In August 2010, I sent an email to pghackers reporting a strange deadlock problem.
I couldn’t understand how it was possible you could get a deadlock by updating
the very same row again later in the same transaction.
I also made a screencast demonstrating the problem:
Simon Riggs came up with a very clever solution to this problem.
By introducing a new type of lock, it would be possible to avoid a lot of deadlocks.
Álvaro Herrera has since then been working on the implementation.
It turned out to be a lot of work, unfortunately the feature didn’t make it into 9.2.
I bet this will be one of the most important performance features in 9.3.
It will greatly improve the performance by increasing the concurrency possible if you have a high transaction load.
I thought it would be fun to test my old example from 2010 using the latest version of the patch.
Let’s checkout Álvaro’s latest version of the fklocks patch:
git remote add fklocks git://github.com/alvherre/postgres.git
git fetch fklocks
git checkout fklocks
CREATE TABLE A ( AID integer not null, Col1 integer, PRIMARY KEY (AID) ); CREATE TABLE B ( BID integer not null, AID integer not null, Col2 integer, PRIMARY KEY (BID), FOREIGN KEY (AID) REFERENCES A(AID) ); INSERT INTO A (AID) VALUES (1); INSERT INTO B (BID,AID) VALUES (2,1); -- Process 1: BEGIN; -- Process 2: BEGIN; UPDATE A SET Col1 = 1 WHERE AID = 1; UPDATE B SET Col2 = 2 WHERE BID = 2; UPDATE B SET Col2 = 3 WHERE BID = 2; -- Process 1 is waiting UPDATE B SET Col2 = 4 WHERE BID = 2; -- Process 2 is allowed to update the same row COMMIT; COMMIT; SELECT * FROM A; aid | col1 -----+------ 1 | 1 (1 row) SELECT * FROM B; bid | aid | col2 -----+-----+------ 2 | 1 | 3 (1 row)
Both transactions were able to COMMIT successfully, hurray!
Without this patch, you would get a “deadlock detected” on row 30,
when Process 2 tries to update the same row again, BID = 2.
However, I was a bit surprised the final value of Col2 is 3 and not 4,
as Process 2 updated the row BID = 2 before Process 1,
and Process 2 updated the row again after Process 1.
This probably has a logical explanation.
I hope to have time to do some concurrency stress testing using
example code and data extracted from our production database.
Would be interesting to see if my old read-life deadlock scenarios
are fixed by this patch and how it affects concurrency and performance.
Optimistic updates result in the second commit overwriting the first. While the update statement for 4 came later the commit for 4 came before the commit for 3.
Or, to put it a different way: Process 1 blocks when it attempts to update Col2, and cannot progress until process 2 commits. Once process 2 commits, process 1 can continue to run where it left off, updating Col2. It does so, then commits. The update for process 1 actually occurs after the update for process 2, because the update doesn’t occur until process 1 unblocks.
The final value of Col2 being 3 is actually correct. Process 1 blocks until the commit of process 2 when the lock on that row is released.
As J Prevost mentioned, the update in process 1 doesn’t happen till *after* the commit in Process 2.
Anonymous is incorrect: it has nothing to do with the order of the commits. And the “update for 4” *didn’t* come later — but it was *submitted* later.
To get around this “problem” of “lost updates” you’ll need to do either of two things:
1. use a serializable transaction isolation level for both processes,
2. rewrite your update statements to include the expected “old” value. Or you can rework your data model and add either of these two columns:
A. changed_on timestamp default now()
B. version integer column default 0
and, again, rewrite your update statements to include the expected “old” changed_on or version value. And update the changed_on = now() or version = version + 1.
Great find, this inability to “reupdate” is something that should be fixed.
I believe everything published made a bunch of sense.
But, think on this, what if you composed a catchier post title?
I am not suggesting your content is not solid, but what if you added
a headline to maybe get a person’s attention? I mean Foreign Key Locks | Joel on SQL is kinda plain. You could glance at Yahoo’s front
page and see how they create news titles to grab people interested.
You might add a video or a related picture or two to grab people excited about
what you’ve written. Just my opinion, it might bring your posts a little bit more interesting.
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