Fix problem with not getting 'No Database Selected' error

Docs/manual.texi:
  Changelog

Docs/manual.texi

@@ -45712,6 +45712,10 @@ Our TODO section contains what we plan to have in 4.0. @xref{TODO MySQL 4.0}.
 
 @itemize @bullet
 @item
+Cleaned up global lock handling for @code{FLUSH TABLES WITH READ LOCK}
+@item
+Fixed problem with @code{DATETIME = constant} in @code{WHERE} optimization.
+@item
 Speed up all internal list handling.
 @item
 Added support for @code{UNION}.

sql/sql_base.cc

@@ -1323,7 +1323,7 @@ int open_tables(THD *thd,TABLE_LIST *start)
   {
     if (!tables->table &&
 	!(tables->table=open_table(thd,
-				   tables->db ? tables->db : thd->db,
+				   tables->db,
 				   tables->real_name,
 				   tables->name, &refresh)))
     {
@@ -1380,7 +1380,7 @@ TABLE *open_ltable(THD *thd, TABLE_LIST *table_list, thr_lock_type lock_type)
   DBUG_ENTER("open_ltable");
 
   thd->proc_info="Opening table";
-  while (!(table=open_table(thd,table_list->db ? table_list->db : thd->db,
+  while (!(table=open_table(thd,table_list->db,
 			    table_list->real_name,table_list->name,
 			    &refresh)) && refresh) ;
   if (table)
@@ -1612,9 +1612,7 @@ find_field_in_tables(THD *thd,Item_field *item,TABLE_LIST *tables)
     for (; tables ; tables=tables->next)
     {
       if (!strcmp(tables->name,table_name) &&
-	  (!db ||
-	   (tables->db && !strcmp(db,tables->db)) ||
-	   (!tables->db && !strcmp(db,thd->db))))
+	  (!db || !strcmp(db,tables->db)))
       {
 	found_table=1;
 	Field *find=find_field_in_table(thd,tables->table,name,length,
@@ -1854,8 +1852,7 @@ insert_fields(THD *thd,TABLE_LIST *tables, const char *db_name,
 	check_grant_all_columns(thd,SELECT_ACL,table) )
       DBUG_RETURN(-1);
     if (!table_name || (!strcmp(table_name,tables->name) &&
-			(!db_name || !tables->db ||
-			 !strcmp(tables->db,db_name))))
+			(!db_name || !strcmp(tables->db,db_name))))
     {
       Field **ptr=table->field,*field;
       thd->used_tables|=table->map;
@@ -2079,7 +2076,8 @@ int mysql_create_index(THD *thd, TABLE_LIST *table_list, List<Key> &keys)
   create_info.db_type=DB_TYPE_DEFAULT;
   DBUG_RETURN(mysql_alter_table(thd,table_list->db,table_list->real_name,
 				&create_info, table_list,
-				fields, keys, drop, alter, (ORDER*)0, FALSE, DUP_ERROR));
+				fields, keys, drop, alter, (ORDER*)0, FALSE,
+				DUP_ERROR));
 }
 
 
@@ -2094,7 +2092,8 @@ int mysql_drop_index(THD *thd, TABLE_LIST *table_list, List<Alter_drop> &drop)
   create_info.db_type=DB_TYPE_DEFAULT;
   DBUG_RETURN(mysql_alter_table(thd,table_list->db,table_list->real_name,
 				&create_info, table_list,
-				fields, keys, drop, alter, (ORDER*)0, FALSE, DUP_ERROR));
+				fields, keys, drop, alter, (ORDER*)0, FALSE,
+				DUP_ERROR));
 }
 
 /*****************************************************************************

sql/sql_parse.cc

@@ -1810,16 +1810,14 @@ mysql_execute_command(void)
     DBUG_VOID_RETURN;
 #else
     {
-      char *db=tables->db ? tables->db : thd->db;
-      if (!db)
+      char *db=tables->db;
+      if (!*db)
       {
 	send_error(&thd->net,ER_NO_DB_ERROR);	/* purecov: inspected */
 	goto error;				/* purecov: inspected */
       }
       remove_escape(db);			// Fix escaped '_'
       remove_escape(tables->name);
-      if (!tables->db)
-	tables->db=thd->db;
       if (check_access(thd,SELECT_ACL | EXTRA_ACL,db,&thd->col_access))
 	goto error;				/* purecov: inspected */
       tables->grant.privilege=thd->col_access;
@@ -1837,7 +1835,7 @@ mysql_execute_command(void)
     DBUG_VOID_RETURN;
 #else
     {
-      char *db=tables->db ? tables->db : thd->db;
+      char *db=tables->db;
       if (!db)
       {
 	send_error(&thd->net,ER_NO_DB_ERROR);	/* purecov: inspected */
@@ -2178,7 +2176,7 @@ check_access(THD *thd,uint want_access,const char *db, uint *save_priv,
   else
     save_priv= &dummy;
 
-  if (!db && !thd->db && !dont_check_global_grants)
+  if (!db[0] && !thd->db && !dont_check_global_grants)
   {
     send_error(&thd->net,ER_NO_DB_ERROR);	/* purecov: tested */
     return TRUE;				/* purecov: tested */
@@ -2725,6 +2723,8 @@ add_proc_to_list(Item *item)
 
 static void remove_escape(char *name)
 {
+  if (!*name)					// For empty DB names
+    return;
   char *to;
 #ifdef USE_MB
   char *strend=name+(uint) strlen(name);