EXPLAIN on UNION"s

mysql-test/r/union.result:
  New result for EXPLAIN UNION's
mysql-test/t/union.test:
  No error when EXPLAIN is done on UNION's

mysql-test/r/union.result

@@ -56,3 +56,6 @@ t2	c	1
 t2	d	1
 t2	e	1
 t2	f	1
+table	type	possible_keys	key	key_len	ref	rows	Extra
+t1	ALL	NULL	NULL	NULL	NULL	4	
+t2	ALL	NULL	NULL	NULL	NULL	4	

mysql-test/t/union.test

@@ -17,7 +17,6 @@ select a,b from t1 union select a,b from t1;
 select 't1',b,count(*) from t1 group by b UNION select 't2',b,count(*) from t2 group by b;
 
 # Test some error conditions with UNION
---error 1215
 explain select a,b from t1 union all select a,b from t2;
 
 --error 1215

sql/sql_select.cc

@@ -6752,23 +6752,25 @@ static void select_describe(JOIN *join, bool need_tmp_table, bool need_order,
 
   /* Don't log this into the slow query log */
   join->thd->lex.select_lex.options&= ~(QUERY_NO_INDEX_USED | QUERY_NO_GOOD_INDEX_USED);
-  field_list.push_back(new Item_empty_string("table",NAME_LEN));
-  field_list.push_back(new Item_empty_string("type",10));
-  field_list.push_back(item=new Item_empty_string("possible_keys",
+  if (join->thd->lex.select == &join->thd->lex.select_lex)
+  {
+    field_list.push_back(new Item_empty_string("table",NAME_LEN));
+    field_list.push_back(new Item_empty_string("type",10));
+    field_list.push_back(item=new Item_empty_string("possible_keys",
 						  NAME_LEN*MAX_KEY));
-  item->maybe_null=1;
-  field_list.push_back(item=new Item_empty_string("key",NAME_LEN));
-  item->maybe_null=1;
-  field_list.push_back(item=new Item_int("key_len",0,3));
-  item->maybe_null=1;
-  field_list.push_back(item=new Item_empty_string("ref",
-						  NAME_LEN*MAX_REF_PARTS));
-  item->maybe_null=1;
-  field_list.push_back(new Item_real("rows",0.0,0,10));
-  field_list.push_back(new Item_empty_string("Extra",255));
-  if (send_fields(thd,field_list,1))
-    return; /* purecov: inspected */
-
+    item->maybe_null=1;
+    field_list.push_back(item=new Item_empty_string("key",NAME_LEN));
+    item->maybe_null=1;
+    field_list.push_back(item=new Item_int("key_len",0,3));
+    item->maybe_null=1;
+    field_list.push_back(item=new Item_empty_string("ref",
+						    NAME_LEN*MAX_REF_PARTS));
+    item->maybe_null=1;
+    field_list.push_back(new Item_real("rows",0.0,0,10));
+    field_list.push_back(new Item_empty_string("Extra",255));
+    if (send_fields(thd,field_list,1))
+      return;
+  }
   char buff[512],*buff_ptr;
   String tmp(buff,sizeof(buff)),*packet= &thd->packet;
   table_map used_tables=0;
@@ -6899,7 +6901,8 @@ static void select_describe(JOIN *join, bool need_tmp_table, bool need_order,
     // For next iteration
     used_tables|=table->map;
   }
-  send_eof(&thd->net);
+  if (!join->thd->lex.select->next)
+    send_eof(&thd->net);
   DBUG_VOID_RETURN;
 }
 

sql/sql_union.cc

@@ -38,12 +38,6 @@ int mysql_union(THD *thd, LEX *lex,select_result *result)
   uint elements;
   DBUG_ENTER("mysql_union");
 
-  if (lex->select_lex.options & SELECT_DESCRIBE)
-  {
-    my_error(ER_WRONG_USAGE,MYF(0),"DESCRIBE","UNION");
-    return -1;
-  } 
-
   /* Fix tables--to-be-unioned-from list to point at opened tables */
   for (sl=&lex->select_lex; sl; sl=sl->next)
   {
@@ -66,6 +60,25 @@ int mysql_union(THD *thd, LEX *lex,select_result *result)
     }
   }
 
+  if (lex->select_lex.options & SELECT_DESCRIBE)
+  {
+    for (sl= &lex->select_lex; sl; sl=sl->next)
+    {
+      lex->select=sl;
+      res=mysql_select(thd, (TABLE_LIST*) sl->table_list.first,
+		       sl->item_list,
+		       sl->where,
+		       sl->ftfunc_list,
+		       (ORDER*) 0,
+		       (ORDER*) sl->group_list.first,
+		       sl->having,
+		       (ORDER*) NULL,
+		       sl->options | thd->options | SELECT_NO_UNLOCK | SELECT_DESCRIBE,
+		       result);
+    }
+    return 0;
+  } 
+
   order = (ORDER *) last_sl->order_list.first;
   {
     Item *item;