What Is the Best Way to See Raw Data Stream Being Passed from SQL to .NET Application?

Question

I am running some functions on application startup for my .NET application. This mostly involves running a bunch of scripts on my SQL database to ensure plans are cached. These queries are generated from EFCore-compiled queries.

In doing this, I noticed that one of my queries was consistently taking 16 seconds. My code is calling FirstOrDefault on the IQueryable to generate the query and subsequent plan. I am able to use EFCore logging to see the query generated, so I have been using that to get started.

When I run the query without a filter (i.e. TOP(1)) it takes ~16 seconds. With a Top(1) it is much faster and what I would expect for application startup after the first try.

This has me worried that somehow the entire dataset is being pulled and/or sent over the wire during my application startup.

I would like to validate this but am not sure of the best way to do so. I would appreciate a pointer on how to proceed doing this. Thank you for any assistance you can provide.

Answer 1

to simplify the thread. if you use FirstOrDefault() on an EF query, under the covers it will read all pending rows via NextResult() processing before the row is returned from FirstOrDefault(). If you want the first row only of an EF query and do not any extra rows returned, you use the .Take();

var author = dbContext.Authors.Where(_ => _.State == "UT")
    .Take(1)
    .FirstOrDefault();

this tells the sqlserver to only return 1 row.

EF has limited support for canceling a pending query. You need access to the DbCommand instance. you can create your own for example:

using (var cmd = dbContext.Authors.Where(_ => _.State == "UT").CreateDbCommand())
{
    await cmd.Connection.OpenAsync();
    using var reader = await cmd.ExecuteReaderAsync();
    if (await reader.ReadAsync())
    {
        // process first row
		var
    }
    // attempt to cancel query
    cmd.Cancel();
}

if you need this low level, you are better off with dapper, where at least the reader can bind to a POCO object.

Answer 2

Thanks for posting your question in the Microsoft Q&A forum.

If you need to debug and monitor the data flow between your application and the database, I would recommend using SQL Profiler. This tool allows you to capture and analyze the main queries executed on the database. You can then copy these queries into SQL Server Management Studio (SSMS) and thoroughly analyze them, including examining their execution plans. This comprehensive approach enables you to gain insights into the performance and behavior of your database interactions, facilitating effective troubleshooting and optimization efforts.

Please don't forget to close up the thread here by upvoting and accept it as an answer if it is helpful

Answer 3

in EF, the FirstOrDefault(), causes EF to run the query (which will return all rows) and loads the entity collection. If tracking is enabled, then it checks each entity to see if already in the collection. Then your FirstOrDefault() get the first Entity of the collection if one exists.

if you used a DataReader directly, you could read one row, then cancel the query (though this is not very efficient and not recommended). Also you would lose messages and row counts. normally you'd read a row, and if you did not need more, call NextResults(), which would read all rows and process all messages.

so if you only want one row, use top(), fetch first row only or EF .Take()

note: if you called a sp, then all the rows need to read before the return value is available.