BUG#18659: In get_part_iter_for_interval_via_walking(), when we need to

obtain partition number, call partition_info->get_part_partition_id() when
the table has subpartitions, and get_partition_id() otherwise. (The bug
was that we were always doing the latter)

mysql-test/r/partition_pruning.result

@@ -583,3 +583,17 @@ explain partitions select * from t1 where a>='a' and a <= 'dddd';
 id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t1	p1,p2,p3,p4,p5	ALL	NULL	NULL	NULL	NULL	5	Using where
 drop table t1;
+create table t1 (f_int1 integer) partition by list(abs(mod(f_int1,2)))
+subpartition by hash(f_int1) subpartitions 2
+(
+partition part1 values in (0),
+partition part2 values in (1),
+partition part4 values in (null)
+);
+insert into t1 set f_int1 = null;
+select * from t1 where f_int1 is null;
+f_int1
+NULL
+explain partitions select * from t1 where f_int1 is null;
+id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	part4_p2sp0	system	NULL	NULL	NULL	NULL	1	

mysql-test/t/partition_pruning.test

@@ -479,5 +479,19 @@ select * from t1 where a>='a' and a <= 'dddd';
 explain partitions select * from t1 where a>='a' and a <= 'dddd';
 drop table t1;
 
+# BUG#18659: Assertion failure when subpartitioning is used and partition is
+# "IS NULL"
+create table t1 (f_int1 integer) partition by list(abs(mod(f_int1,2)))
+  subpartition by hash(f_int1) subpartitions 2
+(
+  partition part1 values in (0),
+  partition part2 values in (1),
+  partition part4 values in (null)
+);
+insert into t1 set f_int1 = null;
+
+select * from t1 where f_int1 is null;
+explain partitions select * from t1 where f_int1 is null;
+
 # No tests for NULLs in RANGE(monotonic_expr()) - they depend on BUG#15447
 # being fixed.

sql/sql_partition.cc

@@ -5521,7 +5521,11 @@ int get_part_iter_for_interval_via_walking(partition_info *part_info,
     else
     {
       longlong dummy;
-      if (!part_info->get_partition_id(part_info, &part_id, &dummy))
+      int res= part_info->is_sub_partitioned() ?
+                  part_info->get_part_partition_id(part_info, &part_id,
+                                                   &dummy):
+                  part_info->get_partition_id(part_info, &part_id, &dummy);
+      if (!res)
       {
         init_single_partition_iterator(part_id, part_iter);
         return 1; /* Ok, iterator initialized */