE TABLE tblName AS SELECT stmt syntax
  • XA syntax (TiDB uses a two-phase commit internally, but this is not exposed via an SQL interface)
  • LOCK TABLE syntax (TiDB uses tidb_snapshot to produce backups)
  • CHECK TABLE syntax
  • CHECKSUM TABLE syntax

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. Currently, TiDB allocates IDs in batches. If you insert data on multiple TiDB servers at the same time, the allocated IDs are not continuous. You can use the tidb_allow_remove_auto_inc system variable to enable or disable deleting the AUTO_INCREMENT attribute of a column. The syntax for deleting this column attribute is alter table modify or alter table change.

Note:

If you use auto-increment IDs in a cluster with multiple tidb-server instances, do not mix default values and custom values. Otherwise, an error might occur in the following situation.

Assume that you have a table with the auto-increment ID:

CREATE TABLE t(id int unique key AUTO_INCREMENT, c int);

The principle of the auto-increment ID in TiDB is that each tidb-server instance caches a section of ID values (currently 30000 IDs are cached) for allocation and fetches the next section after this section is used up.

Assume that the cluster contains two tidb-server instances, namely Instance A and Instance B. Instance A caches the auto-increment ID of [1, 30000], while Instance B caches the auto-increment ID of [30001, 60000].

The operations are executed as follows:

  1. The client issues the INSERT INTO t VALUES (1, 1) statement to Instance B which sets the id to 1 and the statement is executed successfully.
  2. The client issues the INSERT INTO t (c) (1) statement to Instance A. This statement does not specify the value of id, so Instance A allocates the value. Currently, Instances A caches the auto-increment ID of [1, 30000], so it allocates the id value to 1 and adds 1 to the local counter. However, at this time the data with the id of 1 already exists in the cluster, therefore it reports Duplicated Error.

Also, starting from TiDB 2.1.18, TiDB supports using the system variable tidb_allow_remove_auto_inc to control whether the AUTO_INCREMENT property of a column is allowed to be removed by executing ALTER TABLE MODIFY or ALTER TABLE CHANGE statements. It is not allowed by default. Once the AUTO_INCREMENT property is removed, it cannot be recovered, because TiDB does not support adding the AUTO_INCREMENT column attribute.

Note:

If the primary key is not specified, TiDB uses the _tibd_rowid column to identify rows. The values of the _tibd_rowid column and the auto-increment column (if there is) are assigned by the same allocator. If the auto-increment column is specified as the primary key, then TiDB uses this column to identify rows. Therefore, there might be the following situations.

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

Performance schema tables return empty results in TiDB. TiDB uses a combination of Prometheus and Grafana for performance metrics instead.

Query Execution Plan

The output format of Query Execution Plan (EXPLAIN/EXPLAIN FOR) in TiDB is greatly different from that in MySQL. Besides, the output content and the privileges setting of EXPLAIN FOR are not the same as those of MySQL. See Understand the Query Execution Plan for more details.

Built-in functions

TiDB supports most of the MySQL built-in functions, but not all. See TiDB SQL Grammar for the supported functions.

DDL

In TiDB DDL does not block reads or writes to tables while in operation. However, some restrictions currently apply to DDL changes:

  • Add Index:
    • Does not support creating multiple indexes at the same time.
    • Adding an index on a generated column via ALTER TABLE is not supported.
  • Add Column:
    • Does not support creating multiple columns at the same time.
    • Does not support setting a column as the PRIMARY KEY, or creating a unique index, or specifying AUTO_INCREMENT while adding it.
  • Drop Column: Does not support dropping the PRIMARY KEY column or index column.
  • Change/Modify Column:
    • Does not support lossy changes, such as from BIGINT to INTEGER or VARCHAR(255) to VARCHAR(10). Otherwise, the length %d is less than origin %d error might be output.
    • Does not support modifying the precision of DECIMAL data types starting from TiDB v2.1.10.
    • Does not support changing the UNSIGNED attribute.
    • Does not support changing from NULL to NOT NULL.
    • Only supports changing the CHARACTER SET attribute from utf8 to utf8mb4.
  • LOCK [=] {DEFAULT|NONE|SHARED|EXCLUSIVE}: the syntax is supported, but is not applicable to TiDB. All DDL changes that are supported do not lock the table.
  • ALGORITHM [=] {DEFAULT|INSTANT|INPLACE|COPY}: the syntax for ALGORITHM=INSTANT and ALGORITHM=INPLACE is fully supported, but it works differently from MySQL because some operations that are INPLACE in MySQL are INSTANT in TiDB. The syntax ALGORITHM=COPY is not applicable to TIDB and returns a warning.
  • Multiple operations cannot be completed in a single ALTER TABLE statement. For example, it's not possible to add multiple columns or indexes in a single statement.

For more information, see Online Schema Changes.

Analyze table

ANALYZE TABLE 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.

Storage engines

For compatibility reasons, TiDB supports the syntax to create tables with alternative storage engines. Metadata commands describe tables as being of engine InnoDB:

CREATE TABLE t1 (a INT) ENGINE=MyISAM;
Query OK, 0 rows affected (0.14 sec)
SHOW CREATE TABLE t1;
*************************** 1. row ***************************
       Table: t1
Create Table: CREATE TABLE `t1` (
  `a` int(11) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_bin
1 row in set (0.00 sec)

Architecturally, TiDB does support a similar storage engine abstraction to MySQL, and user tables are created in the engine specified by the --store option used when you start tidb-server (typically tikv).

SQL modes

TiDB supports all of the SQL modes from MySQL 5.7 with minor exceptions:

  • The compatibility modes deprecated in MySQL 5.7 and removed in MySQL 8.0 are not supported (such as ORACLE, POSTGRESQL etc).
  • The mode ONLY_FULL_GROUP_BY has minor semantic differences to MySQL 5.7, which we plan to address in the future.
  • The SQL modes NO_DIR_IN_CREATE and NO_ENGINE_SUBSTITUTION are supported for compatibility, but are not applicable to TiDB.

Version-specific comments

TiDB executes all MySQL version-specific comments, regardless of the version they apply to. For example, the comment /*!90000 */ would instruct a MySQL server less than 9.0 to not execute code. In TiDB this code will always be executed:

mysql 8.0.16> SELECT /*!90000 "I should not run", */ "I should run" FROM dual;
+--------------+
| I should run |
+--------------+
| I should run |
+--------------+
1 row in set (0.00 sec)

tidb> SELECT /*!90000 "I should not run", */ "I should run" FROM dual;
+------------------+--------------+
| I should not run | I should run |
+------------------+--------------+
| I should not run | I should run |
+------------------+--------------+
1 row in set (0.00 sec)

Default differences

  • Default character set:
    • The default value in TiDB is utf8mb4.
    • The default value in MySQL 5.7 is latin1, but changes to utf8mb4 in MySQL 8.0.
  • 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, but changes to utf8mb4_0900_ai_ci in MySQL 8.0.
    • You can use the SHOW CHARACTER SET statement to check the default collations of all character sets.
  • 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 supports named timezones such as America/Los_Angeles without having to load the time zone information tables as in MySQL.

Because they are built-in, named time zones in TiDB might behave slightly differently to MySQL, and cannot be modified. For example, in TiDB the names are case-sensitive #8087.

Note:

TiKV calculates time-related expressions that can be pushed down to it. This calculation uses the built-in time zone rule and does not depend on the time zone rule installed in the system. If the time zone rule installed in the system does not match the version of the built-in time zone rule in TiKV, the time data that can be inserted might result in a statement error in a few cases.

For example, if the tzdata 2018a time zone rule is installed in the system, the time 1988-04-17 02:00:00 can be inserted into TiDB of the 3.0.0-rc.1 version when the time zone is set to Asia/Shanghai or the time zone is set to the local time zone and the local time zone is Asia/Shanghai. But reading this record might result in a statement error because this time does not exist in the Asia/Shanghai time zone according to the tzdata 2018i time zone rule used by TiKV 3.0.0-rc.1. Daylight saving time is one hour late.

The named timezone rules in TiKV of two versions are as follows:

Zero month and zero day

It is not recommended to unset the NO_ZERO_DATE and NO_ZERO_IN_DATE SQL modes, which are enabled by default in TiDB as in MySQL. While TiDB supports operating with these modes disabled, the TiKV coprocessor does not. Executing certain statements that push down date and time processing functions to TiKV might result in a statement error.