Introduced optimizer_switch for cost based order by limit optimization

mysql-test/main/mysqld--help.result

@@ -717,7 +717,8 @@ The following specify which files/extra groups are read (specified before remain
  extended_keys, exists_to_in, orderby_uses_equalities, 
  condition_pushdown_for_derived, split_materialized, 
  condition_pushdown_for_subquery, rowid_filter, 
- condition_pushdown_from_having, not_null_range_scan
+ condition_pushdown_from_having, not_null_range_scan, 
+ cost_based_order_by_limit
  --optimizer-trace=name 
  Controls tracing of the Optimizer:
  optimizer_trace=option=val[,option=val...], where option
@@ -1402,7 +1403,6 @@ The following specify which files/extra groups are read (specified before remain
  Prohibit update of a VIEW, which does not contain a key
  of the underlying table and the query uses a LIMIT clause
  (usually get from GUI tools)
- --use-sort-nest     Enable the sort nest
  --use-stat-tables=name 
  Specifies how to use system statistics tables. One of: 
  NEVER, COMPLEMENTARY, PREFERABLY, 
@@ -1782,7 +1782,6 @@ transaction-isolation REPEATABLE-READ
 transaction-prealloc-size 4096
 transaction-read-only FALSE
 updatable-views-with-limit YES
-use-sort-nest FALSE
 use-stat-tables PREFERABLY_FOR_QUERIES
 userstat FALSE
 verbose TRUE

mysql-test/main/opt_trace.result

@@ -2175,8 +2175,8 @@ explain  select * from t1 where a=1 and b=2 order by c limit 1	{
                   "index": "a_c",
                   "can_resolve_order": true,
                   "updated_limit": 47,
+                  "index_scan_time": 47,
                   "range_scan_time": 4.324,
-                  "index_scan_time": 4.324,
                   "records": 180,
                   "chosen": true
                 },