global stats of partition table may be incorrect under dynamic prune mode

[chrysan]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

create table t (a int, b int, c int) partition by range (a) (partition p0 values less than (10), partition p1 values less than (20));
insert into t values (1,1,1), (2,2,2), (11,11,11);
set tidb_partition_prune_mode=dynamic;
analyze table t partition p0 columns b with 1 topn, 1 buckets;
analyze table t partition p1 columns c with 1 topn, 1 buckets;
select * from t where b>1 and c>1;
show stats_histograms where table_name like "t";

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

distinct count should be 3 for both columns b & c

+---------+------------+----------------+-------------+----------+---------------------+----------------+------------+--------------+-------------+
| Db_name | Table_name | Partition_name | Column_name | Is_index | Update_time         | Distinct_count | Null_count | Avg_col_size | Correlation |
+---------+------------+----------------+-------------+----------+---------------------+----------------+------------+--------------+-------------+
| test    | t          | global         | b           |        0 | 2022-05-17 14:44:33 |              3 |          0 |            8 |           0 |
| test    | t          | global         | c           |        0 | 2022-05-17 14:44:33 |              3 |          0 |            8 |           0 |
+---------+------------+----------------+-------------+----------+---------------------+----------------+------------+--------------+-------------+

3. What did you see instead (Required)

distinct count is incorrect for both columns b & c, generate global stats ignoring the missing column stats.

+---------+------------+----------------+-------------+----------+---------------------+----------------+------------+--------------+-------------+
| Db_name | Table_name | Partition_name | Column_name | Is_index | Update_time         | Distinct_count | Null_count | Avg_col_size | Correlation |
+---------+------------+----------------+-------------+----------+---------------------+----------------+------------+--------------+-------------+
| test    | t          | global         | b           |        0 | 2022-05-17 14:41:52 |              2 |          0 |           17 |           0 |
| test    | t          | global         | c           |        0 | 2022-05-17 14:42:39 |              0 |          0 |           10 |           0 |

4. What is your TiDB version? (Required)

v6.0, master