What Does the 'Sending Data' State Really Mean in MySQL?

Understanding the "Sending Data" State in MySQL

The "SHOW PROCESSLIST" command reveals the current state of running processes in MySQL. When this command returns "Sending data" in the State column for a specific query, it signifies that MySQL is in the process of transmitting results to the client. However, it's worth noting that this status can be somewhat misleading.

Contrary to what one might assume, "Sending data" does not necessarily imply that the query has been executed and the data is being actively sent to the client. In reality, this state indicates that MySQL is still in the process of reading and filtering data. This data could be located on disk, in memory, or may even involve sorting or temporary table creation.

The fact that this process is taking an extended period (up to an hour) could be due to various factors, such as a large amount of data being processed or inadequate performance optimization. Despite the fact that MySQL has not yet transmitted any data to the client, it may display the "Sending data" status while scanning through the data.

In MySQL 8.0.17 and later, the "Sending data" state has been incorporated into the "Executing" state and is no longer indicated separately. This change aims to provide a more accurate representation of the ongoing process.

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