运行Linq语句的超时异常

Question

This just started happening. This statement was working for months, now I just keep getting the timeout error below. when I execute the same statement directly on the SSMS it comes back in a second. the table has 44k records and is index on 5 columns state being one of them.

select distinct(state) from [ZipCodeDatabase]

I am running the following linq statement

states = ZipCodeRepository.Get(orderBy: z => z.OrderBy(o => o.State)).Select(z => z.State).Distinct().ToList();

When I run this linq statement I am continually getting a timeout error and have no idea y since it was working correctly before.

I included the Get() function which is a generic repo function, but maybe I am missing something there, that is causing the delay.

Get Function:

public virtual IEnumerable<TEntity> Get(
Expression<Func<TEntity, bool>> filter = null,
Func<IQueryable<TEntity>, IOrderedQueryable<TEntity>> orderBy = null,
string includeProperties = "") //params Expression<Func<TEntity, object>>[] includes
{
    IQueryable<TEntity> query = dbSet;
    if (filter != null)
    {
        query = query.Where(filter);
    }
    foreach (var includeProperty in includeProperties.Split
        (new char[] { ',' }, StringSplitOptions.RemoveEmptyEntries))
    {
        query = query.Include(includeProperty);
    }

    if (orderBy != null)
    {
        return orderBy(query).ToList();
    }
    else
    {
        return query.ToList();
    }
}

The timeout error:

System.Data.SqlClient.SqlException (0x80131904): Timeout expired. The timeout period elapsed prior to completion of the operation or the server is not responding. ---> System.ComponentModel.Win32Exception (0x80004005): The wait operation timed out at System.Data.SqlClient.SqlConnection.OnError(SqlException exception, Boolean breakConnection, Action 1 wrapCloseInAction) at System.Data.SqlClient.SqlInternalConnection.OnError(SqlException exception, Boolean breakConnection, Action 1 wrapCloseInAction) at System.Data.SqlClient.TdsParser.ThrowExceptionAndWarning(TdsParserStateObject stateObj, Boolean callerHasConnectionLock, Boolean asyncClose) at System.Data.SqlClient.TdsParser.TryRun(RunBehavior runBehavior, SqlCommand cmdHandler, SqlDataReader dataStream, BulkCopySimpleResultSet bulkCopyHandler, TdsParserStateObject stateObj, Boolean& dataReady) at System.Data.SqlClient.SqlDataReader.TryCloseInternal(Boolean closeReader) at System.Data.SqlClient.SqlDataReader.Close() at System.Data.Common.DbDataReader.Dispose(Boolean disposing) at System.Data.Common.DbDataReader.Dispose() at System.Data.Common.Internal.Materialization.Shaper 1.Finally() at System.Data.Common.Internal.Materialization.Shaper 1.Finally() at System.Data.Common.Internal.Materialization.Shaper 1.SimpleEnumerator.Dispose() at System.Collections.Generic.List 1..ctor(IEnumerable 1 collection) at System.Linq.Enumerable.ToList[TSource](IEnumerable 1 source) at ClientsToProfitsKendoUI.DAL.GenericRepository 1.Get(Expression 1 filter, Func 2 orderBy, String includeProperties)

Answer 1

Your get method always calls ToList , which materializes the query at that point. Meaning that it will grab all 44k records from the database before doing any filtering.

You're also doing the ordering by state before actually selecting any data, which is a useless exercise in ordering arbitrary data.

From your Get method, remove the ToList calls. And optimize your query by first selecting the State , then calling Distinct and then ordering. This allows the database to do its thing optimally.

If you need the list fully materialized, call ToList at the very end. But until that, keep using the IQueryable which your query returns. That way you can let Entity Framework offload as much as possible to the database.

Answer 2

You're materializing every row in the database just to select distinct states. Let the database do the heavy lifting for you and simply return the states rather than materializing everything and then having .Net do it.

Add this to your ZipCodeRepository

public IEnumerable<string> GetStates()
{
    return dbSet.OrderBy(e => e.State).Select(e => e.State).Distinct();
}

Note that what's causing the materialization (and a ton of overhead) is the ToList() .

In general, keep the collection as an IQueryable until you're actually ready to to use the results. The moment you actually enumerate the collection EF will actually perform the query for you anyway.

Alternatively, if you remove the ToList() calls and change the return type of the Get function to be IQueryable<TEntity> that should also resolve the issue.