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

Understanding "Sending data" State in MySQL

When executing the SHOW PROCESSLIST; query in MySQL, you may encounter the "Sending data" status in the State column. This status indicates that MySQL has begun transmitting data to the client. However, this process can sometimes take a significant amount of time, such as up to an hour.

Contrary to popular assumption, "Sending data" does not imply that the query has been entirely processed. Instead, it signifies that MySQL is still retrieving data from disk or memory to fulfill the query's requirements. This data could be related to the tables, indexes, temporary tables, or even sorted results.

For instance, if you have a table of 1 million records (without an index) and you only require a single record, MySQL will still display the "Sending data" status while scanning the entire table. This is because MySQL has not yet transmitted any data despite actively searching for the requested record.

Changes in MySQL 8.0.17 and Later

Starting with MySQL 8.0.17, the "Sending data" state is no longer reported separately. Instead, it is incorporated into the "Executing" state. This change simplifies the status information and better reflects the ongoing processing activities.

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