MySQL in the first query when the time is very slow, the second query is relatively fast reason?

1, the default query_cache is open

You use show global variables like '%query_cache% ';

Confirm that there is no open, if open. First query read data file, the second time will go query_cache, so it will soon. Of course, if the data is updated, you need to re-cache it.

2, the second case, query table engine: Show variables like '%storage_engine% ', the table engine uses InnoDB. The first query will also walk the data file, the second direct walk Buffer_pool, also faster than querying data files directly

MySQL in the first query when the time is very slow, the second query is relatively fast reason?