EXPLAIN FORMAT=JSON: produce the 'ref' column.

mysql-test/r/explain_json.result

@@ -64,6 +64,7 @@ EXPLAIN
       "key": "a",
       "used_key_parts": "TODO",
       "key_length": "5",
+      "ref": ["test.t0.a"],
       "rows": 1,
       "filtered": 100
     }

sql/sql_explain.cc

@@ -253,7 +253,7 @@ static void push_string(List<Item> *item_list, String *str)
                        system_charset_info));
 }
 
-static void push_string_list(List<Item> *item_list, List<char> &lines, 
+static void push_string_list(List<Item> *item_list, String_list &lines, 
                              String *buf)
 {
   List_iterator_fast<char> it(lines);
@@ -677,10 +677,11 @@ int Explain_table_access::print_explain(select_result_sink *output, uint8 explai
     item_list.push_back(item_null);
 
   /* `ref` */
-  if (ref_set)
-    push_string(&item_list, &ref);
-  else
+  StringBuffer<64> ref_list_buf;
+  if (ref_list.is_empty())
     item_list.push_back(item_null);
+  else
+    push_string_list(&item_list, ref_list, &ref_list_buf);
  
   /* `rows` */
   if (rows_set)
@@ -772,6 +773,18 @@ int Explain_table_access::print_explain(select_result_sink *output, uint8 explai
 }
 
 
+bool String_list::append_str(MEM_ROOT *mem_root, const char *str)
+{
+  size_t len= strlen(str);
+  char *cp;
+  if (!(cp = (char*)alloc_root(mem_root, len)))
+    return 1;
+  memcpy(cp, str, len+1);
+  push_back(cp);
+  return 0;
+}
+
+
 static void write_item(Json_writer *writer, Item *item)
 {
   THD *thd= current_thd;
@@ -857,12 +870,26 @@ void Explain_table_access::print_explain_json(Json_writer *writer,
   /* `used_key_parts` */
   if (key_str.length())
     writer->add_member("used_key_parts").add_str("TODO");
-
+  
+  /* `key_length` */
   StringBuffer<64> key_len_str;
   fill_key_len_str(&key_len_str);
   if (key_len_str.length())
     writer->add_member("key_length").add_str(key_len_str);
+  
+  /* `ref` */
+  // TODO: need to print this as an array.
+  if (!ref_list.is_empty())
+  {
+    List_iterator_fast<char> it(ref_list);
+    const char *str;
+    writer->add_member("ref").start_array();
+    while ((str= it++))
+      writer->add_str(str);
+    writer->end_array();
+  }
 
+  /* `rows` */
   if (rows_set)
     writer->add_member("rows").add_ll(rows);
 
@@ -873,6 +900,7 @@ void Explain_table_access::print_explain_json(Json_writer *writer,
     writer->add_member("r_rows").add_ll(avg_rows);
   }
   
+  /* `filtered` */
   if (filtered_set)
     writer->add_member("filtered").add_double(filtered);
 

sql/sql_explain.h

@@ -464,6 +464,13 @@ private:
 };
 
 
+class String_list: public List<char>
+{
+public:
+  bool append_str(MEM_ROOT *mem_root, const char *str);
+};
+
+
 /*
   EXPLAIN data structure for a single JOIN_TAB.
 */
@@ -489,9 +496,8 @@ public:
   StringBuffer<32> used_partitions;
   bool used_partitions_set;
   
-  /* Empty string means "NULL" will be printed */
-  List<char> possible_keys;
-  //StringBuffer<32> possible_keys_str;
+  /* Empty means "NULL" will be printed */
+  String_list possible_keys;
   
   /*
     Index use: key name and length.
@@ -509,8 +515,7 @@ public:
   */
   Explain_index_use hash_next_key;
   
-  bool ref_set; /* not set means 'NULL' should be printed */
-  StringBuffer<32> ref;
+  String_list ref_list;
 
   bool rows_set; /* not set means 'NULL' should be printed */
   ha_rows rows;

sql/sql_select.cc

@@ -23261,23 +23261,14 @@ void explain_append_mrr_info(QUICK_RANGE_SELECT *quick, String *res)
 
 
 ///////////////////////////////////////////////////////////////////////////////
-// TODO: join with make_possible_keys_line ?
-int append_possible_keys(MEM_ROOT *alloc, List<char> &list, TABLE *table, 
+int append_possible_keys(MEM_ROOT *alloc, String_list &list, TABLE *table, 
                          key_map possible_keys)
 {
   uint j;
   for (j=0 ; j < table->s->keys ; j++)
   {
     if (possible_keys.is_set(j))
-    {
-      const char *key_name= table->key_info[j].name;
-      size_t len= strlen(key_name);
-      char *cp;
-      if (!(cp = (char*)alloc_root(alloc, len)))
-        return 1;
-      memcpy(cp, key_name, len+1);
-      list.push_back(cp);
-    }
+      list.append_str(alloc, table->key_info[j].name);
   }
   return 0;
 }
@@ -23312,13 +23303,10 @@ void JOIN_TAB::save_explain_data(Explain_table_access *eta, table_map prefix_tab
 
   TABLE *table=tab->table;
   TABLE_LIST *table_list= tab->table->pos_in_table_list;
-  char buff4[512];
   my_bool key_read;
   char table_name_buffer[SAFE_NAME_LEN];
-  String tmp4(buff4,sizeof(buff4),cs);
   KEY *key_info= 0;
   uint key_len= 0;
-  tmp4.length(0);
   quick_type= -1;
   QUICK_SELECT_I *quick= NULL;
 
@@ -23456,14 +23444,11 @@ void JOIN_TAB::save_explain_data(Explain_table_access *eta, table_map prefix_tab
       store_key **ref=tab->ref.key_copy;
       for (uint kp= 0; kp < tab->ref.key_parts; kp++)
       {
-        if (tmp4.length())
-          tmp4.append(',');
-
         if ((key_part_map(1) << kp) & tab->ref.const_ref_part_map)
-          tmp4.append("const");
+          eta->ref_list.append_str(thd->mem_root, "const");
         else
         {
-          tmp4.append((*ref)->name(), strlen((*ref)->name()), cs);
+          eta->ref_list.append_str(thd->mem_root, (*ref)->name());
           ref++;
         }
       }
@@ -23477,17 +23462,7 @@ void JOIN_TAB::save_explain_data(Explain_table_access *eta, table_map prefix_tab
                            table->key_info[tab->index].key_length);
   }
 
-  if (key_info)
-  {
-    if (key_info && tab_type != JT_NEXT)
-    {
-      eta->ref.copy(tmp4);
-      eta->ref_set= true;
-    }
-    else
-      eta->ref_set= false;
-  }
-  else
+  if (!key_info)
   {
     if (table_list && /* SJM bushes don't have table_list */
         table_list->schema_table &&
@@ -23517,7 +23492,6 @@ void JOIN_TAB::save_explain_data(Explain_table_access *eta, table_map prefix_tab
       if (key_name_buf.length())
         eta->key.set(thd->mem_root, key_name_buf.c_ptr_safe(), -1);
     }
-    eta->ref_set= false;
   }
   
   /* "rows" */