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

Understanding the "Sending Data" State in MySQL

MySQL's SHOW PROCESSLIST command provides insights into the execution status of running queries. One commonly encountered state is "Sending data."

What does "Sending Data" Indicate?

Contrary to what its name suggests, "Sending data" does not exclusively refer to the transfer of query results. Instead, it represents a broader stage in the query execution cycle, known as the "reading and filtering data" phase.

The Nature of the Reading Phase

During this phase, MySQL is actively accessing and processing data from various sources. These sources can include tables, indexes, temporary tables, or sorted outputs. Even if no actual data transmission has occurred, MySQL reports the state as "Sending data."

Example Scenario

Consider a table with 1 million records and an index. To retrieve a specific record without the index, MySQL must scan the entire table. During this process, the query will be marked as "Sending data," despite not sending any results until the required record is identified.

Additional Note for MySQL 8.0.17 and Later

In MySQL versions 8.0.17 and above, the "Sending data" state has been merged into the "Executing" state, eliminating the separate indication of this phase.

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