MDEV-28285 Unexpected result when combining DISTINCT, subselect and LIMIT

The problem was that when  JOIN_TAB::remove_duplicates() noticed there
can only be one possible row in the output, it adjusted limits but
didn't take into account any possible offset.

Fixed by not adjusting limit offset when setting one-row-limit.

mysql-test/main/distinct.result

@@ -1157,3 +1157,28 @@ explain select * from t1 limit 0 offset 10;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Zero limit
 drop table t1, t2;
+#
+# MDEV-28285 Unexpected result when combining DISTINCT, subselect
+# and LIMIT
+#
+create table t1 (a int primary key);
+create table t2 (a int primary key, b int not null);
+insert into t1 select seq from seq_1_to_10;
+insert into t2 select seq,seq from seq_1_to_10;
+select distinct a from t1 where t1.a=1 and t1.a in (select a from t2 where t2.b in (1,2));
+a
+1
+explain select distinct a from t1 where t1.a=1 and t1.a in (select a+0 from t2 where t2.b in (1,2)) limit 10,10;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t1	const	PRIMARY	PRIMARY	4	const	1	Using index; Using temporary
+1	PRIMARY	<subquery2>	eq_ref	distinct_key	distinct_key	8	func	1	
+2	MATERIALIZED	t2	ALL	NULL	NULL	NULL	NULL	10	Using where
+select distinct a from t1 where t1.a=1 and t1.a in (select a+0 from t2 where t2.b in (1,2)) limit 10,10;
+a
+select distinct a from t1 where t1.a=1 and t1.a in (select a+0 from t2 where t2.b in (1,2)) limit 0,1;
+a
+1
+drop table t1,t2;
+#
+# end of 10.5 tests
+#

mysql-test/main/distinct.test

@@ -892,3 +892,24 @@ explain select * from t1 limit 0;
 explain select * from t1 limit 0 offset 10;
 
 drop table t1, t2;
+
+--echo #
+--echo # MDEV-28285 Unexpected result when combining DISTINCT, subselect
+--echo # and LIMIT
+--echo #
+
+create table t1 (a int primary key);
+create table t2 (a int primary key, b int not null);
+
+insert into t1 select seq from seq_1_to_10;
+insert into t2 select seq,seq from seq_1_to_10;
+
+select distinct a from t1 where t1.a=1 and t1.a in (select a from t2 where t2.b in (1,2));
+explain select distinct a from t1 where t1.a=1 and t1.a in (select a+0 from t2 where t2.b in (1,2)) limit 10,10;
+select distinct a from t1 where t1.a=1 and t1.a in (select a+0 from t2 where t2.b in (1,2)) limit 10,10;
+select distinct a from t1 where t1.a=1 and t1.a in (select a+0 from t2 where t2.b in (1,2)) limit 0,1;
+drop table t1,t2;
+
+--echo #
+--echo # end of 10.5 tests
+--echo #

sql/sql_limit.h

@@ -52,6 +52,15 @@ class Select_limit_counters
      select_limit_cnt= 1;
    }
 
+   /* Send the first row, still honoring offset_limit_cnt */
+   void send_first_row()
+   {
+     /* Guard against overflow */
+     if ((select_limit_cnt= offset_limit_cnt +1 ) == 0)
+       select_limit_cnt= offset_limit_cnt;
+     // with_ties= false;   Remove // on merge to 10.6
+   }
+
    bool is_unlimited()
    { return select_limit_cnt == HA_POS_ERROR; }
    bool is_unrestricted()

sql/sql_select.cc

@@ -24640,7 +24640,7 @@ JOIN_TAB::remove_duplicates()
       !(join->select_options & OPTION_FOUND_ROWS))
   {
     // only const items with no OPTION_FOUND_ROWS
-    join->unit->lim.set_single_row();		// Only send first row
+    join->unit->lim.send_first_row();		// Only send first row
     my_free(sortorder);
     DBUG_RETURN(false);
   }