Understanding basic locking

In this section, you will learn about basic locking mechanisms. The goal is to understand how locking works in general and how to get simple applications right.

To show you how things work, we will create a simple table. For demonstrative purposes, I will add one row to the table using a simple INSERT command:

test=# CREATE TABLE  t_test (id int);  
CREATE TABLE 
test=# INSERT INTO t_test VALUES (0);  
INSERT 0 1 

The first important thing is that tables can be read concurrently. Many users reading the same data at the same time won't block each other. This allows PostgreSQL to handle thousands of users without any problems.

The question now is what happens if reads and writes occur at the same time? Here is an example. Let's assume that the table contains one row and its id = 0:

Transaction 1	Transaction 2
BEGIN;	BEGIN;
UPDATE t_test SET id = id + 1 RETURNING *;
User will see 1	SELECT * FROM t_test;
	User will see 0
...