Long Query
When you see an entry in the log that looks similar to this
[Wed Mar 26 06:26:19 2014] [error] [client 127.0.0.1] LONG QUERY: SELECT DISTINCT user.ID AS user_id, t.* FROM …
We have included it in the log to signify that the length of the query, which would be over 1,024 characters, is a Long Query and may be able to be optimized. We did not take any action on this query, it is just a notice to our customers. These only apply to Select statements, and not any other type of statements.
http://wpengine.com/support/long-query-killed-query-error-logs/