ha_partition and innodb index intersection produce fewer rows (MySQL Bug#70703)

create table t11 (

  a int not null,

  b int not null,

  pk int not null,

  primary key (pk),

  key(a),

  key(b)

) partition by hash(pk) partitions 10;

insert into t11 values (1,2,4); -- both

insert into t11 values (1,0,17);  -- left

insert into t11 values (1,2,25);   -- both

insert into t11 values (10,20,122); 

insert into t11 values (10,20,123);

-- Now, fill in some data so that the optimizer choses index_merge

create table t12 (a int);

insert into t12 values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);

insert into t11 select 1,2, 200 + A.a + 10*B.a + 100*C.a from t12 A, t12 B, t12 C;

insert into t11 select 10+A.a + 10*B.a + 100*C.a + 1000*D.a,

                       10+A.a + 10*B.a + 100*C.a  + 1000*D.a, 

                       2000 + A.a + 10*B.a + 100*C.a + 1000*D.a

                       from t12 A, t12 B, t12 C ,t12 D;

-- This should show index_merge, using intersect

explain select * from t11 where a=1 and b=2 and  pk between 1 and 999999 ;

-- 794 rows in output

select * from t11 where a=1 and b=2 and  pk between 1 and 999 ;

-- 802 rows in output

select * from t11 ignore index(a,b)  where a=1 and b=2 and  pk between 1 and 999 ;