3 files changed, 34 insertions, 15 deletions

diff --git a/sql/opt_trace.cc b/sql/opt_trace.cc
index d7b3d83bb18..01819314bb3 100644
--- a/sql/opt_trace.cc
+++ b/sql/opt_trace.cc
@@ -562,13 +562,10 @@ void Opt_trace_stmt::set_allowed_mem_size(size_t mem_size)
   current_json->set_size_limit(mem_size);
 }
 
-/*
-  Prefer this when you are iterating over JOIN_TABs
-*/
 
-void Json_writer::add_table_name(const JOIN_TAB *tab)
+void get_table_name_for_trace(const JOIN_TAB *tab, String *out)
 {
-  char table_name_buffer[SAFE_NAME_LEN];
+  char table_name_buffer[64];
   DBUG_ASSERT(tab != NULL);
   DBUG_ASSERT(tab->join->thd->trace_started());
 
@@ -578,7 +575,7 @@ void Json_writer::add_table_name(const JOIN_TAB *tab)
     size_t len= my_snprintf(table_name_buffer, sizeof(table_name_buffer)-1,
                             "<derived%u>",
                             tab->table->derived_select_number);
-    add_str(table_name_buffer, len);
+    out->copy(table_name_buffer, len, &my_charset_bin);
   }
   else if (tab->bush_children)
   {
@@ -587,15 +584,27 @@ void Json_writer::add_table_name(const JOIN_TAB *tab)
                             sizeof(table_name_buffer)-1,
                             "<subquery%d>",
                             ctab->emb_sj_nest->sj_subq_pred->get_identifier());
-    add_str(table_name_buffer, len);
+    out->copy(table_name_buffer, len, &my_charset_bin);
   }
   else
   {
     TABLE_LIST *real_table= tab->table->pos_in_table_list;
-    add_str(real_table->alias.str, real_table->alias.length);
+    out->set(real_table->alias.str, real_table->alias.length, &my_charset_bin);
   }
 }
 
+/*
+  Prefer this when you are iterating over JOIN_TABs
+*/
+
+void Json_writer::add_table_name(const JOIN_TAB *tab)
+{
+  String sbuf;
+  get_table_name_for_trace(tab, &sbuf);
+  add_str(sbuf.ptr(), sbuf.length());
+}
+
+
 void Json_writer::add_table_name(const TABLE *table)
 {
   add_str(table->pos_in_table_list->alias.str);
@@ -642,18 +651,27 @@ void add_table_scan_values_to_trace(THD *thd, JOIN_TAB *tab)
     analysis of the various join orders.
 */
 
-void trace_plan_prefix(JOIN *join, uint idx, table_map join_tables)
+void trace_plan_prefix(Json_writer_object *jsobj, JOIN *join, uint idx,
+                       table_map join_tables)
 {
   THD *const thd= join->thd;
   DBUG_ASSERT(thd->trace_started());
 
-  Json_writer_array plan_prefix(thd, "plan_prefix");
+  String prefix_str;
+  prefix_str.length(0);
   for (uint i= 0; i < idx; i++)
   {
     TABLE_LIST *const tr= join->positions[i].table->tab_list;
     if (!(tr->map & join_tables))
-      plan_prefix.add_table_name(join->positions[i].table);
+    {
+      String str;
+      get_table_name_for_trace(join->positions[i].table, &str);
+      if (prefix_str.length() != 0)
+        prefix_str.append(',');
+      prefix_str.append(str);
+    }
   }
+  jsobj->add("plan_prefix", prefix_str.ptr(), prefix_str.length());
 }
 
 
diff --git a/sql/opt_trace.h b/sql/opt_trace.h
index a43c1dde54b..c6b5c4ea338 100644
--- a/sql/opt_trace.h
+++ b/sql/opt_trace.h
@@ -107,7 +107,8 @@ void opt_trace_print_expanded_query(THD *thd, SELECT_LEX *select_lex,
                                     Json_writer_object *trace_object);
 
 void add_table_scan_values_to_trace(THD *thd, JOIN_TAB *tab);
-void trace_plan_prefix(JOIN *join, uint idx, table_map join_tables);
+void trace_plan_prefix(Json_writer_object *jsobj, JOIN *join, uint idx,
+                       table_map join_tables);
 void print_final_join_order(JOIN *join);
 void print_best_access_for_table(THD *thd, POSITION *pos);
 
diff --git a/sql/sql_select.cc b/sql/sql_select.cc
index 17068c51e83..61f47ea045f 100644
--- a/sql/sql_select.cc
+++ b/sql/sql_select.cc
@@ -9670,7 +9670,7 @@ optimize_straight_join(JOIN *join, table_map remaining_tables)
     double original_record_count, current_record_count;
 
     if (unlikely(thd->trace_started()))
-      trace_plan_prefix(join, idx, remaining_tables);
+      trace_plan_prefix(&trace_one_table, join, idx, remaining_tables);
     /* Find the best access method from 's' to the current partial plan */
     best_access_path(join, s, remaining_tables, join->positions, idx,
                      disable_jbuf, record_count,
@@ -10877,7 +10877,7 @@ best_extension_by_limited_search(JOIN      *join,
     Json_writer_object trace_one_table(thd);
     JOIN_TAB **best_ref= join->best_ref + idx;
     if (unlikely(thd->trace_started()))
-      trace_plan_prefix(join, idx, remaining_tables);
+      trace_plan_prefix(&trace_one_table, join, idx, remaining_tables);
 
     Json_writer_array arr(thd, "get_costs_for_tables");
 
@@ -10944,7 +10944,7 @@ best_extension_by_limited_search(JOIN      *join,
 
       if (unlikely(thd->trace_started()))
       {
-        trace_plan_prefix(join, idx, remaining_tables);
+        trace_plan_prefix(&trace_one_table, join, idx, remaining_tables);
         trace_one_table.add_table_name(s);
       }