In my DB there is View "RqstLst"
I create EF Model from DB. Now I have entity RqstLst.
There is two variant of the same query
public void MyMethod()
{
context = new WaterMEntities();
var query = context.RqstLst;
dgRqstLst.ItemsSource = query; //dgRqstLst - DataGrid in WPF
}
and
public void MyMethod()
{
dgRqstLst.ItemsSource = this.GetRqstLst();
}
private IEnumerable<RqstLst> GetRqstLst()
{
context = new WaterMEntities();
string nativeSQLQuery = "SELECT * " +
"FROM dbo.RqstLst ";
ObjectResult<RqstLst> requestes =
context.ExecuteStoreQuery<RqstLst>(nativeSQLQuery);
return requestes;
}
execution time for first variant(LINQ to Entities) is 19 sec, for second, less then 1 sec.
I look it in sql server profiler. What i do wrong in first variant?
One big difference is that ExecuteStoreQuery doesn't attach the returned objects to the context (at least not the overload you are using) but your first query does (which costs time).
Try to define the same tracking behaviour in your first query like you have in the second query (= NoTracking):
context = new WaterMEntities();
context.RqstLst.MergeOption = MergeOption.NoTracking; // in System.Data.Objects
var query = context.RqstLst;
dgRqstLst.ItemsSource = query;
You didn't do anything wrong with the first option, but depending on your configuration, the query generated by the first one could be much more complex than your straight SQL execution. Have you used the profiler to see exactly what SQL the first query generates? For example, if RqstLst happens to be an abstract base class using TPT Inheritance, the generated SQL could be huge.
Related
In the Dapper documentation, it says you can use an IEnumerable parameter to execute a command multiple times. It gives the following example:
connection.Execute(#"insert MyTable(colA, colB) values (#a, #b)",
new[] { new { a=1, b=1 }, new { a=2, b=2 }, new { a=3, b=3 } }
).IsEqualTo(3); // 3 rows inserted: "1,1", "2,2" and "3,3"
Will this result in multiple round-trips to the database (i.e. one for each T in the IEnumerable<T>)? Or is Dapper smart enough to transform the multiple queries into a batch and just do one round-trip? The documentation says an example usage is batch loading, so I suspect it only does one round-trip, but I want to be sure before I use it for performance-sensitive code.
As a follow-up question, and depending on the answer to the first, I'd be curious how transactions are handled? That is, is there one transaction for the whole set of Ts, or one transaction per T?
I finally got around to looking at this again. Looking at the source code (in \Dapper\SqlMapper.cs), I found the following snippet in method ExecuteImpl:
// ...
foreach (var obj in multiExec)
{
if (isFirst)
{
masterSql = cmd.CommandText;
isFirst = false;
identity = new Identity(command.CommandText, cmd.CommandType, cnn, null, obj.GetType(), null);
info = GetCacheInfo(identity, obj, command.AddToCache);
}
else
{
cmd.CommandText = masterSql; // because we do magic replaces on "in" etc
cmd.Parameters.Clear(); // current code is Add-tastic
}
info.ParamReader(cmd, obj);
total += cmd.ExecuteNonQuery();
}
// ...
The interesting part is on the second-last line where ExecuteNonQuery is called. That method is being called on each iteration of the for loop, so I guess it is not being batched in the sense of a set-based operation. Therefore, multiple round-trips are required. However, it is being batched in the sense that all operations are performed on the same connection, and within the same transaction if so specified.
The only way I can think of to do a set-based operation is to create a custom table-valued type (in the database) for the object of interest. Then, in the .NET code pass a DataTable object containing matching names and types as a command parameter. If there were a way to do this without having to create a table-valued type for every object, I'd love to hear about it.
When you use NHibernate to "fetch" a mapped object, it outputs a SELECT query to the database. It outputs this using parameters; so if I query a list of cars based on tenant ID and name, I get:
select Name, Location from Car where tenantID=#p0 and Name=#p1
This has the nice benefit of our database creating (and caching) a query plan based on this query and the result, so when it is run again, the query is much faster as it can load the plan from the cache.
The problem with this is that we are a multi-tenant database, and almost all of our indexes are partition aligned. Our tenants have vastly different data sets; one tenant could have 5 cars, while another could have 50,000. And so because NHibernate does this, it has the net effect of our database creating and caching a plan for the FIRST tenant that runs it. This plan is likely not efficient for subsequent tenants who run the query.
What I WANT to do is force NHibernate NOT to parameterize certain parameters; namely, the tenant ID. So I'd want the query to read:
select Name, Location from Car where tenantID=55 and Name=#p0
I can't figure out how to do this in the HBM.XML mapping. How can I dictate to NHibernate how to use parameters? Or can I just turn parameters off altogether?
OK everyone, I figured it out.
The way I did it was overriding the SqlClientDriver with my own custom driver that looks like this:
public class CustomSqlClientDriver : SqlClientDriver
{
private static Regex _partitionKeyReplacer = new Regex(#".PartitionKey=(#p0)", RegexOptions.Compiled);
public override void AdjustCommand(IDbCommand command)
{
var m = _tenantIDReplacer.Match(command.CommandText);
if (!m.Success)
return;
// replace the first parameter with the actual partition key
var parameterName = m.Groups[1].Value;
// find the parameter value
var tenantID = (IDbDataParameter ) command.Parameters[parameterName];
var valueOfTenantID = tenantID.Value;
// now replace the string
command.CommandText = _tenantIDReplacer.Replace(command.CommandText, ".TenantID=" + valueOfTenantID);
}
} }
I override the AdjustCommand method and use a Regex to replace the tenantID. This works; not sure if there's a better way, but I really didn't want to have to open up NHibernate and start messing with core code.
You'll have to register this custom driver in the connection.driver_class property of the SessionFactory upon initialization.
Hope this helps somebody!
When I have a query generated like this:
var query = from x in Entities.SomeTable
select x;
I can set a breakpoint and after hovering cursor over query I can see what will be the SQL command sent to database. Unfortunately I cannot do it when I use Count
var query = (from x in Entities.SomeTable
select x).Count();
Of course I could see what comes to SqlServer using profiler but maybe someone has any idea how to do it (if it is possible) in VS.
You can use ToTraceString():
ObjectQuery<SomeTable> query = (from x in Entities.SomeTable select x).Count();
Console.WriteLine(query.ToTraceString());
You can use the Database.Log to log any query made like this :
using (var context = new MyContext())
{
context.Database.Log = Console.Write;
// Your code here...
}
Usually, in my context's constructor, I set that to my logger (whether it is NLog, Log4Net, or the stock .net loggers) and not the console, but actual logging tool is irrelevant.
For more information
In EF6 and above, you can use the following before your query:
context.Database.Log = s => System.Diagnostics.Debug.WriteLine(s);
I've found this to be quicker than pulling up SQL Profiler and running a trace.
Also, this post talks more about this topic:
How do I view the SQL generated by the Entity Framework?
I am using MS SQL server 2008 with Hibernate. the question I have is how Hibernate implements setMaxResults
Take the following simple scenario.
If I have a query that returns 100 rows and if I pass 1 to setMaxResults, will this affect the returned result from the SQL server itself(as if running a select top 1 statement) or does Hibernate get all the results first (all 100 rows in this case) and pick the top one itself?
Reason I am asking is that it would have a huge performance issue when the number of rows starts to grow.
Thank you.
Hibernate will generate a limit-type query, for all dialects which supports limit query. As the SQLServerDialect supports this (see org.hibernate.dialect.SQLServerDialect.supportsLimit(), and .getLimitString()), you will get a select top 1-query.
If you would like to be absolutly sure, you may turn on debug-logging, or enable the showSql-option and test.
May be following snippet will help. Assume we have a managed Bean class EmpBean and we want only first 5 records. So following is the code
public List<EmpBean> getData()
{
Session session = null;
try
{
session = HibernateUtil.getSession();
Query qry = session.createQuery("FROM EmpBean");
qry.setMaxResults(5);
return qry.list();
}
catch(HibernateException e)
{}
finally
{
HibernateUtil.closeSession(session);
}
return null;
}
Here getSession and closeSession are static utility methods which will take care of creating and closing session
I am creating a task-scheduler on SQL Server 2008.
I have a table that I use to store tasks. Each task is a task name (e.g. ImportFile) and arguments. I store arguments in XML column, since different tasks have different signatures.
Table is as follows:
Id:integer(PK) | operation:nvarchar | Arguments:xml
Before queuing a task, I often need to verify that given task hasn't been scheduled yet. The lookup is done based on both operation and args.
Question: Using Linq-to-Sql how can I check if given operation+args is present in the queue already?
I am looking for something like:
var isTaskScheduled = db.Tasks.Any(t =>
t.Opearation == task.Operation &&
t.Arguments == task.ArgumentsAsXElement);
(which doesn't work because SQL Server can't compare XML type)
Any alternative implementation suggestions?
You might want to surface e.g. a string property that encapsultes your Arguments, or maybe it would be sufficient to have e.g. the length and a CRC of your Arguments as extra properties on your class:
public partial class Task
{
public int ArgumentLength
{ .... }
public int ArgumentCRC
{ .... }
}
That way, if you can compare length (of your XML) and the CRC and they match, you can be pretty sure and safe to assume the two XML's are identical. Your check would then be something like:
var isTaskScheduled =
db.Tasks.Any(t => t.Operation == task.Operation &&
t.ArgumentLength == task.ArgumentLength &&
t.ArgumentCRC == task.ArgumentCRC);
or something like that.
This may be a stretch, but you could use a "Hashcode" when saving the data to the database, then query on the hashcode value at a later date / time.
This assumes that you have a class that represents your task entity and that you have overridden the GetHashCode method of said class.
Now, when you go to query the database to see if the task is in the scheduled queue, you simply query on the hashcode, thus avoiding the need to do any xml poking at query time.
var t1 = new Task{Operation="Run", Arguments="someXElement.value"};
var t2 = new Task{Operation="Run", Arguments="someXElement.value"};
in the code above t1 == t2 because you are overriding GetHashCode and computing the hash for Operation+Arguments.Value. if you store the hashcode in the db, then you can easily tell if you have an object in the DB that equals the hash code that you are checking for.
This may be similar to what marc_s was talking about.
You can write a class which implements IComparable:
public class XMLArgument : IComparable
{
public XMLArgument(string argument)
{
}
public int CompareTo(object obj)
{
...
}
}
var isTaskScheduled = db.Tasks.Any(t =>
t.Opearation == task.Operation &&
(new XMLArgument(t.Arguments)).CompareTo(new XMLArgument(task.ArgumentsAsXElement)) == 0);