isolation-level

问题 I am attempting to set the Transaction Isolation Level in .NET/C# for a Transaction. I am using the following code to set up the transaction: using (var db = new DbContext("ConnectionString")) { using (var transaction = new TransactionScope(TransactionScopeOption.RequiresNew, new TransactionOptions() { IsolationLevel = IsolationLevel.Snapshot })) { ...code here transaction.Complete(); } } Using SQL Server Profiler, this produces the following: set quoted_identifier on set arithabort off set

问题 I want to insert a record to an RDBMS table only if the table does not consist of any rows which are "similar" (according to some specific, irrelevant criteria) to the said row. I have a simple SELECT query for checking if any "similar" rows exist. I originally figured that to achieve my target, it'd be enough to run the SELECT query conditionally followed by the INSERT query, both together inside one transaction. However, I've been reading about different isolation levels, and it seems like

问题 I know the default TRANSACTION ISOLATION LEVEL in SQL Server is "read committed". If I want to change it to "READ UNCOMMITTED", how may i make this configuration change? note: I cannot use SET TRANSACTION ISOLATION LEVEL, which only apply in the current session. I cannot add NOLOCK in the queries because there are thousands of queries involved. Thanks Thanks for your answer. We are ok with reading dirty rows. Update is not a problem in our case as well. but, I really want to change this

问题 I have been looking for definitive documentation regarding the isolation level ( or concurrency or scope ... I'm not sure EXACTLY what to call it) of triggers in SQL Server. I have found the following sources which indicate that what I believe is true (which is to say that two users, executing updates to the same table --even the same rows-- will then have independent and isolated triggers executed): https://social.msdn.microsoft.com/Forums/sqlserver/en-US/601977fb-306c-4888-a72b-3fbab6af0cdc

问题 I have a function, that does multiple queries on the same dataset and I want to ensure all the queries would see exactly the same data. In terms of SQL, this means REPEATABLE READ isolation level for the databases that support it. I don't mind having higher level or even a complete lockdown if the database isn't capable. As far as I see, this isn't the case. I.e. if I run something like this code in one Python shell: with transaction.atomic(): for t in range(0, 60): print("{0}: {1}".format(t,

问题 if we have an UPDATE with a sub-SELECT, can the subquery execute concurrently or not under READ COMMITTED isolation? In other words, is there a race condition present in the following: update list set [state] = 'active' where id = (select top 1 id from list where [state] = 'ready' order by id) In yet other words, if many connections are simulataneously executing this SQL, can we guarantee that one row is in fact updated per invocation (so long as rows in 'ready' state exist)? 回答1: The answer

问题 When measuring performance on my query I came up with a dependency between isolation level and elapsed time that was surprising to me READUNCOMMITTED - 409024 READCOMMITTED - 368021 REPEATABLEREAD - 358019 SERIALIZABLE - 348019 Left column is table hint, and the right column is elapsed time in microseconds (sys.dm_exec_query_stats.total_elapsed_time). Why better isolation level gives better performance? This is a development machine and no concurrency whatsoever happens. I would expect

问题 This is a followup question from this one so I know I can use (blocking) LOCKs but I want to use predicate locks and serializable transaction isolation. What I'd like to have is a generic handler of serialization failures that would retry the function/query X number of times. As example, I have this: CREATE SEQUENCE account_id_seq; CREATE TABLE account ( id integer NOT NULL DEFAULT nextval('account_id_seq'), title character varying(40) NOT NULL, balance integer NOT NULL DEFAULT 0, CONSTRAINT

问题 I have a tasks thread running in two separate instances of tomcat. The Task threads concurrently reads (using select) TASKS table on certain where condition and then does some processing. Issue is ,sometimes both the threads pick the same task , because of which the task is executed twice. My question is how do i make both thread not to read the same set of data from the TASKS table 回答1: I think you need have some variable (column) where you keep last modified date of rows. Your threads can

问题 I have a question. Transaction isolation level is set to serializable. When the one user opens a transaction and INSERTs or UPDATEs data in "table1" and then another user opens a transaction and tries to INSERT data to the same table, does the second user need to wait 'til the first user commits the transaction? 回答1: Generally, no. The second transaction is inserting only, so unless there is a unique index check or other trigger that needs to take place, the data can be inserted