Run one query and execution time is very different

[Metatrader EA <metatraderea@xxxxxxxxx>]

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