Merge 10.3 into 10.4

In main.index_merge_myisam we remove the test that was added in
commit a2d24def8c because
it duplicates the test case that was added in
commit 5af12e4635.

mysql-test/suite/versioning/t/partition.test

@@ -558,4 +558,44 @@ update t1, t2 set a = 1;
 # cleanup
 drop table t1, t2;
 
+--echo #
+--echo # MDEV-20515 multi-update tries to position updated table by null reference
+--echo #
+create or replace table t1 (a int);
+insert into t1 values (0), (1);
+
+create or replace table t2 (b int) with system versioning
+partition by system_time
+(partition p1 history, partition pn current);
+
+insert into t2 values (0), (2);
+update t1 left join t2 on a > b set b= 2 order by b;
+
+# cleanup
+drop table t1, t2;
+
 --source suite/versioning/common_finish.inc
+--echo #
+--echo # MDEV-17091 Assertion `old_part_id == m_last_part' failed in
+--echo # ha_partition::update_row or `part_id == m_last_part' in
+--echo # ha_partition::delete_row upon UPDATE/DELETE after dropping versioning
+--echo #
+create or replace table t1 (pk int primary key, f int) engine=innodb
+        with system versioning
+        partition by key() partitions 2;
+insert into t1 values (1,10),(2,20);
+--echo # expected to hit same partition
+select * from t1 partition (p0);
+alter table t1 drop system versioning;
+
+--echo # 1 and 2 are expected to be in different partitions
+select * from t1 partition(p0);
+select * from t1 partition(p1);
+
+update t1 set f=pk;
+delete from t1;
+drop table t1;
+
+--echo # Test cleanup
+drop database test;
+create database test;