What does it mean when MySQL is in the state “Sending data”?

Question

What does it mean if the Mysql query:

SHOW PROCESSLIST;

returns \"Sending data\" in the State column?

I imagine it means the quer

Answer 1

This is quite a misleading status. It should be called "reading and filtering data".

This means that MySQL has some data stored on the disk (or in memory) which is yet to be read and sent over. It may be the table itself, an index, a temporary table, a sorted output etc.

If you have a 1M records table (without an index) of which you need only one record, MySQL will still output the status as "sending data" while scanning the table, despite the fact it has not sent anything yet.