There is an alternative solution to this problem. Consider an example where you are asked to write an application to generate invoice numbers. The tax office might require you to create invoice numbers without gaps and without duplicates. How would you do this? Of course, one solution would be a table lock. However, you can really do better. Here is what you can do to handle the numbering problem we are trying to solve:

test=# CREATE TABLE t_invoice (id int PRIMARY KEY);
CREATE TABLE
test=# CREATE TABLE  t_watermark (id int);  
CREATE TABLE
test=# INSERT INTO  t_watermark VALUES (0);  
INSERT 0  
test=# WITH  x AS (UPDATE t_watermark SET id = id + 1 RETURNING *)  
         INSERT INTO  t_invoice 
         SELECT * FROM  x RETURNING *; 
id 
----
  1
(1 row)

In this case, we introduced a table called t_watermark. It contains just one row. The WITH command will be executed first. The row will be locked and incremented, and the new value will be returned. Only one person can do this at a time. The value returned by the CTE is then used in the invoice table. It is guaranteed to be unique. The beauty is that there is only a simple row lock on the watermark table, which leads to no reads being blocked in the invoice table. Overall, this way is more scalable.