![]() You'll just have to take my word on SELECT ) but it will depend on your DB and settings. So basically I think you have a few incorrect assumptions. Oracle & I think Postgres will never lock on SELECT (unless you have FOR UPDATE which is clearly reserving for an update anyway). Fortunately, SQL Server detects this deadlock condition, and selects one of the transactions to be the deadlock victim, terminating that statement and rolling back the changes that it has made. SQL Server will not lock on a SELECT if you use SNAPSHOT ISOLATION. That's just crappy database engine design though. At this stage, the SQL Engine will detect a deadlock and roll back one of the two open transactions. INSERT dbo.Tbl2 (id, col) VALUES (111, 555) This script attempts to create a record with a primary key field value of 111 in Tbl2 as part of the open transaction in Session 1. SELECT should never deadlock but on some databases it will because the locks it uses interfere with consistent reads. Add the script shown below to the first Query window and execute it. So that proved inserts & deletes can deadlock. This is very simple example just to get the concept right, but there are other types of deadlocks and they sometimes become very. The best way to create a deadlock is to do the same thing in a different order. You can somewhat ignore select depending on your database and settings but the others will give you deadlocks.Generally all modifications can cause a deadlock and selects will not (get to that later). Is there a way to fix this example for REPEATABLE_READ or SERIALIZABLE isolation? In the above example, the INSERT locking order is whereas the DELETE locking order is. DELETE FROM permissions WHERE id = 100.SELECT permission_id FROM companies WHERE id = 200 - returns permission_id = 100.INSERT INTO companies (name, permission_id) VALUES ('Nintendo', 100) - Inserts companies.id = 200.INSERT INTO permissions - Inserts permissions.id = 100.companies [id BIGINT PRIMARY KEY, name VARCHAR(30), permission_id BIGINT NOT NULL, FOREIGN KEY (permission_id) REFERENCES permissions(id)). ![]() For super bonus points: how can I avoid a deadlock in the following scenario? For bonus points: answer the same question for all other operations (e.g.Is it possible for an INSERT operation to cause a deadlock? If so, please provide a detailed scenario demonstrating how a deadlock may occur (e.g.We are talking about multiple threads accessing multiple tables simultaneously.I am using REPEATABLE_READ or SERIALIZABLE transaction isolation (locks get retained every time I access a row).Or if someone inserts into t1 between our statements we did it.The above is bad code and is bound to cause trouble eventually. SQL Server deadlock is essentially a standoff between two processes that are competing for exclusive access to the same resource. Simple example INSERT INTO t1(b, c, d) VALUES(1, 2, 3) - where a is an identity columnDeclare intSELECT = MAX(a) FROM t1 INSERT INTO t2 (a) VALUES anything goes wrong with the insert, we just put the wrong value in t2. If you aren't, then you have what is called a race condition, and it doesn't take a deadlock to end up with inconsistent data. In this case, as long as you code your updates to be ATOMic where necessary, then there's nothing to worry about. Thanks Russell and tkizer.Yes, data inconsistency is the term I intended to use. Data corruption involves internal system objects.Tara Kizer Microsoft MVP for Windows Server System - SQL Server Subscribe to my blog 29 Assuming: I am using REPEATABLEREAD or SERIALIZABLE transaction isolation (locks get retained every time I access a row) We are talking about multiple threads accessing multiple tables simultaneously. Data corruption is different, at least in SQL Server terms. If it hasn't been coded properly and the query is chosen as the deadlock victim, then data inconsistency is certainly possible. Both are only inside a transaction if it's been coded properly. If you are updating two tables and both need to be updated or none, then both would be inside a transaction and both would participate in the deadlock checking. Whatever has been committed is not involved in the deadlock. Short answer is no, a deadlock won't result in corruption.Ī deadlock involves two sessions that each have an open transaction. If it's already been committed, then there's nothing to rollback (or kill). does SQL Server always perform a rollback of record A? if it has already been committed, it cant rollback, correct?Thanks for your opinions. Hi experts.Sorry to have to post so many questions about transaction deadlocks this week.Even though SQl Server 2005 attempts to rollback transactions for the "victim" process that gets killed when a deadlock occurs, is it still possible for some data to be corrupted? For example if a long-running process updates Record A, but it can't get a lock on record B to update B and the SPID eventually gets killed. We've got lots of great SQL ServerĮxperts to answer whatever question you can come up with.
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