*: fix 'db' and 'Info' column of 'show processlist'

[wshwsh12]

What problem does this PR solve?

#10903
The Info column of show processlist should be NULL if it is not executing any statement.
The db column of show processlist should be NULL if it is not select one.

What is changed and how it works?

open 2 mysql client and run show processlist; both.

Before this pr:

mysql> show processlist;
+------+------+-----------+----+---------+------+-------+------------------+
| Id   | User | Host      | db | Command | Time | State | Info             |
+------+------+-----------+----+---------+------+-------+------------------+
|    2 | root | 127.0.0.1 |    | Query   |    0 | 2     | show processlist |
|    1 | root | 127.0.0.1 |    | Sleep   |   10 | 2     |                  |
+------+------+-----------+----+---------+------+-------+------------------+
2 rows in set (0.00 sec)

This pr:

mysql> show processlist;
+------+------+-----------+------+---------+------+-------+------------------+
| Id   | User | Host      | db   | Command | Time | State | Info             |
+------+------+-----------+------+---------+------+-------+------------------+
|    3 | root | 127.0.0.1 | NULL | Sleep   |    3 | 2     | NULL             |
|    4 | root | 127.0.0.1 | NULL | Query   |    0 | 2     | show processlist |
+------+------+-----------+------+---------+------+-------+------------------+
2 rows in set (0.01 sec)

Check List

Tests

• Unit test
• Manual test

Code changes

• Has exported function/method change

Side effects

• Increased code complexity

Related changes

[codecov]

Codecov Report

Merging #10985 into master will decrease coverage by 0.0046%.
The diff coverage is 70%.

@@               Coverage Diff                @@
##             master     #10985        +/-   ##
================================================
- Coverage   80.9364%   80.9317%   -0.0047%     
================================================
  Files           418        418                
  Lines         89212      89227        +15     
================================================
+ Hits          72205      72213         +8     
- Misses        11777      11783         +6     
- Partials       5230       5231         +1

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@1916eff). Click here to learn what that means.
The diff coverage is 70%.

@@             Coverage Diff             @@
##             master     #10985   +/-   ##
===========================================
  Coverage          ?   80.9615%           
===========================================
  Files             ?        418           
  Lines             ?      89256           
  Branches          ?          0           
===========================================
  Hits              ?      72263           
  Misses            ?      11756           
  Partials          ?       5237

[zz-jason]

How about not changing the type of Info from string to interface{}. We can do it this way:

info := "NULL"
if len(sql) > 0 {
	info = sql
}

[zz-jason]

Never mind. Could you add a test like:

select * from information_schema.processlist where db is null;
select * from information_schema.processlist where info is null;

[wshwsh12]

/run-all-tests

[XuHuaiyu]

We've already used order by ID in the sql, thus the .Sort() is not needed.

[wshwsh12]

/run-all-tests

[wshwsh12]

/run-all-tests

[XuHuaiyu]

LGTM

[zz-jason]

LGTM

[zz-jason]

@wshwsh12 please cherry pick this commit to release-2.1 and release-3.0

[tiancaiamao]

Use interface{} here is a bad idea

[bb7133]

Any better idea? @tiancaiamao