Select unlocked row in Postgresql

Question

Is there a way to select rows in Postgresql that aren\'t locked? I have a multi-threaded app that will do:

Select... order by id desc limit 1 for update
         


        

Answer 1

My solution is to use the UPDATE statement with the RETURNING clause.

Users

-----------------------------------
ID        | Name       |      flags
-----------------------------------
1         |  bob       |        0  
2         |  fred      |        1  
3         |  tom       |        0   
4         |  ed        |        0   

Instead of SELECT .. FOR UPDATE use

BEGIN; 

UPDATE "Users"
SET ...
WHERE ...;
RETURNING ( column list );

COMMIT;

Because the UPDATE statement obtains a ROW EXCLUSIVE lock on the table its updating you get serialized updates. Reads are still allowed, but they only see data before the start of the UPDATE transaction.

Reference: Concurrency Control Chapter of Pg docs.