incorrect correlation of histogram when column is loaded ad hoc

[eurekaka]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?

mysql> create table t(c int);
Query OK, 0 rows affected (0.01 sec)

mysql> insert into t values(1),(2),(3),(4),(5);
Query OK, 5 rows affected (0.01 sec)
Records: 5  Duplicates: 0  Warnings: 0

mysql> analyze table t;
Query OK, 0 rows affected (0.01 sec)

mysql> show stats_histograms where table_name = 't';
Empty set (0.00 sec)

-- reboot TiDB server and use a new connection

mysql> show stats_histograms where table_name = 't';
Empty set (0.00 sec)

-- use this query to mark column histogram as needed
mysql> explain select * from t where c = 1;
+---------------------+-------+------+----------------------------------------------+
| id                  | count | task | operator info                                |
+---------------------+-------+------+----------------------------------------------+
| TableReader_7       | 0.01  | root | data:Selection_6                             |
| └─Selection_6       | 0.01  | cop  | eq(test.t.c, 1)                              |
|   └─TableScan_5     | 10.00 | cop  | table:t, range:[-inf,+inf], keep order:false |
+---------------------+-------+------+----------------------------------------------+
3 rows in set (0.00 sec)

mysql> show stats_histograms where table_name = 't';
+---------+------------+----------------+-------------+----------+---------------------+----------------+------------+--------------+-------------+
| Db_name | Table_name | Partition_name | Column_name | Is_index | Update_time         | Distinct_count | Null_count | Avg_col_size | Correlation |
+---------+------------+----------------+-------------+----------+---------------------+----------------+------------+--------------+-------------+
| test    | t          |                | c           |        0 | 2019-03-13 18:14:55 |              5 |          0 |            8 |           0 |
+---------+------------+----------------+-------------+----------+---------------------+----------------+------------+--------------+-------------+
1 row in set (0.00 sec)

2. What did you expect to see?

Correlation field should be 1.

3. What did you see instead?

Correlation field is 0.

4. What version of TiDB are you using (tidb-server -V or run select tidb_version(); on TiDB)?

Release Version: v3.0.0-beta-211-g09beefbe0
Git Commit Hash: 09beefbe045011e3c77608c9ed33da87c11efa94
Git Branch: master
UTC Build Time: 2019-03-13 10:13:24
GoVersion: go version go1.12 darwin/amd64
Race Enabled: false
TiKV Min Version: 2.1.0-alpha.1-ff3dd160846b7d1aed9079c389fc188f7f5ea13e
Check Table Before Drop: false