planner: add protection to avoid setting tot_col_size to negative numbers (#55327)

[ti-chi-bot]

This is an automated cherry-pick of #55327

What problem does this PR solve?

Issue Number: close #55126

Problem Summary: planner: add protection to avoid setting tot_col_size to negative numbers

What changed and how does it work?

See more details and analysis in the issue.
When updating tot_col_size, use GREATEST to protect it from becoming a negative number.

Hard to use UT to test it, so I tested it manually locally:

First I hacked the code below to simulate all positive updates on tot_col_size are lost:

Then execute those SQLs:

mysql> create table t (a varchar(100), key(a));
Query OK, 0 rows affected (0.01 sec)

mysql> analyze table t;
Query OK, 0 rows affected, 2 warnings (0.03 sec)

mysql> insert into t values (space(100));
Query OK, 1 row affected (0.01 sec)

mysql> update t set a="";
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

Then we can see the tot_col_size becomes a negative number:

mysql> show stats_histograms;
+---------+------------+----------------+-------------+----------+---------------------+----------------+------------+--------------+-------------+-------------+-----------------+----------------+----------------+---------------+
| Db_name | Table_name | Partition_name | Column_name | Is_index | Update_time         | Distinct_count | Null_count | Avg_col_size | Correlation | Load_status | Total_mem_usage | Hist_mem_usage | Topn_mem_usage | Cms_mem_usage |
+---------+------------+----------------+-------------+----------+---------------------+----------------+------------+--------------+-------------+-------------+-----------------+----------------+----------------+---------------+
| test    | t          |                | a           |        0 | 2024-08-09 14:39:42 |              0 |          0 |         -101 |           0 | allLoaded   |               0 |              0 |              0 |             0 |
+---------+------------+----------------+-------------+----------+---------------------+----------------+------------+--------------+-------------+-------------+-----------------+----------------+----------------+---------------+

After this PR, this case won't happen, at least we can ensure it's equal or larger than 0:

mysql> show stats_histograms;
+---------+------------+----------------+-------------+----------+---------------------+----------------+------------+--------------+-------------+-------------+-----------------+----------------+----------------+---------------+
| Db_name | Table_name | Partition_name | Column_name | Is_index | Update_time         | Distinct_count | Null_count | Avg_col_size | Correlation | Load_status | Total_mem_usage | Hist_mem_usage | Topn_mem_usage | Cms_mem_usage |
+---------+------------+----------------+-------------+----------+---------------------+----------------+------------+--------------+-------------+-------------+-----------------+----------------+----------------+---------------+
| test    | t          |                | a           |        0 | 2024-08-09 14:47:16 |              0 |          0 |            0 |           0 | allLoaded   |               0 |              0 |              0 |             0 |
+---------+------------+----------------+-------------+----------+---------------------+----------------+------------+--------------+-------------+-------------+-----------------+----------------+----------------+---------------+

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No need to test

  • I checked and no code files have been changed.

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[ti-chi-bot]

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idc-jenkins-ci-tidb/unit-test	2f1fbd4	link	true	/test unit-test
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[qw4990]

Pick it later manually