Results of count(col) and count(1) may be different

[zyguan]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

drop table if exists t1;
create table t1 (c_int int, primary key (c_int));
insert into t1 values (1), (2), (3);
drop table if exists t2;
create table t2 (c_int int, primary key (c_int));
insert into t2 values (1);
select (select count(c_int) from t2 where c_int = t1.c_int) c1, (select count(1) from t2 where c_int = t1.c_int) c2 from t1;

2. What did you expect to see? (Required)

+------+------+
| c1   | c2   |
+------+------+
|    1 |    1 |
|    0 |    0 |
|    0 |    0 |
+------+------+

3. What did you see instead (Required)

+----+----+
| c1 | c2 |
+----+----+
|  1 |  1 |
|  1 |  0 |
|  1 |  0 |
+----+----+

4. What is your TiDB version? (Required)

master @ 602afed
release-4.0 @ 397fce9