histogram buckets has NULL value crated by fast analyze

[rebelice]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

create table t (a int);
set @@tidb_enable_fast_analyze=1;
insert into t values(1), (2), (3), (NULL);
analyze table t with 0 topn, 2 buckets;
select * from mysql.stats_buckets;

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

+----------+----------+---------+-----------+-------+---------+-------------+-------------+-----+
| table_id | is_index | hist_id | bucket_id | count | repeats | upper_bound | lower_bound | ndv |
+----------+----------+---------+-----------+-------+---------+-------------+-------------+-----+
|       53 |        0 |       1 |         0 |     3 |       1 | 2           |             |   0 |
|       53 |        0 |       1 |         1 |     1 |       1 | 3           | 3           |   0 |
+----------+----------+---------+-----------+-------+---------+-------------+-------------+-----+
2 rows in set (0.002 sec)

3. What did you see instead (Required)

The lower_bound is not NULL

4. What is your TiDB version? (Required)

| tidb_version() |
+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Release Version: v4.0.0-beta.2-2663-gf23e34965
Edition: Community
Git Commit Hash: f23e349
Git Branch: HEAD
UTC Build Time: 2021-04-20 08:45:18
GoVersion: go1.16.1
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false |

[rebelice]

/sig planner

[rebelice]

/assign @rebelice

[ti-srebot]

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