BUG#20484: "Partitions: crash with EXPLAIN and UNION"

 - Don't forget to produce "partitions" column for "UNION RESULT" row of EXPLAIN output

mysql-test/r/partition_pruning.result

@@ -697,3 +697,10 @@ ref	NULL
 rows	2
 Extra	
 drop table t2;
+create table t1 (s1 int);
+explain partitions select 1 from t1 union all select 2;
+id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t1	NULL	system	NULL	NULL	NULL	NULL	0	const row not found
+2	UNION	NULL	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
+NULL	UNION RESULT	<union1,2>	NULL	ALL	NULL	NULL	NULL	NULL	NULL	
+drop table t1;

mysql-test/t/partition_pruning.test

@@ -590,3 +590,10 @@ explain partitions select * from t2;
 --horizontal_results
 drop table t2;
 
+
+# BUG#20484 "Partitions: crash with explain and union"
+create table t1 (s1 int);
+explain partitions select 1 from t1 union all select 2;
+drop table t1;
+
+

sql/sql_select.cc

@@ -13938,6 +13938,9 @@ static void select_describe(JOIN *join, bool need_tmp_table, bool need_order,
       }
       item_list.push_back(new Item_string(table_name_buffer, len, cs));
     }
+    /* partitions */
+    if (join->thd->lex->describe & DESCRIBE_PARTITIONS)
+      item_list.push_back(item_null);
     /* type */
     item_list.push_back(new Item_string(join_type_str[JT_ALL],
 					  strlen(join_type_str[JT_ALL]),