recreate table tmp(s varchar(80) primary key using index tmp_s); commit;
insert into tmp select rpad('', 80, uuid_to_char(gen_uuid()))
from rdb$types,rdb$types
rows 1000;
commit;

execute block returns( via_table_sort int, via_plan_order int, via_tree_navigate int )
as
  declare t0 timestamp;
  declare t1 timestamp;
  declare t2 timestamp;
  declare t3 timestamp;
  declare i int;
  declare n int = 1000;
begin
  for select s from tmp as cursor c
  do begin
    execute statement( 'select rdb$set_context(''USER_TRANSACTION'', :ctx_name, :ctx_value) from rdb$database' ) (ctx_name := c.s, ctx_value := 1) into i;
  end
  t0='now';
  i=0;
  while(i<n) do select 0*count(*)+:i+1 from (select s from tmp order by s||'') into i;

  i=0;
  t1='now';
  while(i<n) do select 0*count(*)+:i+1 from (select s from tmp order by s) into i;

  i=0;
  t2='now';
  while(i<n) do select 0*count(*)+:i+1 from ( select c.mon$variable_name from mon$context_variables c where c.mon$transaction_id=current_transaction) into i;
  t3='now';

  via_table_sort = datediff(millisecond from t0 to t1);
  via_plan_order = datediff(millisecond from t1 to t2);
  via_tree_navigate = datediff(millisecond from t2 to t3);
  suspend;
end

VIA_TABLE_SORT VIA_PLAN_ORDER VIA_TREE_NAVIGATE
============== ============== =================
          2251           1717               509

PLAN (C TMP NATURAL)
PLAN SORT (TMP NATURAL)
PLAN (TMP ORDER TMP_S)
 PLAN (C NATURAL) 
1 records fetched
   4546 ms, 5053067 fetch(es)

Table                             Natural     Index    Update    Insert    Delete   Backout     Purge   Expunge
***************************************************************************************************************
RDB$RELATIONS                                     8
TMP                               1001000   1000000