Understand EXPLAIN mysql -

i trying interpret explain of mysql on query,this table:

    create table text_mess(     datamess timestamp(3) default 0,     sender bigint ,     recipient bigint ,     roger boolean,     msg char(255),     foreign key(recipient)              references users (tel)                          on delete cascade                         on update cascade, primary key(datamess,sender) ) engine = innodb 

this first type of query :

    explain     select /*!straight_join*/datamess, sender,recipient,roger,msg     text_mess join (select max(datamess)as dmess                     text_mess                      roger = true                     group sender,recipient) max                     on text_mess.datamess=max.dmess ;  

and second:explain

    explain     select /*!straight_join*/datamess, sender,recipient,roger,msg      (select max(datamess)as dmess                     text_mess                      roger = true                     group sender,recipient) max       join     text_mess     on max.dmess = text_mess.datamess ; 

the 2 queries asking same thing, difference order of ref_table (driving_table), in first case text_mess, in second case sub query : ![first , second query][1]

as can see difference in order of first 2 lines, question in particular on second (the faster query ) second line should inner-table, if so, why column ref tells me: max.dmess, should column of ref-table (sub-query). and, last row referred on how first built? , in end think there more efficient query?