Bug Report: bad count result of cross shard outer join

[eseokoh]

Overview of the Issue

count of cross shard outer join shows incorrect result.

Reproduction Steps

1. Deploy the following vschema:

{
  "sharded": true,
  "vindexes": {
    "hash": {
      "type": "xxhash"
    },
  "tables": {
    "x0": {
      "column_vindexes": [
        {
          "column": "a",
          "name": "xxhash"
        }
      ]
    },
    "x1": {
      "column_vindexes": [
        {
          "column": "c",
          "name": "xxhash"
        }
      ]
    }
  }
}

2. Deploy the following schema:

create table x0(a int primary key, b int);
create table x1(c int primary key, b int);
insert into x0 (a, b) values (1, 11), (2, null), (3, 0);
insert into x1 (c, b) values (1, 11);

3. Run Select:

select x0.a from x0 left outer join x1 on x0.b = x1.b;

+---+
| a |
+---+
| 1 |
| 2 |
| 3 |
+---+
3 rows in set (0.01 sec)

As you see, this is good, however the followings are bad.

select count(x0.a) from x0 left outer join x1 on x0.b = x1.b;
+-------------+
| count(x0.a) |
+-------------+
|           1 |
+-------------+
1 row in set (0.01 sec)

select count(*) from x0 left outer join x1 on x0.b = x1.b;

+----------+
| count(*) |
+----------+
|        1 |
+----------+
1 row in set (0.09 sec)

Binary Version

14.0.4

Operating System and Environment details

N/A

Log Fragments

N/A