*: Hide "Mem" column from SHOW PROCESSSLIST

[morgo]

What problem does this PR solve?

#8482

What is changed and how it works?

The command SHOW PROCESSLIST has a column called 'Mem', which at least for me has always showed a zero value. This patch removes the column completely, but an alternative fix (beyond my current skill) could be to fix the output to be correct.

In MySQL the memory info is available in perfschema (try for example: SELECT * FROM sys.session) , but since TiDB does not implement perfschema maybe extending processlist is the best way to present it? I could be convinced either way, but I am submitting this patch because a zero value is a bad first experience.

MySQL 5.7.40:

mysql57> mysql57> SHOW PROCESSLIST;
+----+----------+-----------+------+---------+------+----------+------------------+
| Id | User     | Host      | db   | Command | Time | State    | Info             |
+----+----------+-----------+------+---------+------+----------+------------------+
|  2 | msandbox | localhost | NULL | Query   |   -1 | starting | SHOW PROCESSLIST |
+----+----------+-----------+------+---------+------+----------+------------------+
1 row in set (0.00 sec)

TiDB master:

mysql> SHOW PROCESSLIST;
+------+------+-----------+------+---------+------+-------+------------------+------+
| Id   | User | Host      | db   | Command | Time | State | Info             | Mem  |
+------+------+-----------+------+---------+------+-------+------------------+------+
|    1 | root | 127.0.0.1 |      | Query   |    0 | 2     | SHOW PROCESSLIST |    0 |
+------+------+-----------+------+---------+------+-------+------------------+------+
1 row in set (0.00 sec)

TiDB master + this patch:

mysql> SHOW PROCESSLIST;
+------+------+-----------+------+---------+------+-------+------------------+
| Id   | User | Host      | db   | Command | Time | State | Info             |
+------+------+-----------+------+---------+------+-------+------------------+
|    2 | root | 127.0.0.1 |      | Query   |    0 | 2     | SHOW PROCESSLIST |
+------+------+-----------+------+---------+------+-------+------------------+
1 row in set (0.00 sec)

Check List

Tests

• Manual test

Ran manual test + fixed tests that broke.

Code changes

• Has interface methods change

Side effects

• Breaking backward compatibility

Related changes

• Need to update the documentation
• Need to be included in the release note

---

This change is 

[shenli]

@morgo Will this break the compatibility with MySQL. AFAK, some admin tools rely some specific columns of SHOW PROCESSSLIST.

[morgo]

@shenli I've updated the description to make it clearer. 'Mem' is not a column in MySQL - it was a TiDB extension not fully implemented.

[eurekaka]

The Mem is not 0 if you run a memory consuming query simultaneously with SHOW PROCESSLIST. For example:

mysql> SHOW PROCESSLIST;
+------+------+-----------+------+---------+------+-------+-----------------------------------------------------------------+--------+
| Id   | User | Host      | db   | Command | Time | State | Info                                                            | Mem    |
+------+------+-----------+------+---------+------+-------+-----------------------------------------------------------------+--------+
|    1 | root | 127.0.0.1 | tmp  | Query   |    0 | 2     | SHOW PROCESSLIST                                                |      0 |
|    2 | root | 127.0.0.1 | tmp  | Query   |    2 | 2     | insert into tbl select t1.a, t1.b, t1.c from tbl t1 join tbl t2 | 245616 |
+------+------+-----------+------+---------+------+-------+-----------------------------------------------------------------+--------+
2 rows in set (0.02 sec)

[morgo]

Interesting it is not accurate for my select and load data statements:

mysql> SHOW PROCESSLIST;
+------+------+-----------+-----------+---------+------+-------+------------------------------------------------------------------------------------------------------+------+
| Id   | User | Host      | db        | Command | Time | State | Info                                                                                                 | Mem  |
+------+------+-----------+-----------+---------+------+-------+------------------------------------------------------------------------------------------------------+------+
|    5 | root | 127.0.0.1 | bikeshare | Query   |  153 | 2     | LOAD DATA LOCAL INFILE './2011-capitalbikeshare-tripdata.csv' INTO TABLE trips FIELDS TERMINATED BY  |    0 |
|    6 | root | 127.0.0.1 | bikeshare | Query   |  153 | 2     | LOAD DATA LOCAL INFILE './2012Q4-capitalbikeshare-tripdata.csv' INTO TABLE trips FIELDS TERMINATED B |    0 |
|    4 | root | 127.0.0.1 | bikeshare | Query   |  153 | 2     | LOAD DATA LOCAL INFILE './2012Q1-capitalbikeshare-tripdata.csv' INTO TABLE trips FIELDS TERMINATED B |    0 |
|    7 | root | 127.0.0.1 | bikeshare | Query   |  153 | 2     | LOAD DATA LOCAL INFILE './2016Q3-capitalbikeshare-tripdata.csv' INTO TABLE trips FIELDS TERMINATED B |    0 |
|   11 | root | 127.0.0.1 | bikeshare | Query   |  153 | 2     | LOAD DATA LOCAL INFILE './2013Q3-capitalbikeshare-tripdata.csv' INTO TABLE trips FIELDS TERMINATED B |    0 |
|    9 | root | 127.0.0.1 | bikeshare | Query   |  153 | 2     | LOAD DATA LOCAL INFILE './2014Q3-capitalbikeshare-tripdata.csv' INTO TABLE trips FIELDS TERMINATED B |    0 |
|   12 | root | 127.0.0.1 |           | Query   |   53 | 2     | SELECT SLEEP(100)                                                                                    |    0 |
|   13 | root | 127.0.0.1 | bikeshare | Query   |    7 | 2     | SELECT * FROM trips WHERE sleep(10)=0 LIMIT 10                                                       |    0 |
|    1 | root | 127.0.0.1 |           | Query   |    0 | 2     | SHOW PROCESSLIST                                                                                     |    0 |
|    8 | root | 127.0.0.1 | bikeshare | Query   |  153 | 2     | LOAD DATA LOCAL INFILE './2014Q1-capitalbikeshare-tripdata.csv' INTO TABLE trips FIELDS TERMINATED B |    0 |
|   10 | root | 127.0.0.1 | bikeshare | Query   |  153 | 2     | LOAD DATA LOCAL INFILE './2013Q2-capitalbikeshare-tripdata.csv' INTO TABLE trips FIELDS TERMINATED B |    0 |
+------+------+-----------+-----------+---------+------+-------+------------------------------------------------------------------------------------------------------+------+
11 rows in set (0.01 sec)

[zz-jason]

For now, we only trace the memory usage in some memory intensive SQL operators, for example, HashJoin/IndexLookupJoin/IndexLookupReader, etc.

[eurekaka]

Oh, then I prefer removing this Mem field since it may confuse users without this knowledge.

[eurekaka]

LGTM

[jackysp]

How to get the values of Mem if we remove it? @morgo

[jackysp]

/run-all-tests

[morgo]

@jackysp When I had written the patch, I thought they were always zero. It turns out there are cases of non-zero :(

How about adding an extension SHOW PROCESSLIST MEMINFO, where the column is shown?

[zz-jason]

@jackysp When I had written the patch, I thought they were always zero. It turns out there are cases of non-zero :(

How about adding an extension SHOW PROCESSLIST MEMINFO, where the column is shown?

Glade to see this new feature. 😄

[zz-jason]

/run-integration-ddl-test

[zz-jason]

LGTM