recreate table td(id int primary key, pid int , s01 varchar(16) ); commit;
insert into td select row_number()over(), 1+rand()*49999, left(uuid_to_char(gen_uuid()),16)
from rdb$types,rdb$types,rdb$types rows 1000000;
commit;
create index td_pid_id on td(pid, id); 
commit;

 var-1. 
SQL> set explain on;
SQL> out /dev/null;
SQL> select pid,max(id) from td group by pid;

Select Expression
    -> Aggregate
        -> Table "TD" Access By ID
            -> Index "TD_PID_ID" Full Scan

 var-2. 
SQL> select pid+0 pid, max(id) from td group by pid+0;

Select Expression
    -> Aggregate
        -> Sort (record length: 40, key length: 12)
            -> Table "TD" Full Scan

2014-04-06T02:14:26.2540 (6403:0x7f309ceda100) EXECUTE_STATEMENT_FINISH
        sqlex30 (ATT_397, SYSDBA:NONE, NONE, TCPv4:127.0.0.1)
        /opt/fb30trnk/bin/isql:361
                (TRA_12824, CONCURRENCY | WAIT | READ_WRITE)

Statement 137:
-------------------------------------------------------------------------------
select pid,max(id) from td group by pid
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
PLAN (TD ORDER TD_PID_ID)
50000 records fetched
    3712 ms, 893948 read(s), 3002232 fetch(es) 

Table                             Natural     Index    Update    Insert    Delete   Backout     Purge   Expunge
***************************************************************************************************************
TD                                          1000000                                                           

2014-04-06T02:14:59.5580 (6403:0x7f309ceda100) EXECUTE_STATEMENT_FINISH
        sqlex30 (ATT_397, SYSDBA:NONE, NONE, TCPv4:127.0.0.1)
        /opt/fb30trnk/bin/isql:361
                (TRA_12824, CONCURRENCY | WAIT | READ_WRITE)

Statement 138:
-------------------------------------------------------------------------------
select pid+0 pid, max(id) from td group by pid+0
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
PLAN SORT (TD NATURAL)
50000 records fetched
    1694 ms, 18106 read(s), 2036385 fetch(es) 

Table                             Natural     Index    Update    Insert    Delete   Backout     Purge   Expunge
***************************************************************************************************************
TD                                1000000