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Troubleshooting Slow SQL Issues for RDS for MySQL DB Instances

Updated on 2025-02-13 GMT+08:00

This section describes how to troubleshoot slow SQL statements on RDS for MySQL DB instances. For any given service scenario, query efficiency depends on the architecture and on the database table and index design. Poorly designed architecture and indexes will cause many slow SQL statements.

Slow SQL Statements Caused by SQL Exceptions

  • Causes and symptoms

    There are many causes for SQL exceptions, for example, unsuitable database table structure design, missing indexes, or too many rows that need to be scanned.

    On the slow query logs page of the management console, you can download slow query logs to identify the slow SQL statements and see how long they took to execute. For details, see Viewing and Downloading Slow Query Logs.

  • Solution

    Optimize the SQL statements that you need to execute.

Slow SQL Statements Caused by DB Instance Limits

  • Causes and symptoms

    DB instance performance can be limited because:

    • Your workloads have been increasing but the storage has not been scaled up accordingly.
    • The performance of your DB instance has been deteriorating as the physical server of the instance ages.
    • The amount of data has been increasing, and the data structure has been changing.

    You can view the resource usage of the DB instance on the console. If the values of all resource usage metrics are close to 100%, your DB instance may reach its maximum performance. For details, see Viewing Performance Metrics of a DB Instance.

  • Solution

    Upgrade the instance class. For details, see Changing a DB Instance Class.

Slow SQL Statements Caused by Version Upgrades

  • Causes and symptoms

    Upgrading your DB instance may change the SQL execution plan. The join types determined in the execution plan are, in descending order of efficiency:

    system > const > eq_ref > ref > fulltext > ref_or_null > index_merge > unique_subquery > index_subquery > range > index > all

    For more information, see official MySQL documentation.

    If your application frequently resends query requests that specify range and index joins but RDS processes these query requests slowly, a number of SQL statements are parallelized. In this case, your application is slow to release threads. As a result, the connections in the connection pool get depleted, affecting all the workloads on your DB instance.

    You can log in to the console to see how many current connections your DB instance has established. For details, see Viewing Performance Metrics of a DB Instance.

  • Solution

    Analyze the index usage and the number of rows to scan, estimate the query efficiency, reconstruct SQL statements, and adjust indexes. For details, see Executing SQL Plan.

    NOTE:

    Incorrectly optimizing slow queries may cause service exceptions. Exercise caution when performing this operation.

Slow SQL Statements Caused by Inappropriate Parameter Settings

Slow SQL Statements Caused by Batch Operations

  • Causes and symptoms

    A large number of operations are performed to import, delete, and query data.

    You can view Total Storage Space, Storage Space Usage, and IOPS on the console. For details, see Viewing Performance Metrics of a DB Instance.

  • Solution

    Perform batch operations during off-peak hours, or split them.

Slow SQL Statements Caused by Scheduled Tasks

  • Causes and symptoms

    If the load of your DB instance changes regularly over time, there may be scheduled tasks causing this.

    You can view DELETE Statements per Second, INSERT Statements per Second, INSERT_SELECT Statements per Second, REPLACE Statements per Second, REPLACE_SELECTION Statements per Second, SELECT Statements per Second, and UPDATE Statements per Second on the console to determine whether the load has been changing regularly. For details, see Viewing Monitoring Metrics.

  • Solution

    Adjust the time when scheduled tasks are run. You are advised to run scheduled tasks during off-peak hours and change the maintenance window to off-peak hours. For details, see Changing the Maintenance Window.

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