MDEV-18700 EXPLAIN EXTENDED shows a wrong operation for query

           with UNION ALL after INTERSECT

EXPLAIN EXTENDED erroneously showed UNION instead of UNION ALL in
the warning if UNION ALL followed INTERSECT or EXCEPT operations.
The bug was in the function st_select_lex_unit::print() that printed
the text of the query used in the warning.

mysql-test/main/union.result

@@ -2568,5 +2568,42 @@ c1
 -10
 drop table t1,t2;
 #
+# MDEV-18700: EXPLAIN EXTENDED for query with UNION ALL
+#             after INTERSECT/EXCEPT operations
+#
+create table t1 (a int);
+insert into t1 values (3), (1), (7), (3), (2), (7), (4);
+create table t2 (a int);
+insert into t2 values (4), (5), (9), (1), (8), (9);
+create table t3 (a int);
+insert into t3 values (8), (1), (8), (2), (3), (7), (2);
+explain extended
+select * from t2 where a < 5
+intersect
+select * from t3 where a < 5
+union all
+select * from t1 where a > 4;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
+1	PRIMARY	t2	ALL	NULL	NULL	NULL	NULL	6	100.00	Using where
+2	INTERSECT	t3	ALL	NULL	NULL	NULL	NULL	7	100.00	Using where
+3	UNION	t1	ALL	NULL	NULL	NULL	NULL	7	100.00	Using where
+NULL	UNIT RESULT	<unit1,2,3>	ALL	NULL	NULL	NULL	NULL	NULL	NULL	
+Warnings:
+Note	1003	/* select#1 */ select `test`.`t2`.`a` AS `a` from `test`.`t2` where `test`.`t2`.`a` < 5 intersect /* select#2 */ select `test`.`t3`.`a` AS `a` from `test`.`t3` where `test`.`t3`.`a` < 5 union all /* select#3 */ select `test`.`t1`.`a` AS `a` from `test`.`t1` where `test`.`t1`.`a` > 4
+explain extended
+select * from t2 where a < 5
+except
+select * from t3 where a < 5
+union all
+select * from t1 where a > 4;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
+1	PRIMARY	t2	ALL	NULL	NULL	NULL	NULL	6	100.00	Using where
+2	EXCEPT	t3	ALL	NULL	NULL	NULL	NULL	7	100.00	Using where
+3	UNION	t1	ALL	NULL	NULL	NULL	NULL	7	100.00	Using where
+NULL	UNIT RESULT	<unit1,2,3>	ALL	NULL	NULL	NULL	NULL	NULL	NULL	
+Warnings:
+Note	1003	/* select#1 */ select `test`.`t2`.`a` AS `a` from `test`.`t2` where `test`.`t2`.`a` < 5 except /* select#2 */ select `test`.`t3`.`a` AS `a` from `test`.`t3` where `test`.`t3`.`a` < 5 union all /* select#3 */ select `test`.`t1`.`a` AS `a` from `test`.`t1` where `test`.`t1`.`a` > 4
+drop table t1,t2,t3;
+#
 # End of 10.3 tests
 #

mysql-test/main/union.test

@@ -1811,6 +1811,36 @@ SELECT c1 FROM t1 UNION SELECT - @a FROM t2;
 
 drop table t1,t2;
 
+--echo #
+--echo # MDEV-18700: EXPLAIN EXTENDED for query with UNION ALL
+--echo #             after INTERSECT/EXCEPT operations
+--echo #
+
+create table t1 (a int);
+insert into t1 values (3), (1), (7), (3), (2), (7), (4);
+
+create table t2 (a int);
+insert into t2 values (4), (5), (9), (1), (8), (9);
+
+create table t3 (a int);
+insert into t3 values (8), (1), (8), (2), (3), (7), (2);
+
+explain extended
+select * from t2 where a < 5
+intersect
+select * from t3 where a < 5
+union all
+select * from t1 where a > 4;
+
+explain extended
+select * from t2 where a < 5
+except
+select * from t3 where a < 5
+union all
+select * from t1 where a > 4;
+
+drop table t1,t2,t3;
+
 --echo #
 --echo # End of 10.3 tests
 --echo #

sql/sql_lex.cc

@@ -2897,8 +2897,6 @@ void st_select_lex_unit::print(String *str, enum_query_type query_type)
         str->append(STRING_WITH_LEN(" union "));
         if (union_all)
           str->append(STRING_WITH_LEN("all "));
-        else if (union_distinct == sl)
-          union_all= TRUE;
         break;
       case INTERSECT_TYPE:
         str->append(STRING_WITH_LEN(" intersect "));
@@ -2907,6 +2905,8 @@ void st_select_lex_unit::print(String *str, enum_query_type query_type)
         str->append(STRING_WITH_LEN(" except "));
         break;
       }
+      if (sl == union_distinct)
+        union_all= TRUE;
     }
     if (sl->braces)
       str->append('(');