Understanding MySQL show processlist output

By gerald, 31 March, 2023
server

Photo by Brett Sayles: https://bit.ly/3K4NfjN

The MySQL command "SHOW PROCESSLIST" is used to display a list of active client connections to the MySQL server, along with details about the SQL statements being executed by each connection.

When you execute the SHOW PROCESSLIST command, you will receive a result set with the following columns:

  1. Id: This column displays the unique identifier for each client connection to the MySQL server.

  2. User: This column displays the username of the client connection.

  3. Host: This column displays the hostname or IP address of the client connection.

  4. db: This column displays the name of the database that the client connection is currently using.

  5. Command: This column displays the type of command being executed by the client connection. The most common commands are Sleep, Query, Connect, Binlog Dump, and Table Dump.
    Here are some common values that you may see in this column:

    • Sleep: This means that the client connection is idle and not executing any command.

    • Query: This means that the client connection is executing a query. The actual SQL query being executed can be found in the "Info" column.

    • Connect: This means that the client connection is in the process of connecting to the MySQL server.

    • Binlog Dump: This means that the client connection is reading binary logs from the MySQL server.

    • Table Dump: This means that the client connection is executing a table dump.

    • Init DB: This means that the client connection is in the process of switching to a different database.

    • Quit: This means that the client connection has sent a disconnect request to the MySQL server.

      Understanding the Command column can help you identify which clients use the most resources or causing performance issues on the MySQL server. For example, if you see many clients with a Query command that takes a long time to complete, you may need to optimize those queries to improve server performance.

  6. Time: This column displays the number of seconds that the client connection has been executing its current command.
    The Time column can be useful for identifying long-running queries or commands that may be causing performance issues on the MySQL server. For example, if you see a client connection with a Query command that has been executing for a long time in the Time column, it may indicate that the query is not optimized or that the server is experiencing resource contention.

    Note that the Time column does not necessarily indicate the total execution time for a query or command. Instead, it represents the time since the client connection started executing the current command. If a client connection has executed multiple commands during its session, the Time column will only reflect the time for the current command.

    It's also important to keep in mind that the Time column may not be accurate for very short-running commands or for very high-traffic servers with many client connections. In these cases, the Time column may not provide a reliable indication of query or command performance.

  7. State: This column displays the current state of the client connection. This can provide insight into what the client is currently doing and what it is waiting for.
    The State column can be useful for identifying what a client connection is currently doing and what it is waiting for. For example, a client connection with a Locked state may indicate that the query is waiting for a lock on a table or row before it can proceed. A client connection with a Sending Data state may indicate that the query is sending results back to the client.

    Here are some common values that you may see in the State column:

    The State column can provide valuable information for troubleshooting performance issues on the MySQL server. By identifying client connections with long wait times or frequent waits on specific locks or resources, you may need to optimize the queries or adjust the configuration to improve overall performance.

    • Init: This means that the client connection is initializing.

    • Waiting for table metadata lock: This means that the client connection is waiting for a lock on a table metadata.

    • Locked: This means that the client connection is waiting for a lock on a table or row.

    • Sending data: This means that the client connection is sending data back to the client.

    • Sorting result: This means that the client connection is sorting query results.

    • Copying to tmp table: This means that the client connection is copying data to a temporary table for further processing.

    • Query end: This means that the client connection has finished executing its current query.

    • InnoDB purge worker: This means that the thread is part of the InnoDB storage engine's internal background process for purging old undo log entries.

      InnoDB is a storage engine used by MySQL that provides features such as transactional processing and crash recovery. InnoDB uses an undo log to store the previous versions of modified records in case they need to be rolled back in the event of a transaction rollback or a system crash.

      The InnoDB purge worker is responsible for removing old undo log entries that are no longer needed by any active transaction. This is an important background task, as it helps to keep the size of the undo log under control and avoid running out of disk space.

      When you see the InnoDB purge worker state in the State column of the SHOW PROCESSLIST output, it means that the InnoDB storage engine is actively running a thread to perform this background task. This is a normal and expected state for an InnoDB-based MySQL server, and you should not typically need to take any action in response to it.

      However, if the database is experiencing performance issues or resource contention, it is possible that the InnoDB purge worker may be contributing to the problem. In such cases, you may want to investigate further to determine if there are any configuration changes or optimizations that can be made to improve performance.

    • InnoDB purge coordinator: This means that the thread is part of the InnoDB storage engine's internal background process for coordinating the purge of old undo log entries.

      InnoDB uses an undo log to store the previous versions of modified records in case they need to be rolled back in the event of a transaction rollback or a system crash. The InnoDB purge coordinator is responsible for coordinating the purge of old undo log entries with the InnoDB purge workers.

      When you see the InnoDB purge coordinator state in the State column of the SHOW PROCESSLIST output, it means that the InnoDB storage engine is actively running a thread to perform this background task. This is a normal and expected state for an InnoDB-based MySQL server, and you should not typically need to take any action in response to it.

      However, if you are experiencing performance issues or resource contention on the MySQL server, it is possible that the InnoDB purge coordinator may be contributing to the problem. In such cases, you may want to investigate further to determine if there are any configuration changes or optimizations that can be made to improve performance.

    • InnoDB shutdown handler: This means that the thread is part of the InnoDB storage engine's internal process for handling server shutdown.

      When MySQL shutdown, it needs to ensure that all transactions are properly committed or rolled back before the server exits. The InnoDB storage engine has a shutdown handler thread that is responsible for coordinating this process.

      When you see the InnoDB shutdown handler state in the State column of the SHOW PROCESSLIST output, it means that the InnoDB storage engine is actively running a thread to handle the shutdown process. This is a normal and expected state during server shutdown, and you should not typically need to take any action in response to it.

      However, if the InnoDB shutdown handler state persists for an unusually long time, or if the server shutdown process appears to be taking an excessive amount of time, it may indicate a problem with the server or the InnoDB storage engine. In such cases, you may want to investigate further to determine the cause of the problem and take appropriate action to resolve it.

    • Master has sent all binlog to slave; waiting for binlog to be updated: This means that a MySQL replication slave is currently waiting for new updates from the replication master.

      MySQL replication is a process that allows MySQL to create copies of a database on multiple servers, called replication slaves. The replication master records all changes to the database in a log called the binary log, and the replication slaves read this log and apply the changes to their own copies of the database.

      When the replication master has sent all of its binary log files to the replication slave, the slave enters the Master has sent all binlog to slave; waiting for binlog to be updated state. This means that the slave has caught up with the master and is now waiting for new updates to be sent.

      During this time, the slave is actively connected to the master and waiting for any new changes to be written to the binary log. Once new changes are available, the slave will read them and apply them to its own copy of the database.

      This state is a normal and expected part of the MySQL replication process, and you should not typically need to take any action in response to it. However, if the slave remains in this state for an extended period of time or if replication appears to be stalled, you may want to investigate further to determine the cause of the problem and take appropriate action to resolve it.

  8. Info: This column displays additional information about the current command being executed by the client connection. This can include the actual SQL statement being executed or other relevant details.

By using the SHOW PROCESSLIST command, you can gain a better understanding of the current state of the MySQL server and identify any problematic or resource-intensive client connections that may need attention.

When you run the SHOW PROCESSLIST command in MySQL, it returns a list of all currently executing processes on the MySQL server, including all running queries. If you are seeing a lot of queries in the output, it could mean that there are many active connections to the MySQL server, or that some queries are taking a long time to execute.

Here are a few things you can do to investigate and address the issue:

  1. Check the number of active connections: Use the command SHOW STATUS LIKE 'Threads_connected'; to check how many connections are currently active on the MySQL server. If this number is high, it could be causing performance issues. Consider optimizing the queries or adding more server resources to handle the load.

  2. Identify long-running queries: Look for queries that have been running for a long time in the Time column of the SHOW PROCESSLIST output. These could be causing performance issues and should be investigated further. Use the EXPLAIN command to analyze the query execution plan and identify any bottlenecks.

  3. Optimize the queries: Review the queries that are being executed and ensure that they are properly optimized. Make sure that you are using appropriate indexes, avoiding unnecessary joins, and optimizing subqueries.

  4. Consider adding caching: If the site is a high-traffic website or application, consider adding caching to reduce the number of queries that need to be executed on each request. You use Redis to store frequently accessed data in memory and serve it quickly.

By following these steps, you should be able to identify and address the cause of the high number of queries returned by SHOW PROCESSLIST.

Why are there so many Sleeping commands in [show processlist] result?

There can be several reasons why there are many Sleeping threads in the output of the SHOW PROCESSLIST command in MySQL:

  1. Connection Pooling: Many web applications use connection pooling, where idle connections are kept open and reused for subsequent requests. When a connection is not being used, it will be in the Sleeping state.

  2. Long-Running Transactions: If a transaction takes a long time to complete, the connection will be in the "Sleeping" state until the transaction is finished.

  3. Delayed Inserts: MySQL allows us to use the DELAYED keyword to insert data into a table, which can result in the connection being in the "Sleeping" state while waiting for the insert to occur.

  4. Waiting for Client: A connection will be in the Sleeping state if it is waiting for the client to send a query or request.

  5. Waiting for Locks: If a connection is waiting for a table or row lock, it will be in the Sleeping state until the lock is released.

In general, having a large number of Sleeping threads in the SHOW PROCESSLIST output is not necessarily a cause for concern. However, if you notice that the number of "Sleeping" threads is increasing over time, it may be an indication of connection pooling or transaction issues that require further investigation. In such cases, you may want to consider optimizing your application's database connections or reviewing the database transactions to improve performance.

 

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