Engine Variables

SingleStore DB defines engine variables as either sync or non-sync. The following sections discuss these variables and how to set them.

Sync Variables

Sync variables are cluster-wide variables that you set on the master aggregator. After you set a sync variable, your update is propagated to all aggregators, all leaves, or both. The nodes affected by your update depend on the variable’s type and the command you use to set the variable. Your update takes effect immediately on the affected nodes (unless the variable’s scope is Session that can also be set globally) and persists if the affected nodes are restarted.

Types of Sync Variables

There are three types of sync variables:

  • Variables that can sync to all aggregators.

  • Variables that can sync to all leaves.

  • Variables that can sync to all nodes (leaves and aggregators).

The last sentence of the Description column in the list of sync variables specifies each variable’s type.

Sync Variable Scopes

Every variable has one of the following scopes.

  • Global. When you set a global variable, its value is effective for your current connection to the node and any other existing or subsequent connections, initiated by any user.

  • Session that can also be set globally. When set for the session, the variable’s value is effective for your current connection to the node. When set globally, this variable’s value is NOT effective for your current connection to the node but is effective for any other existing or subsequent connections to the node, initiated by any user.

Important

SingleStore recommends using a global setting for session variables whose values must be propagated to leaf nodes from the aggregator nodes on which the variable is set. For session variables that are only referenced by the aggregator nodes, the SET SESSION statement can be used instead.

Most sync variables have global scope. Those that instead have the scope session that can also be set globally are indicated in the Description column in the list of sync variables.

Note: Use the @@ selector to read the value of a variable. See Reading Variables for more information.

Setting Sync Variables

Sync variables with global or session scope can be set globally in the following way.

  • Using a SingleStore DB client, run one of the following three commands: run SET CLUSTER (the equivalent of SET GLOBAL) to sync a variable to all nodes or to all aggregators; run SET AGGREGATOR to sync a variable to all aggregators; run SET LEAF to sync a variable to all leaves.Connecting to SingleStore

    Important

    When you set a session variable using SET CLUSTER (or its equivalent SET GLOBAL) or SET AGGREGATOR, you need to additionally run FLUSH CONNECTION POOLS on every aggregator node in the cluster. This will clear the session variable value saved earlier in the connection pools and enable the aggregator and leaf nodes to use the new value.

Caution

You can set a sync variable on the Master Aggregator only. You will receive an error if you attempt to set a sync variable on any other type of node.

Non-Sync Variables

Non-sync variables are set to take effect on individual nodes. They have one the following four states:

  • Settable to take effect when a node starts

  • Settable to take effect while a node is running

  • Settable to take effect both when a node starts and while the node is running

  • Read-only (not settable)

Most non-sync variables are settable to take effect both when a node starts and while the node is running. Exceptions are indicated in the Exceptions to When Variable can be Set column in the list of non-sync variables.

Non-Sync Variable Scopes

Every non-sync variable has one of the following scopes.

  • Global. When you set a global variable, its value is effective for your current connection to the node and any other existing or subsequent connections, initiated by any user.

  • Session that can also be set globally. When set for the session, the variable’s value is effective for your current connection to the node. When set globally, this variable’s value is NOT effective for your current connection to the node but is effective for any other existing or subsequent connections to the node, initiated by any user.

Important

SingleStore recommends using a global setting for session variables whose values must be propagated to leaf nodes from the aggregator nodes on which the variable is set.

For session variables that are only referenced by the aggregator nodes, the SET SESSION statement can be used instead.

Most non-sync variables have global scope. Those that instead have the scope session that can also be set globally are indicated in the Description column in the list of non-sync variables.

Note: Use the @@ selector to read the value of a variable. See Reading Variables for more information.

Setting Non-Sync Variables

Non-sync variables with global or session scope can be set globally in the following way.

  • SET GLOBAL sets a non-sync variable globally and the variable’s value will not persist if the node is restarted.

    Important

    When you set a session variable using the SET GLOBAL command, you need to additionally run FLUSH CONNECTION POOLS on every aggregator node in the cluster. This will clear the session variable value saved earlier in the connection pools and enable the aggregator and leaf nodes to use the new value.

Caution

When you add new nodes to a cluster, non-sync variables that you set previously are not automatically applied to the new nodes.

Reading Variables

To retrieve the value of a variable, use the @@ selector with the variable in a SELECT statement. For example,

SELECT @@sql_mode;
****
+-------------------+
| @@sql_mode        |
+-------------------+
| STRICT_ALL_TABLES |
+-------------------+
1 row in set (0.27 sec)