"select * from a left join b on a.col1=b.col1 where b.col1 is null" can't filter out expect line

[irene-coming]

• dble version：
  5.6.29-dble-9.9.9.9-287a2f4-20180921052651

• preconditions ：
  prepare a separate mysql database for compare with dble

• configs：

schema.xml

<table name="table_a" primaryKey="id" rule="hash-four" dataNode="dn1,dn2,dn3,dn4"></table>
<table name="table_b" primaryKey="id" rule="hash-four" dataNode="dn1,dn2,dn3,dn4"></table>
<table name="table_c" primaryKey="id" rule="hash-four" dataNode="dn1,dn2,dn3,dn4"></table>

• steps：
  step1. create table and insert data in both dble and compare mysql server :

create table table_a(id int, name varchar(20));
create table table_b(id int, name varchar(20));
insert into table_a value(1,'a');
insert into table_b values(1,'d'),(2,'b'),(3,'c');

step2. execute query with compare mysql server:

mysql> select b.* from table_b b left join table_a a on a.id=b.id where a.id is NULL;
+------+------+
| id   | name |
+------+------+
|    3 | c    |
|    2 | b    |
+------+------+
2 rows in set (0.00 sec)

step3. execute query with dble business port:

mysql> select b.* from table_b b left join table_a a on a.id=b.id where a.id is NULL;
Empty set (0.02 sec)

• expect result：
  1. step3(dble's resultset) get the same result with step2(mysql's resultset)
• real result：
  1.step3 get different result with step2
• supplements：
  1.

If there is no matching row for the right table in the ON or USING part in a LEFT JOIN, a row with all columns set to NULL is used for the right table. You can use this fact to find rows in a table that have no counterpart in another table
https://dev.mysql.com/doc/refman/8.0/en/join.html

/label ~BUG

[sunsun314]

When route complex join with partition value is null ,route to all the node,it may cause some performance problem

[yexiaoli88]

verified on version: 5.6.29-dble-9.9.9.9-edff869-20180928061234