The System Variables
The system variables in MySQL are the system parameters that modify the operation of the database runtime. These variables have two types of scope, Global Scope and Session Scope. TiDB supports all the system variables in MySQL 5.7. Most of the variables are only supported for compatibility and do not affect the runtime behaviors.
Set the system variables
You can use the SET statement to change the value of the system variables. Before you change, consider the scope of the variable. For more information, see MySQL Dynamic System Variables.
Set Global variables
Add the GLOBAL keyword before the variable or use @@global. as the modifier:
SET GLOBAL autocommit = 1;
SET @@global.autocommit = 1;Set Session Variables
Add the SESSION keyword before the variable, use @@session. as the modifier, or use no modifier:
SET SESSION autocommit = 1;
SET @@session.autocommit = 1;
SET @@autocommit = 1;Note:
LOCALand@@local.are the synonyms forSESSIONand@@session.
The fully supported MySQL system variables in TiDB
The following MySQL system variables are fully supported in TiDB and have the same behaviors as in MySQL.
| Name | Scope | Description |
|---|---|---|
| autocommit | GLOBAL | SESSION | whether automatically commit a transaction |
| sql_mode | GLOBAL | SESSION | support some of the MySQL SQL modes |
| time_zone | GLOBAL | SESSION | the time zone of the database |
| tx_isolation | GLOBAL | SESSION | the isolation level of a transaction |
| hostname | NONE | the hostname of the TiDB server |