Identifying root cause of slow query outliers from pt-query-digest? I've run pt-query-digest and noticed many outliers with very high query times. Many of these queries do not perform slowly when I run them myself, and I'm having trouble figuring out the root cause of these sporadic slow queries. Is there another tool I might use to shed more light on this situation? What might cause the same query to only be slow some of the time?
Queries can become slow in such cases:


server is shorted out with disk i/o
server is shorted out with memory and swapped
server have too small key_buffer_size and buffer should be refreshed prior to perform the query
server have too small (join|sort)_buffer_size and on-disk temporary tables are created


Any of the mentioned above can cause sporadical slowdown. List is not exhaustive, sure.