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POUse SqlTransaction & IsolationLevel for lengthy read operation?
 

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    <p>I am executing several long-running SQL queries as part of a reporting module. These queries are constructed dynamically at run-time. Depending on the user's input, they may be single or multi-statement, have one or more parameters and operate on one or more database tables - in other words, their form cannot be easily anticipated.</p> <p>Currently, I am just executing these statements on an ordinary <code>SqlConnection</code>, i.e.</p> <pre><code>using (SqlConnection cn = new SqlConnection(ConnectionString)) { cn.Open(); // command 1 // command 2 // ... // command N } </code></pre> <p>Because these queries (really query batches) can take a while to execute, I am concerned about locks on tables holding up reads/writes for other users. It is not a problem if the data for these reports changes during the execution of the batch; the report queries should never take precedence over other operations on those tables, nor should they lock them.</p> <p>For most long-running/multi-statement operations that involve modifying data, I would use transactions. The difference here is that these report queries are not modifying any data. Would I be correct in wrapping these report queries in an <code>SqlTransaction</code> in order to control their isolation level?</p> <p>i.e:</p> <pre><code>using (SqlConnection cn = new SqlConnection(ConnectionString)) { cn.Open(); using (SqlTransaction tr = cn.BeginTransaction(IsolationLevel.ReadUncommitted)) { // command 1 // command 2 // ... // command N tr.Commit(); } } </code></pre> <p>Would this achieve my desired outcome? Is it correct to commit a transaction, even though no data has been modified? Is there another approach?</p>
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    1. CODo the reports have to be [correct](http://blogs.msdn.com/b/sqlcat/archive/2007/02/01/previously-committed-rows-might-be-missed-if-nolock-hint-is-used.aspx) or not?
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      1. USRemus Rusanu
    2. CO@RemusRusanu The reports provide an instantaneous snapshot of the data over a long period of time; they do not need to take into account changes that may occur mid-transaction. The underlying queries are likely to be run multiple times by the user in short succession, and the user will not expect the results to be identical each time.
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    3. COThis is exactly the kind of reports that get affected (badly) by dirty reads. Counts and aggregates will jump randomly up and down and data within a single run (a report) will not be consistent (eg. sum debit != sum credit). Your users will loose confidence in the report as it will seem to produce random data. Have you considered [`SNAPSHOT`](http://msdn.microsoft.com/en-us/library/ms345124(v=sql.90).aspx) instead?
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