when pid column of sys.session will be null in mysql? - mysql

Question

Am using mysql 5.7.19-log server, i am working on application monitor tool where i need to monitor session details of a mysql server. In the process came to know that pid column can be null in sys.session table. i want to know what are the cases in which pid can be null?
when i exec desc session in mysql shell result as follows:
mysql>desc session;
+------------------------+------------------------------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+------------------------+------------------------------------------+------+-----+---------+-------+
| thd_id | bigint(20) unsigned | NO | | NULL | |
| conn_id | bigint(20) unsigned | YES | | NULL | |
| user | varchar(128) | YES | | NULL | |
| db | varchar(64) | YES | | NULL | |
| command | varchar(16) | YES | | NULL | |
| state | varchar(64) | YES | | NULL | |
| time | bigint(20) | YES | | NULL | |
| current_statement | longtext | YES | | NULL | |
| statement_latency | text | YES | | NULL | |
| progress | decimal(26,2) | YES | | NULL | |
| lock_latency | text | YES | | NULL | |
| rows_examined | bigint(20) unsigned | YES | | NULL | |
| rows_sent | bigint(20) unsigned | YES | | NULL | |
| rows_affected | bigint(20) unsigned | YES | | NULL | |
| tmp_tables | bigint(20) unsigned | YES | | NULL | |
| tmp_disk_tables | bigint(20) unsigned | YES | | NULL | |
| full_scan | varchar(3) | NO | | | |
| last_statement | longtext | YES | | NULL | |
| last_statement_latency | text | YES | | NULL | |
| current_memory | text | YES | | NULL | |
| last_wait | varchar(128) | YES | | NULL | |
| last_wait_latency | text | YES | | NULL | |
| source | varchar(64) | YES | | NULL | |
| trx_latency | text | YES | | NULL | |
| trx_state | enum('ACTIVE','COMMITTED','ROLLED BACK') | YES | | NULL | |
| trx_autocommit | enum('YES','NO') | YES | | NULL | |
| pid | varchar(1024) | YES | | NULL | |
| program_name | varchar(1024) | YES | | NULL | |
+------------------------+------------------------------------------+------+-----+---------+-------+
28 rows in set (0.00 sec)```

Answer 1

In your screenshot, You need to check the column with the name Null. It says that whether the value of a field can be null or not. In your case, pid can be null because as per the description of a table, pid allows Null.
Possibilities of pid can be the null are followed.
Inserting pid as null value
INSERT INTO session (pid, program_name) values (null, 'test1'); // pid: null
Don't inserting pid
INSERT INTO session (program_name) values ('test1'); // pid: null
Updating pid as null value
UPDATE session SET pid = null WHERE `thd_id` = 1; // pid: null