Fixed the bug mdev-7791.

When an IN subquery predicate was converted to a semi-join that were
materialized and the result of the materialization happened to be
the last in the execution plan then any conjunctive condition with RAND()
turned out to be lost.

Fixed by attaching this condition to the last top base table.

mysql-test/r/subselect_sj2_mat.result

@@ -1556,3 +1556,26 @@ i1
 DROP TABLE t1,t2,t3;
 set join_cache_level= @save_join_cache_level;
 set optimizer_switch=@save_optimizer_switch;
+#
+# mdev-7791: materialization of a semi-join subquery +
+#            RAND() in WHERE
+#            (materialized table is accessed last)
+#
+set @save_optimizer_switch=@@optimizer_switch;
+set optimizer_switch='materialization=on';
+create table t1(i int);
+insert into t1 values (1), (2), (3), (7), (9), (10);
+create table t2(i int);
+insert into t2 values (1), (2), (3), (4), (5), (6), (7), (8), (9), (10);
+select * from t1 where (rand() < 0) and i in (select i from t2);
+i
+explain extended
+select * from t1 where (rand() < 0) and i in (select i from t2);
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
+1	PRIMARY	t1	ALL	NULL	NULL	NULL	NULL	6	100.00	Using where
+1	PRIMARY	<subquery2>	eq_ref	distinct_key	distinct_key	4	func	1	100.00	
+2	MATERIALIZED	t2	ALL	NULL	NULL	NULL	NULL	10	100.00	
+Warnings:
+Note	1003	select `test`.`t1`.`i` AS `i` from `test`.`t1` semi join (`test`.`t2`) where ((rand() < 0))
+drop table t1,t2;
+set optimizer_switch=@save_optimizer_switch;

mysql-test/t/subselect_sj2_mat.test

@@ -263,3 +263,23 @@ DROP TABLE t1,t2,t3;
 set join_cache_level= @save_join_cache_level;
 set optimizer_switch=@save_optimizer_switch;
 
+--echo #
+--echo # mdev-7791: materialization of a semi-join subquery +
+--echo #            RAND() in WHERE
+--echo #            (materialized table is accessed last)
+--echo #
+
+set @save_optimizer_switch=@@optimizer_switch;
+set optimizer_switch='materialization=on';
+
+create table t1(i int);
+insert into t1 values (1), (2), (3), (7), (9), (10);
+create table t2(i int);
+insert into t2 values (1), (2), (3), (4), (5), (6), (7), (8), (9), (10);
+
+select * from t1 where (rand() < 0) and i in (select i from t2);
+explain extended
+select * from t1 where (rand() < 0) and i in (select i from t2);
+
+drop table t1,t2;
+set optimizer_switch=@save_optimizer_switch;

sql/sql_select.cc

@@ -8761,12 +8761,20 @@ make_join_select(JOIN *join,SQL_SELECT *select,COND *cond)
     /*
       Step #2: Extract WHERE/ON parts
     */
+    uint i;
+    for (i= join->top_join_tab_count - 1; i >= join->const_tables; i--)
+    {
+      if (!join->join_tab[i].bush_children)
+        break;
+    }
+    uint last_top_base_tab_idx= i;
+
     table_map save_used_tables= 0;
     used_tables=((select->const_tables=join->const_table_map) |
 		 OUTER_REF_TABLE_BIT | RAND_TABLE_BIT);
     JOIN_TAB *tab;
     table_map current_map;
-    uint i= join->const_tables;
+    i= join->const_tables;
     for (tab= first_depth_first_tab(join); tab;
          tab= next_depth_first_tab(join, tab), i++)
     {
@@ -8804,7 +8812,7 @@ make_join_select(JOIN *join,SQL_SELECT *select,COND *cond)
 	Following force including random expression in last table condition.
 	It solve problem with select like SELECT * FROM t1 WHERE rand() > 0.5
       */
-      if (tab == join->join_tab + join->top_join_tab_count - 1)
+      if (tab == join->join_tab + last_top_base_tab_idx)
         current_map|= RAND_TABLE_BIT;
       used_tables|=current_map;