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This document describes how to enable or disable the parallel query feature of TencentDB for MySQL via the console or command line.

Prerequisites

Database version: MySQL 8.0 on kernel version 20220831 or later.

Parameters

Note：

The parallel query feature can be enabled for both source and read-only instances, as long as their number of CPU cores is greater than or equal to 4.

You can enable the parallel query feature for the current instance by setting the txsql_max_parallel_worker_threads and txsql_parallel_degree parameters to a value other than 0 via the console or command line. Parameters and suggested settings are as follows:
Parameter information

Suggested settings

• Parallelism limit: txsql_parallel_degree indicates the maximum number of threads for the parallel query of a single statement, i.e., the default parallelism. We recommend that you limit this value to half of the CPU core quantity of the instance. To ensure the stability, the parallel query feature is disabled for small clusters with fewer than four CPU cores, and you cannot adjust parallel query parameters via the console or command line.
• During the parallel query of a SQL statement, the parallelism set by txsql_parallel_degree will be used by default, which can be adjusted through the HINT statement. For more information, see HINT Statement Control.
• txsql_max_parallel_worker_threads indicates the number of threads of the instance that can be used for parallel query, and txsql_max_parallel_worker_threads / txsql_parallel_degree indicates the maximum number of SQL statements allowed in a parallel query.
• txsql_max_parallel_worker_threads and txsql_parallel_degree control the status of the parallel query feature. When either of them is 0, the feature is disabled.

TencentDB for MySQL offers various parameters for you to set the execution conditions of parallel query for business adaptation and stability. After conditions are set, the database will check whether each SQL statement can be executed against such conditions like execution cost, number of table rows, and memory usage for the parallel statement execution. Parameters are described as follows:

Note：

• Parallel query parameters take effect immediately after being set, with no instance restart required.
• If the scope of a parameter is session, it takes effect only for the statement.

Enabling or disabling parallel query in the console

You can enable or disable the feature by setting parameters on the Parameter Settings page in the TencentDB for MySQL console.

• Set txsql_max_parallel_worker_threads and txsql_parallel_degree to a value other than 0 to enable parallel query.
• Set txsql_max_parallel_worker_threads or txsql_parallel_degree to 0 to disable parallel query.

You can also set execution conditions on the Parameter Settings page. For detailed directions, see Setting Instance Parameters.

Specifying the parallel execution mode of a SQL statement through the HINT statement

TencentDB for MySQL allows you to specify the parallel execution mode of a SQL statement through the HINT statement. For detailed directions, see HINT Statement Control.

References

• Viewing Parallel Query
• HINT Statement Control