MDEV-19179 Regression: SELECT ... UNION ... with inconsistent column names fails

A bogus error message was issued when a condition was pushed into a
materialized derived table or view specified as union of selects with
aggregation when the corresponding columns of the selects had different
names. This happened because the expression pushed into having clauses of
the selects was adjusted for the names of the first select of the union.
The easiest solution was to rename the columns of the other selects to be
name compatible with the columns of the first select.

Approved by Oleksandr Byelkin <sanja@mariadb.com>

mysql-test/r/derived_cond_pushdown.result

@@ -10593,4 +10593,45 @@ a
 abc
 DROP VIEW v1;
 DROP TABLE t1;
+#
+# MDEV-19179: pushdown into UNION of aggregation selects whose
+#             corresponding columns have different names
+#
+create table t1 (a int);
+insert into t1 values (3), (7), (1);
+select *
+from (select min(a) as x from t1 union all select max(a) as y from t1) t
+where x>0;
+x
+1
+7
+explain extended select *
+from (select min(a) as x from t1 union all select max(a) as y from t1) t
+where x>0;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
+1	PRIMARY	<derived2>	ALL	NULL	NULL	NULL	NULL	6	100.00	Using where
+2	DERIVED	t1	ALL	NULL	NULL	NULL	NULL	3	100.00	
+3	UNION	t1	ALL	NULL	NULL	NULL	NULL	3	100.00	
+Warnings:
+Note	1003	select `t`.`x` AS `x` from (select min(`test`.`t1`.`a`) AS `x` from `test`.`t1` having `x` > 0 union all select max(`test`.`t1`.`a`) AS `x` from `test`.`t1` having `x` > 0) `t` where `t`.`x` > 0
+prepare stmt from "select *
+from (select min(a) as x from t1 union all select max(a) as y from t1) t
+where x>0";
+execute stmt;
+x
+1
+7
+execute stmt;
+x
+1
+7
+deallocate prepare stmt;
+create view v1(m) as
+select min(a) as x from t1 union all select max(a) as y from t1;
+select * from v1 where m > 0;
+m
+1
+7
+drop view v1;
+drop table t1;
 # End of 10.2 tests

mysql-test/t/derived_cond_pushdown.test

@@ -2184,4 +2184,32 @@ SELECT * FROM v1 WHERE IF( a REGEXP 'def', 'foo', a ) IN ('abc', 'foobar');
 DROP VIEW v1;
 DROP TABLE t1;
 
+--echo #
+--echo # MDEV-19179: pushdown into UNION of aggregation selects whose
+--echo #             corresponding columns have different names
+--echo #
+
+create table t1 (a int);
+insert into t1 values (3), (7), (1);
+
+let $q=
+select *
+from (select min(a) as x from t1 union all select max(a) as y from t1) t
+where x>0;
+
+eval $q;
+eval explain extended $q;
+
+eval prepare stmt from "$q";
+execute stmt;
+execute stmt;
+deallocate prepare stmt;
+
+create view v1(m) as
+select min(a) as x from t1 union all select max(a) as y from t1;
+select * from v1 where m > 0;
+
+drop view v1;
+drop table t1;
+
 --echo # End of 10.2 tests

sql/item.h

@@ -818,6 +818,12 @@ class Item: public Value_source,
   void set_name_for_rollback(THD *thd, const char *str, uint length,
                              CHARSET_INFO *cs);
   void rename(char *new_name);
+  void share_name_with(Item *item)
+  {
+    name= item->name;
+    name_length= item->name_length;
+    is_autogenerated_name= item->is_autogenerated_name;
+  }
   void init_make_field(Send_field *tmp_field,enum enum_field_types type);
   virtual void cleanup();
   virtual void make_field(THD *thd, Send_field *field);

sql/sql_derived.cc

@@ -1199,7 +1199,8 @@ bool pushdown_cond_for_derived(THD *thd, Item *cond, TABLE_LIST *derived)
     DBUG_RETURN(false);
 
   st_select_lex_unit *unit= derived->get_unit();
-  st_select_lex *sl= unit->first_select();
+  st_select_lex *first_sl= unit->first_select();
+  st_select_lex *sl= first_sl;
 
   if (derived->prohibit_cond_pushdown)
     DBUG_RETURN(false);
@@ -1311,7 +1312,24 @@ bool pushdown_cond_for_derived(THD *thd, Item *cond, TABLE_LIST *derived)
       if (!extracted_cond_copy)
         continue;
     }
-    
+
+    /*
+      Rename the columns of all non-first selects of a union to be compatible
+      by names with the columns of the first select. It will allow to use copies
+      of the same expression pushed into having clauses of different selects.
+    */
+    if (sl != first_sl)
+    {
+      DBUG_ASSERT(sl->item_list.elements == first_sl->item_list.elements);
+      List_iterator_fast<Item> it(sl->item_list);
+      List_iterator_fast<Item> nm_it(unit->types);
+      Item * item;
+      while((item= it++))
+      {
+        item->share_name_with(nm_it++);
+      }
+    }
+
     /*
       Transform the references to the 'derived' columns from the condition
       pushed into the having clause of sl to make them usable in the new context