MySQL Compatibility

VTGate servers speak both gRPC and the MySQL server protocol. This allows you to connect to Vitess as if it were a MySQL Server without any changes to application code. This document refers to known compatibility issues where Vitess differs from MySQL.

Transaction Model

Vitess provides READ-COMMITTED semantics when executing cross-shard queries. This differs to MySQL, which defaults to REPEATABLE-READ.

SQL Syntax

The following describes some of the major differences in SQL Syntax handling between Vitess and MySQL. For a list of unsupported queries, check out the test-suite cases.

DDL

Vitess supports MySQL DDL, and will send ALTER TABLE statements to each of the underlying tablet servers. For large tables it is recommended to use an external schema deployment tool and apply directly to MySQL. This is discussed further in Applying MySQL Schema.

Join Queries

Vitess supports INNER JOIN including cross-shard joins. LEFT JOIN is supported as long as long as there are not expressions that compare columns on the outer table to the inner table in sharded keyspaces.

Aggregation

Vitess supports a subset of GROUP BY operations, including cross-shard operations. The VTGate servers are capable of scatter-gather operations, but can only stream results. Thus, a query that performs a GROUP BY colx ORDER BY coly may be refused if the intermediate result set is larger than VTGate’s in-memory limit.

Subqueries

Vitess supports a subset of subqueries. For example, a subquery combined with a GROUP BY operation is not supported.

Window Functions and CTEs

Vitess does not yet support Window Functions or Common Table Expressions.

Cross-shard Transaction

By default, Vitess does not support transactions that span across shards. While Vitess can support this with the use of Two-Phase Commit, it is usually recommended to design the VSchema in such a way that cross-shard modifications are not required.

OLAP Workload

By default, Vitess sets some intentional restrictions on the execution time and number of rows that a query can return. This can be disabled by setting the workload to OLTP:

SET workload='olap'

Network Protocol

Prepared Statements

Starting with version 4.0, Vitess features experimental support for prepared statements. Session-based statements to PREPARE and EXECUTE prepared statements are not supported.

Authentication Plugins

Vitess supports the mysql_native_password authentication plugin. Support for caching_sha2_password can be tracked in #5399.

Transport Security

To configure VTGate to support TLS set -mysql_server_ssl_cert and -mysql_server_ssl_key. Client certificates can also be mandated by setting -mysql_server_ssl_ca. If there is no CA specified then TLS is optional.

Session Scope

Vitess uses a connection pool to fan-in connections from VTGate servers to Tablet servers. VTGate servers will refuse statements that make changes to the connection’s session scope. This includes:

  • SET SESSION var=x
  • CREATE TEMPORARY TABLE
  • SET @var=x

The exception to this, is that Vitess maintains a whitelist statements that MySQL connectors may use when they first connect to Vitess. Vitess will ignore these noisy statements when it knows it is safe to do so.

Character Set and Collation

Vitess only supports utf8 and variants such as utf8mb4.

SQL Mode

Vitess behaves similar to the STRICT_TRANS_TABLES sql mode, and does not recommend changing the SQL Mode setting.

Data Types

Vitess supports all of the data types available in MySQL. Using the FLOAT data type as part of a PRIMARY KEY is strongly discouraged, since features such as filtered replication and VReplication will not correctly be able to detect which rows should be included as part of a modification.

Auto Increment

Tables in sharded keyspaces do not support the auto_increment, as the values generated would be local only to each shard. Vitess Sequences are provided as an alternative, which have very close semantics to auto_increment.

Extensions to MySQL Syntax

SHOW Statements

Vitess supports a few additional options with the SHOW statement.

  • SHOW keyspaces – A list of keyspaces available.
  • SHOW vitess_tablets – Information about the current Vitess tablets such as the keyspace, key ranges, tablet type, hostname, and status.
  • SHOW vitess_shards – A list of shards that are available.
  • SHOW vschema tables – A list of tables available in the current keyspace’s vschema.
  • SHOW vschema vindexes – Information about the current keyspace’s vindexes such as the keyspace, name, type, params, and owner. Optionally supports an “ON” clause with a table name.

USE Statements

Support for selecting a shard and tablet type via the USE statement can be achived with the following (backticks are important):

-- `schemaName:shardKeyRange@tabletType`
USE `schema:-80@rdonly`