Fixed bug #21231: wrong results for a simple query with a

a non-correlated single-row subquery over information schema.

The function get_all_tables filling all information schema
tables reset lex->sql_command to SQLCOM_SHOW_FIELDS. After
this the function could evaluate partial conditions related to
some columns. If these conditions contained a subquery over
information schema it led to a wrong evaluation and a wrong 
result set.
This bug was already fixed in 5.1.
This patch follows the way how it was done in 5.1 where
the value of lex->sql_command is set to SQLCOM_SHOW_FIELDS
in get_all_tables only for the calls of the function 
open_normal_and_derived_tables and is restored after these
calls.

mysql-test/r/information_schema.result

@@ -1232,3 +1232,11 @@ TABLE_PRIVILEGES	TABLE_SCHEMA
 TRIGGERS	TRIGGER_SCHEMA
 USER_PRIVILEGES	GRANTEE
 VIEWS	TABLE_SCHEMA
+SELECT MAX(table_name) FROM information_schema.tables;
+MAX(table_name)
+VIEWS
+SELECT table_name from information_schema.tables
+WHERE table_name=(SELECT MAX(table_name)
+FROM information_schema.tables);
+table_name
+VIEWS

mysql-test/t/information_schema.test

@@ -920,4 +920,14 @@ SELECT t.table_name, c1.column_name
                   c2.column_name LIKE '%SCHEMA%'
         );
 
+#
+# Bug#21231: query with a simple non-correlated subquery over
+#            INFORMARTION_SCHEMA.TABLES 
+#
+
+SELECT MAX(table_name) FROM information_schema.tables;
+SELECT table_name from information_schema.tables
+  WHERE table_name=(SELECT MAX(table_name)
+                      FROM information_schema.tables);
+
 # End of 5.0 tests.

sql/sql_show.cc

@@ -2116,12 +2116,6 @@ int get_all_tables(THD *thd, TABLE_LIST *tables, COND *cond)
   LINT_INIT(end);
   LINT_INIT(len);
 
-  /*
-    Let us set fake sql_command so views won't try to merge
-    themselves into main statement.
-  */
-  lex->sql_command= SQLCOM_SHOW_FIELDS;
-
   lex->reset_n_backup_query_tables_list(&query_tables_list_backup);
 
   /*
@@ -2144,8 +2138,16 @@ int get_all_tables(THD *thd, TABLE_LIST *tables, COND *cond)
       I_S tables will be done.
     */
     thd->temporary_tables= open_tables_state_backup.temporary_tables;
+    /*
+      Let us set fake sql_command so views won't try to merge
+      themselves into main statement. If we don't do this,
+      SELECT * from information_schema.xxxx will cause problems.
+      SQLCOM_SHOW_FIELDS is used because it satisfies 'only_view_structure()' 
+    */
+    lex->sql_command= SQLCOM_SHOW_FIELDS;
     res= open_normal_and_derived_tables(thd, show_table_list,
                                         MYSQL_LOCK_IGNORE_FLUSH);
+    lex->sql_command= save_sql_command;
     /*
       get_all_tables() returns 1 on failure and 0 on success thus
       return only these and not the result code of ::process_table()
@@ -2267,8 +2269,10 @@ int get_all_tables(THD *thd, TABLE_LIST *tables, COND *cond)
             TABLE_LIST *show_table_list= (TABLE_LIST*) sel.table_list.first;
             lex->all_selects_list= &sel;
             lex->derived_tables= 0;
+            lex->sql_command= SQLCOM_SHOW_FIELDS;
             res= open_normal_and_derived_tables(thd, show_table_list,
                                                 MYSQL_LOCK_IGNORE_FLUSH);
+            lex->sql_command= save_sql_command;
             /*
               We should use show_table_list->alias instead of 
               show_table_list->table_name because table_name