Row Locking in Postgres

Question

I have table named 'car'

              Table "db145004db.car"
  Column  |  Type  | Collation | Nullable | Default 
----------+--------+-----------+----------+---------
 car_no   | bigint |           | not null | 
 car_name | text   |           |          | 
Indexes:
    "car_pkey" PRIMARY KEY, btree (car_no)

Table Content:

 car_no |  car_name  
--------+------------
      1 | first_car
      2 | second_car
      3 | third_car

I ran this following two transaction,

Transaction 1:

begin ;
select * from car where car_no=2 for update; 

Transaction 2:

begin ;
select * from car where car_no=2 for update; (waiting for first transaction to release lock)

My 'pg_locks' table looks like:

  locktype    | database | relation | page | tuple | virtualxid | transactionid | classid | objid | objsubid | virtualtransaction |  pid  |        mode         | granted | fastpath 
---------------+----------+----------+------+-------+------------+---------------+---------+-------+----------+--------------------+-------+---------------------+---------+----------
 relation      |   103777 |   143022 |      |       |            |               |         |       |          | 4/75               | 11043 | AccessShareLock     | t       | t
 relation      |   103777 |   143016 |      |       |            |               |         |       |          | 4/75               | 11043 | RowShareLock        | t       | t
 virtualxid    |          |          |      |       | 4/75       |               |         |       |          | 4/75               | 11043 | ExclusiveLock       | t       | t
 relation      |   103777 |   143022 |      |       |            |               |         |       |          | 5/144              | 11150 | AccessShareLock     | t       | t
 relation      |   103777 |   143016 |      |       |            |               |         |       |          | 5/144              | 11150 | RowShareLock        | t       | t
 virtualxid    |          |          |      |       | 5/144      |               |         |       |          | 5/144              | 11150 | ExclusiveLock       | t       | t
 relation      |   103777 |    11577 |      |       |            |               |         |       |          | 3/141              | 11263 | AccessShareLock     | t       | t
 virtualxid    |          |          |      |       | 3/141      |               |         |       |          | 3/141              | 11263 | ExclusiveLock       | t       | t
 transactionid |          |          |      |       |            |         10204 |         |       |          | 5/144              | 11150 | ShareLock           | f       | f
 tuple         |   103777 |   143016 |    0 |     2 |            |               |         |       |          | 5/144              | 11150 | AccessExclusiveLock | t       | f
 transactionid |          |          |      |       |            |         10204 |         |       |          | 4/75               | 11043 | ExclusiveLock       | t       | f

I can't see any locks related to rows/tuples (car_no=2). Also, locktype and mode are little confusing(I can't understand what they tend to convey). As a newbie to psql, I am totally confused. Can anyone help interpreting 'pg_locks', with respect to row-level locking, with respect to above scenario.

Answer 1

I came across the postgres docs regarding locking , which states that

Although tuples are a lockable type of object, information about row-level locks is stored on disk, not in memory, and therefore row-level locks normally do not appear in this view. If a transaction is waiting for a row-level lock, it will usually appear in the view as waiting for the permanent transaction ID of the current holder of that row lock.

I got my answer.