do not re-populate I_S tables in subqueries

mysql-test/r/information_schema2.result

@@ -6,3 +6,15 @@ select variable_name from information_schema.session_variables where variable_na
 (select variable_name from information_schema.session_variables where variable_name = 'basedir');
 variable_name
 BASEDIR
+create table t1 (a char);
+insert t1 values ('a'),('t'),('z');
+flush status;
+select a, exists (select 1 from information_schema.columns where table_schema=concat('tes',a)) from t1;
+a	exists (select 1 from information_schema.columns where table_schema=concat('tes',a))
+a	0
+t	1
+z	0
+show status like 'created_tmp_tables';
+Variable_name	Value
+Created_tmp_tables	38
+drop table t1;

mysql-test/t/information_schema2.test

@@ -7,3 +7,16 @@ select variable_name from information_schema.session_status where variable_name
 select variable_name from information_schema.session_variables where variable_name =
 (select variable_name from information_schema.session_variables where variable_name = 'basedir');
 
+#
+# information_schema tables inside subqueries, they should not be re-populated
+# (i_s.columns needs to scan i_s itself, creating a tmp table for every i_s
+# table. if it's re-populated, it'll do that multiple times)
+#
+create table t1 (a char);
+insert t1 values ('a'),('t'),('z');
+flush status;
+select a, exists (select 1 from information_schema.columns where table_schema=concat('tes',a)) from t1;
+# fix the result in ps-protocol
+--replace_result 39 38
+show status like 'created_tmp_tables';
+drop table t1;

sql/sql_show.cc

@@ -7720,15 +7720,20 @@ bool get_schema_tables_result(JOIN *join,
     TABLE_LIST *table_list= tab->table->pos_in_table_list;
     if (table_list->schema_table && thd->fill_information_schema_tables())
     {
+#if MYSQL_VERSION_ID > 100105
+#error I_S tables only need to be re-populated if make_cond_for_info_schema() will preserve outer fields
       bool is_subselect= (&lex->unit != lex->current_select->master_unit() &&
                           lex->current_select->master_unit()->item);
+#else
+#define is_subselect false
+#endif
 
       /* A value of 0 indicates a dummy implementation */
       if (table_list->schema_table->fill_table == 0)
         continue;
 
       /* skip I_S optimizations specific to get_all_tables */
-      if (thd->lex->describe &&
+      if (lex->describe &&
           (table_list->schema_table->fill_table != get_all_tables))
         continue;