Prior to PostgreSQL 9.3, there are two levels of locks allowing to control DML operations on a given set of rows for a transaction by using SELECT FOR SHARE and FOR UPDATE. Such locks taken on rows in a transaction block allow blocking INSERT/DELETE/UPDATE on those rows. There is also a protocol between those lock levels. FOR UPDATE is equivalent to an exclusive lock on the row selected, meaning that no other backend can take a FOR UPDATE lock on the same row and waits until the other other transaction finishes. FOR SHARE means that all the other backends can take a FOR SHARE lock on those rows. No FOR UPDATE locks can be taken on rows already locked with FOR SHARE. It is also possible to use the NOWAIT option, making the server return an error if there is a wait situation.
PostgreSQL 9.3 introduces two new levels of locks: FOR KEY SHARE and FOR NO KEY UPDATE. This feature has been committed thanks to the perseverance of Alvaro Herrera after two years of effort. Really congratulations to Alvaro!
commit 0ac5ad5134f2769ccbaefec73844f8504c4d6182 Author: Alvaro Herrera <email@example.com> Date: Wed Jan 23 12:04:59 2013 -0300 Improve concurrency of foreign key locking This patch introduces two additional lock modes for tuples: "SELECT FOR KEY SHARE" and "SELECT FOR NO KEY UPDATE". These don't block each other, in contrast with already existing "SELECT FOR SHARE" and "SELECT FOR UPDATE". UPDATE commands that do not modify the values stored in the columns that are part of the key of the tuple now grab a SELECT FOR NO KEY UPDATE lock on the tuple, allowing them to proceed concurrently with tuple locks of the FOR KEY SHARE variety. Foreign key triggers now use FOR KEY SHARE instead of FOR SHARE; this means the concurrency improvement applies to them, which is the whole point of this patch.
The main point of this feature is to reduce lock contention for foreign key triggers, as now those ones use FOR KEY SHARE instead of FOR SHARE. Also, UPDATE commands that do not update columns related to the key of the tuple now take now a FOR NO KEY UPDATE, explaining the name of the lock. With this level of locking, UPDATE queries that do not involve columns of the tuple key can perform concurrently.
Honestly, with now 4 levels of locks, it is becoming complicated to remember which operation blocks or allows the other on the same tuple. So let's make a couple of tests to determine what blocks what with a simple table with some data:
postgres=# CREATE TABLE aa AS SELECT 1 AS a; SELECT 1
The test scenario is pretty simple: two client sessions trying to take a lock on the same tuple. Session 1 launches its commands first, then session 2, the goal being to see if session 2 takes the lock or waits for it. Session 1:
BEGIN; SELECT * FROM aa FOR $LOCK;
Then session 2 does that
SELECT * FROM aa FOR $LOCK;
$LOCK can be either FOR SHARE, FOR UPDATE, FOR NO KEY UPDATE or FOR KEY SHARE.
Here are the results:
|Locks||UPDATE||NO KEY UPDATE||SHARE||KEY SHARE|
|NO KEY UPDATE||Waits||Waits||Waits||OK|
I hope this table helps. Have fun.