![]() ![]() If you use it at the top of a stored proc it will apply to every statement in the proc. Therefore it will apply to every table used in the SELECT. SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED will apply to all statements issued on that connection until it is cancelled. Is there an additional saving in SQL Server resource overhead of applying the lowest level isolation? AFAIK It only gets stuck when a user is modifying table design and at that point a read uncommitted blocks. but read uncommitted doesn't lock anything. You guys are talking about its usefullness in very large tables where lots of joins are made. I wish I was kidding but I am not! See They claim to have fixed it eg in but my experience says "not in all circumstances".Ĭan someone explain the benefit of NOLOCK over Isolation Level READ UNCOMMITTED? ![]() Also, complex SQL queries may join 10 or more tables and you can't tell in which order the query engine will choose to access them.Ī single SELECT query can block itself in SQL Server! If the query engine decides to use intra-query parallelism it is possible for the separate threads that it spawns to deadlock each other. This is good advice and would eliminate most deadlocks, but it is not possible to do this analysis in non-trivial applications. I think we all know the definition of a deadlock, I think Frans is suggesting that BEGAN TRAN/COMMIT blocks are written such that access to potentially common tables is always done in the same order (A, B, C, D, E.). SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED Then before your 30 seconds are up, try the following variations in another window: ![]() INSERT INTO #FOO(FOOID) VALUES ('NOTSEEME') INSERT INTO #FOO(FOOID) VALUES ('CANSEEME') Run this in one window in Query Analyzer:ĬREATE TABLE #FOO (FOOID CHAR(10) NOT NULL PRIMARY KEY) Thomas is describing one, for example if you have a transaction that INSERTs into a table as part of a long running transaction then the default isolation level will block SELECTs upon that table. NOLOCK (or SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED - whew!) can certainly be useful in circumstances where you do not have deadlocks. So I'm not sure what you mean by your suggestion about the logic being off. Its the job of the db engine to execute them in order and without contention. If you have a webfarm with 3 instances of an app hitting the same db you'll get all sorts of requests for selects and inserts. So I'm not quite sure what to make of your suggestion there. Which meant we did not touch the SQL Server engine in terms of the decision and resource usage whilst it ran through a normal day of operations. In both apps I've worked on we simply developed to what we understood to be best practices. In your prev post you suggested that deadlocks are purely the result of the timing of a write versus a read operation. One of the systems used compound PK's the other used Identity PK's and both had clustered indexes on them (the default behavior of SQL Server). In both systems, I suppose partially because there were 10+ people coding to them, we ended up using NOLOCK on all SELECT procs. The current one has a couple of table with 130M rows. Frans - I've worked on 2 systems that had considerable amounts of data. ![]()
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