PostgreSQL 9.6 introduced a pg_blocking_pids() to get a blocking process id of your running transaction.

When multiple sessions are trying to access a table, and that table is locked. At that time, we need to find that blocking process id which blocked that table so that we can take necessary steps.

Using pg_blocking_pids(), we can get that head process id which is blocking other connections.

Below is a full demonstration of this:

First, save your current process id:

SELECT pg_backend_pid();
-- my process id is: 6720

Create a sample table:

CREATE TABLE tbl_students(rno int, name character varying(10));

Put your table in lock mode:

BEGIN TRANSACTION;
LOCK tbl_students IN ACCESS EXCLUSIVE MODE;

Open a new session, and save process id for that session also:

SELECT pg_backend_pid();
-- my process id is: 5936

Now in that new session, try to insert below record:
Your result will be in waiting mode…

INSERT INTO tbl_students VALUES (1,'Anvesh');

Open a third new session, and execute below command with process id of your second session:

SELECT pg_blocking_pids(5936);

The result is:

pg_blocking_pids
--------------------
{6720}