What is difference between write_lock and read_lock in concurency control? How to implement read_lock in PostgreSQL?

Question

What is difference between write_lock and read_lock in concurency? What is allowed in write_lock? What is allowed in read_lock? In PostgreSQL, write_lock can be implemented with FOR UPDATE

SELECT salary 
FROM staff 
WHERE worker_id=1 
FOR UPDATE

How to implement read_lock in PostgreSQL?

Answer 1

SELECT salary 
FROM staff 
WHERE worker_id=1 
FOR SHARE;

There are 4 modes possible here. Per documentation:

UPDATE
NO KEY UPDATE
SHARE
KEY SHARE

All the details in the manual.