List<Item> *fields= (this)->fields;

(gdb) p *this->fields

$29 = {<base_list> = {<Sql_alloc> = {<No data fields>}, first = 0x7fffd4013ed8, last = 0x7fffd4013fe0, elements = 2}, <No data fields>}

(gdb) p *(this-1)->fields

$27 = {<base_list> = {<Sql_alloc> = {<No data fields>}, first = 0x555556dbd9d8 <vtable for Window_spec+16>, last = 0xa5a5a5a5a5a5a501, elements = 0}, <No data fields>}

(gdb) p this->fields

$28 = (List<Item> *) 0x7fffd4013648

List_iterator<Item> it(*fields);

(gdb) p it

$20 = {<base_list_iterator> = {list = 0x7fffd4012660, el = 0x7fffd4012660, prev = 0x0, current = 0x0}, <No data fields>}

#3  <signal handler called>

#4  0x00007fce9ade0390 in st_join_table::remove_duplicates (this=0x7fce80013ac0) at /data/src/bb-10.2-mdev13355/sql/sql_select.cc:21792

#7  0x00007fce9add81f4 in sub_select_postjoin_aggr (join=0x7fce80013198, join_tab=0x7fce80013ac0, end_of_records=true) at /data/src/bb-10.2-mdev13355/sql/sql_select.cc:18270

#8  0x00007fce9add7d30 in do_select (join=0x7fce80013198, procedure=0x0) at /data/src/bb-10.2-mdev13355/sql/sql_select.cc:18101

#9  0x00007fce9adb2589 in JOIN::exec_inner (this=0x7fce80013198) at /data/src/bb-10.2-mdev13355/sql/sql_select.cc:3484

#10 0x00007fce9adb1a38 in JOIN::exec (this=0x7fce80013198) at /data/src/bb-10.2-mdev13355/sql/sql_select.cc:3279

#11 0x00007fce9adb2c01 in mysql_select (thd=0x7fce80000b00, tables=0x7fce800129f0, wild_num=0, fields=..., conds=0x7fce80013020, og_num=0, order=0x0, group=0x0, having=0x0, proc_param=0x0, select_options=2147748609, result=0x7fce80013178, unit=0x7fce800045e8, select_lex=0x7fce80004d20) at /data/src/bb-10.2-mdev13355/sql/sql_select.cc:3679

#12 0x00007fce9ada750c in handle_select (thd=0x7fce80000b00, lex=0x7fce80004520, result=0x7fce80013178, setup_tables_done_option=0) at /data/src/bb-10.2-mdev13355/sql/sql_select.cc:373

#13 0x00007fce9ad733ea in execute_sqlcom_select (thd=0x7fce80000b00, all_tables=0x7fce800129f0) at /data/src/bb-10.2-mdev13355/sql/sql_parse.cc:6443

#14 0x00007fce9ad69429 in mysql_execute_command (thd=0x7fce80000b00) at /data/src/bb-10.2-mdev13355/sql/sql_parse.cc:3458

#15 0x00007fce9ad76daa in mysql_parse (thd=0x7fce80000b00, rawbuf=0x7fce80012378 "SELECT DISTINCT MAX(i), ROW_NUMBER() OVER () FROM t1 WHERE 0", length=60, parser_state=0x7fce92960200, is_com_multi=false, is_next_command=false) at /data/src/bb-10.2-mdev13355/sql/sql_parse.cc:7879

#16 0x00007fce9ad64e56 in dispatch_command (command=COM_QUERY, thd=0x7fce80000b00, packet=0x7fce80163bd1 "", packet_length=60, is_com_multi=false, is_next_command=false) at /data/src/bb-10.2-mdev13355/sql/sql_parse.cc:1817

#17 0x00007fce9ad63797 in do_command (thd=0x7fce80000b00) at /data/src/bb-10.2-mdev13355/sql/sql_parse.cc:1362

#18 0x00007fce9aeaf549 in do_handle_one_connection (connect=0x7fce9e6e4a50) at /data/src/bb-10.2-mdev13355/sql/sql_connect.cc:1354

#19 0x00007fce9aeaf2d6 in handle_one_connection (arg=0x7fce9e6e4a50) at /data/src/bb-10.2-mdev13355/sql/sql_connect.cc:1260

#20 0x00007fce9b1f6984 in pfs_spawn_thread (arg=0x7fce9e6c7620) at /data/src/bb-10.2-mdev13355/storage/perfschema/pfs.cc:1862

#21 0x00007fce9a386064 in start_thread (arg=0x7fce92961700) at pthread_create.c:309

#22 0x00007fce9856b62d in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:111

CREATE TABLE t1 (i INT);

INSERT INTO t1 VALUES (1),(2);

SELECT DISTINCT MAX(i), ROW_NUMBER() OVER () FROM t1 WHERE 0;

CREATE TABLE t1 (i INT NOT NULL);

INSERT INTO t1 VALUES (1),(2);

SELECT DISTINCT MAX(i), ROW_NUMBER() OVER () FROM t1 WHERE i IS NULL;

--- /home/psergey/dev-git/10.2-r5/mysql-test/r/win.result       2017-12-26 13:32:38.182427886 +0300

+++ /home/psergey/dev-git/10.2-r5/mysql-test/r/win.reject       2017-12-26 14:12:48.455733684 +0300

@@ -2782,7 +2782,7 @@

 NULL   NULL

 select max(a), row_number() over () from t1 where 1 = 2;

 max(a) row_number() over ()

-NULL   1

+NULL   NULL

 select max(a), sum(max(a)) over () from t1 where 1 = 2;

 max(a) sum(max(a)) over ()

 NULL   NULL

diff --git a/sql/sql_select.cc b/sql/sql_select.cc

index 676b26e..e82e130 100644

--- a/sql/sql_select.cc

+++ b/sql/sql_select.cc

@@ -2654,7 +2654,15 @@ bool JOIN::make_aggr_tables_info()

         curr_tab->having= having;

         having->update_used_tables();

       }

-      curr_tab->distinct= true;

+      /*

+        We only need DISTINCT operation if the join is not degenerate.

+        If it is, we must not request DISTINCT processing, because

+        remove_duplicates() assumes there is a preceding computation step (and

+        in the degenerate join, there's none)

+      */

+      if (top_join_tab_count)

+        curr_tab->distinct= true;

+

       having= NULL;

       select_distinct= false;

     }

 /* Number of tables actually joined at the top level */

uint exec_join_tab_cnt() { return tables_list ? top_join_tab_count : 0; }

curr_tab= join_tab + exec_join_tab_cnt();

select distinct max(a) from t10 group by b;

join_tab[0]

  table = {t10}

  aggr= NULL

  distinct= false

join_tab[1]  (call it $join_tab_1)

  table= <ha_heap object, $work_table_1>

  aggr= pointer

  distinct= true

select distinct row_number() over (order by a ) from t1;

join_tab[0]

  table=t1

  distinct= false

  window_funcs_step= NULL

join_tab[1]

  table= <temp_table>

  window_funcs_step=<pointer>

  distinct= true

select max(b), row_number() over (order by a ) from t1 where 1 = 2 ;

join_tab[0]

  // join->first_select = sub_select_postjoin_aggr

  table= <temp. table>

  window_funcs_step = <pointer>

select distinct max(b), row_number() over (order by a ) from t1 where 1 = 2 ;