enh: last function optimize

tests/script/tsim/parser/limit_stb.sim

@@ -542,7 +542,7 @@ if $data09 != 4 then
   return -1
 endi
 
-sql select _wstart, max(c1), min(c2), avg(c3), sum(c5), spread(c6), first(c7), last(c8), first(c9), t1 from $stb where ts >= $ts0 and ts <= $tsu and t1 > 1 and t1 < 5 and c1 > 0 and c2 < 9 and c3 > 1 and c4 < 7 and c5 > 4  partition by t1 interval(5m) order by t1 desc, _wstart limit 1 offset 0
+sql select _wstart, max(c1), min(c2), avg(c3), sum(c5), spread(c6), first(c7), last(c8), first(c9), t1 from $stb where ts >= $ts0 and ts <= $tsu and t1 > 1 and t1 < 5 and c1 > 0 and c2 < 9 and c3 > 1 and c4 < 7 and c5 > 4  partition by t1 interval(5m) order by t1 desc limit 1 offset 0
 if $rows != 1 then
   return -1
 endi