Parallel query

Prerequisites

For more information about how to view the version, see Query the kernel version number.

Scenarios

Parallel queries are applicable to most SELECT statements, such as queries on large tables, multi-table queries that use JOINs, and queries on a large amount of data. The effect of parallel queries is insignificant for short queries.

• Lightweight analysis business

  Report queries are often complex and time-consuming. The parallel query feature can accelerate individual queries.

• More available system resources

  Parallel queries consume more system resources. You can use parallel queries to improve resource usage and query efficiency only when your system has sufficient CPU resources, low I/O loads, and sufficient memory.

• Combine online and offline queries

  You can use different endpoints to connect services to different database nodes. This ensures that the services do not affect each other. If you enable the parallel query feature for a single-node endpoint, the read-only node that corresponds to the single-node endpoint is accessed through the endpoint that is used for online analytical processing (OLAP).

  

How to use parallel queries

• Use system parameters

  PolarDB allows you to use the global parameter max_parallel_degree to specify the maximum number of threads that each SQL statement can use for parallel processing. The default value is 0. You can modify the parameter when this parameter is being used during parallel processing. After you modify the parameter, you do not need to restart the database. For more information, see Specify cluster parameters.

  We recommend that you configure the following parallel query settings:

  • Gradually increase the value of the max_parallel_degree parameter. We recommend that you set a value that does not exceed a quarter of CPU cores. Enable the parallel query feature only when your cluster has at least eight CPU cores. Do not enable the parallel query feature for small clusters. Set the max_parallel_degree parameter to 2 at the beginning when you use the parallel query feature. Check the CPU usage one day later. If the CPU usage is low, you can gradually increase the value. If the CPU usage is high, do not increase the value.
  • To disable the parallel query feature, set the max_parallel_degree parameter to 0. If the max_parallel_degree parameter is set to 1, the parallel query feature is enabled and the DOP is 1.
  • In the PolarDB console, the max_parallel_degree parameter is prefixed with loose: loose_max_parallel_degree. This way, the max_parallel_degree parameter can be supported by programs of other versions and is consistent with the default settings in the MySQL configuration file. This also ensures that no error is returned when programs of other versions recognize the parameter. For more information, see Program Option Modifiers.
  • When you enable the parallel query feature, set the innodb_adaptive_hash_index parameter to OFF. If the innodb_adaptive_hash_index parameter is set to ON, the parallel query performance is degraded.
  You can modify the global parameter to adjust the cluster-level DOP. You can also adjust the DOP for SQL statements in a specific session. For example, if you add a session-level environment variable to the setting of the Java Database Connectivity (JDBC) connection string, you can set the DOP for a specific application.

  set max_parallel_degree = n 

• Use hints

  You can optimize an individual statement by using hints. For example, if the parallel query feature is disabled by the system, you can use hints to accelerate a slow SQL query that is frequently executed. For more information, see Parallel hints.

• Use a threshold to specify whether the optimizer enables parallel queries

  PolarDB provides two thresholds to specify whether the optimizer enables parallel queries. Parallel queries are enabled by the optimizer when SQL statements meet one of the conditions that are specified by these thresholds.

  • records_threshold_for_parallelism

    If the number of scanned rows in the statement exceeds the value of records_threshold_for_parallelism, the optimizer enables parallel queries. The default value is 10000. If your services require a small volume of data or low concurrency of complex queries, we recommend that you set the threshold to a value greater than or equal to 2000.

    Note The number of scanned rows is estimated based on the statistics of tables. It may be different from the actual number of rows.
  • cost_threshold_for_parallelism

    If the overhead of sequential queries exceeds the value of cost_threshold_for_parallelism, the optimizer enables parallel queries. The default value is 50000.

Parameters and variables

Performance metrics

The following tests use 100 GB of data that is generated based on TPC Benchmark™H (TPC-H) to test the performance of a PolarDB for MySQL 8.0 cluster. In the test, the PolarDB cluster has a 32-core CPU and 256 GB of memory (). The max_parallel_degree parameter is set to 32 or 0. For more information, see Parallel query performance (OLAP).

The test result shows that 95% of the SQL queries in TPC-H are accelerated. For 70% of the accelerated SQL queries, the query speed is more than eight times that of SQL queries that are not executed in parallel.

Terms

• Parallel scans

  In a parallel scan, workers simultaneously scan the data of a table. Each worker produces a partial result and returns the partial result to a leader thread. The leader thread gathers the results by using the Gather node and returns the final result to the client.

• Parallel joins on multiple tables

  When parallel queries are enabled, the complete multi-table join operation is sent to workers for parallel processing. The PolarDB optimizer selects only the optimal table for parallel scanning. Non-parallel scanning is performed on all other tables. Each worker produces a partial result and returns the partial result to a leader thread. The leader thread gathers the results by using the Gather node and returns the final result to the client.

• Parallel sorting

  The PolarDB optimizer sends the ORDER BY operation to all workers for parallel processing. Each worker produces a partial result. The leader thread gathers, merges, and sorts all partial results by using the Gather Merge node, and returns the final sorting result to the client.

• Parallel grouping

  The PolarDB optimizer sends the GROUP BY operation to all workers for parallel processing. Each worker performs the GROUP BY operation on a portion of data. Each worker produces a partial result of GROUP BY. The leader thread gathers the results from all workers by using the Gather node. The PolarDB optimizer checks whether to perform a GROUP BY operation in the leader thread again based on the query plan. For example, if a loose index scan is used to run a GROUP BY query, the leader thread does not perform a GROUP BY operation. If the loose index scan is not used, the leader thread performs a GROUP BY operation and returns the final result to the client.

• Parallel aggregation

  The execution of the aggregate function is sent to all parallel workers. PolarDB supports parallel aggregation in two stages. Each worker that performs parallel queries first performs an aggregation. In this case, the leader thread gathers results from all workers by using the Gather or Gather Merge node. Then, the leader thread aggregates the results from all workers to the final result.

• Support for subqueries
  In parallel queries, subqueries can run by using one of the following policies:

  • Sequential execution in the leader thread

    If subqueries do not support parallel processing, the subqueries in the leader thread are performed in sequence. For example, if two tables are joined by using a JOIN clause that references user-defined functions (UDFs), the subqueries are performed in sequence.

  • Parallel execution in the leader thread. (Another group of workers is used)

    After a parallel plan is generated, the execution plan of the leader thread includes the subqueries that support parallel processing. However, these parallel subqueries cannot run in advance or cannot run in parallel based on the shared access policy. For example, if the subqueries include window functions, the subqueries cannot run in parallel based on the shared access policy.

  • Shared access

    After a parallel plan is generated, if the execution plans of workers reference the subqueries that support parallel processing, the PolarDB optimizer runs these parallel subqueries in advance. This allows the workers to directly access the results of the subqueries.

  • Pushed down

    After a parallel plan is generated, if the execution plans of workers reference correlated subqueries, the workers run these correlated subqueries.