What Does 'Sending Data' Really Mean in a MySQL SHOW PROCESSLIST Output?

Understanding the "Sending Data" State in MySQL

Introduction:
The MySQL SHOW PROCESSLIST command provides insights into the current state of running queries. When a query displays "Sending data" in the State column, it's crucial to understand what it truly signifies.

Interpretation of "Sending Data":
Contrary to its literal meaning, "Sending data" does not imply that the query has completed execution and MySQL is transmitting results to the client. Instead, it represents an intermediate state where MySQL is:

1. Reading and Filtering Data:
MySQL is accessing and processing data stored on disk or in memory. This can include reading from the table itself, an index, a temporary table, or a sorted output.

2. Pending Data Transmission:
Even though MySQL is actively reading and processing data, it may not yet have sent any results to the client. This is because MySQL may be optimizing the data transmission process or waiting for additional resources to become available.

3. Extended Duration:
Due to the nature of data reading and filtering, this state can persist for a significant duration, especially for large datasets or complex queries. In such cases, it's important to note that MySQL has not yet sent any results, despite the display of "Sending data."

Change in MySQL 8.0.17:
In MySQL versions 8.0.17 onward, the "Sending data" state is no longer indicated separately. Instead, it is incorporated into the broader "Executing" state, providing a more comprehensive view of the query's progress.

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