Bug#21299665 INORDERNATE MEMORY USAGE QUERYING I_S.COLUMNS

Some queries using the INFORMATION_SCHEMA can consume excessive
amount of memory because of a suboptimal query plan and lack of
proper materialization. This patch applies a temporary mem_root
on during the JOIN-execution phase when the I_S tables are
materialized as temporary tables.

Author: Kristofer Pettersson

sql/sql_show.cc

@@ -3683,14 +3683,15 @@ make_table_name_list(THD *thd, List<LEX_STRING> *table_names, LEX *lex,
   @retval TRUE  - Failure.
 */
 static bool
-fill_schema_table_by_open(THD *thd, bool is_show_fields_or_keys,
+fill_schema_table_by_open(THD *thd, MEM_ROOT *mem_root, 
+                          bool is_show_fields_or_keys,
                           TABLE *table, ST_SCHEMA_TABLE *schema_table,
                           LEX_STRING *orig_db_name,
                           LEX_STRING *orig_table_name,
                           Open_tables_backup *open_tables_state_backup,
                           bool can_deadlock)
 {
-  Query_arena i_s_arena(thd->mem_root,
+  Query_arena i_s_arena(mem_root,
                         Query_arena::STMT_CONVENTIONAL_EXECUTION),
               backup_arena, *old_arena;
   LEX *old_lex= thd->lex, temp_lex, *lex;
@@ -4390,7 +4391,7 @@ int get_all_tables(THD *thd, TABLE_LIST *tables, Item *cond)
     table_name.str= const_cast<char*>(lsel->table_list.first->table_name);
     table_name.length= lsel->table_list.first->table_name_length;
 
-    error= fill_schema_table_by_open(thd, TRUE,
+    error= fill_schema_table_by_open(thd, &tmp_mem_root, TRUE,
                                      table, schema_table,
                                      &db_name, &table_name,
                                      &open_tables_state_backup,
@@ -4532,7 +4533,7 @@ int get_all_tables(THD *thd, TABLE_LIST *tables, Item *cond)
 
             DEBUG_SYNC(thd, "before_open_in_get_all_tables");
 
-            if (fill_schema_table_by_open(thd, FALSE,
+            if (fill_schema_table_by_open(thd, &tmp_mem_root, FALSE,
                                           table, schema_table,
                                           db_name, table_name,
                                           &open_tables_state_backup,