Optimizer Trace: make plan_prefix not show const/eliminated tables

Author: spetrunia

mysql-test/main/opt_trace.result

@@ -2701,7 +2701,7 @@ select t1.a from t1 left join t2 on t1.a=t2.a	{
           {
             "considered_execution_plans": [
               {
-                "plan_prefix": "t2",
+                "plan_prefix": "",
                 "get_costs_for_tables": [
                   {
                     "best_access_path": {
@@ -2732,7 +2732,7 @@ select t1.a from t1 left join t2 on t1.a=t2.a	{
                 ]
               },
               {
-                "plan_prefix": "t2",
+                "plan_prefix": "",
                 "table": "t1",
                 "rows_for_plan": 4,
                 "cost_for_plan": 0.01066122
@@ -3108,7 +3108,7 @@ explain select t1.a from t1 left join (t2 join t3 on t2.b=t3.b) on t2.a=t1.a and
           {
             "considered_execution_plans": [
               {
-                "plan_prefix": "t3,t2",
+                "plan_prefix": "",
                 "get_costs_for_tables": [
                   {
                     "best_access_path": {
@@ -3139,7 +3139,7 @@ explain select t1.a from t1 left join (t2 join t3 on t2.b=t3.b) on t2.a=t1.a and
                 ]
               },
               {
-                "plan_prefix": "t3,t2",
+                "plan_prefix": "",
                 "table": "t1",
                 "rows_for_plan": 4,
                 "cost_for_plan": 0.01066122

sql/opt_trace.cc

@@ -658,7 +658,7 @@ void trace_plan_prefix(Json_writer_object *jsobj, JOIN *join, uint idx,
 
   String prefix_str;
   prefix_str.length(0);
-  for (uint i= 0; i < idx; i++)
+  for (uint i= join->const_tables; i < idx; i++)
   {
     TABLE_LIST *const tr= join->positions[i].table->tab_list;
     if (!(tr->map & join_tables))