Run one query and execution time is very different

Hi,

I run one query and the execution is very different

Select messages
from mails_hist
join mails using (messages)
where message_date > '2016-07-19 00:00:00'
and message_date < '2016-10-17 00:00:00'
Group by message;

message_date is one timestamp

Query gives around 6300 rows

What can I look for ?

//Bill