Merge pull request #824 from pingcap/coocood/fix-is-null-panic

plan/plans: fix is null panic.

plan/plans/from.go

@@ -244,7 +244,9 @@ func (r *TableDefaultPlan) filterIsNull(ctx context.Context, x *expression.IsNul
 	cn := cns[0]
 	t := r.T
 	col := column.FindCol(t.Cols(), cn)
-
+	if col == nil {
+		return r, false, nil
+	}
 	if col.IsPKHandleColumn(t.Meta()) {
 		if x.Not {
 			// PK handle column never be null.

session_test.go

@@ -611,6 +611,8 @@ func (s *testSessionSuite) TestSelect(c *C) {
 	row, err = r.FirstRow()
 	c.Assert(err, IsNil)
 	match(c, row, 3)
+
+	mustExecSQL(c, se, `select * from t1, t2 where t1.c1 is null`)
 }
 
 func (s *testSessionSuite) TestSubQuery(c *C) {