Through my research I've discovered that, since at least EF 4.1, the .ToString() method on an EF query will return the SQL to be run. Indeed, this very frequently works for me, using Entity Framework 5 and 6.
However, occasionally I call this method and get the runtime type of the query object.
Here is my specific example:
Entity input = ...;
IQueryable<Entity> query = dbContext.SetOfEntity.Where(e => e.Prop == input.Prop);
More specifically, I'm setting a breakpoint in VS2013 and hovering over the query object, and seeingSystem.Data.Entity.Infrastructure.DbQuery<Namespace.Entity> instead of the SQL to run that query. Interestingly enough, if I hover over the DBSet property (dbContext.SetOfEntity), I do see the basic select SQL for the associated table. It's only when I filter the results that I lose the SQL.
Obviously, this is a pretty simple query and I could work out the SQL for myself, but this problem has happened on more complex queries, and it would be nice to be able to debug the SQL being sent to the server without running a database trace.
Some background
A while back, I was using EF5 and the ToString() seemed to work. Shortly before switching to EF6, it seemed that none of the queries showed me SQL, but after switching to EF6, it went back to the correct behavior.
Additionally, whenever I hover over IQueryable queries and try to use the IDE's "Results View" feature, it tells me "Children could not be evaluated". This may be a separate issue, but I figure I'd include it in case it had a common cause.
If you do not need the SQL BEFORE it is executed against the DB you can do the following:
dbContext.Database.Log = s => Debug.WriteLine(s);
This would print the SQL (and some additional data) to the debug output.
See the following link for details: http://msdn.microsoft.com/de-DE/data/dn469464
Also check, as martin_costello suggested, that you are not querying the DB before you try to get the SQL via ToString(). It happened to me too, that I already got objects, because of using IEnumerable<> "to early" (instead if IQueryable<>) and so got way to many entities from the DB and did some filtering "in code" instead of "in SQL"...
Related
As part of a small .NET Core 3 project, I'm trying to use the data model based in Entity Framework, but I'm having some troubles related with queries on joined tables.
When looking for data matching a condition in a single table, the model is easy to understand
List<Element> listOfElements = context.Elements.Where(predicate).ToList();
However, when this query requires joined tables, I'm not sure how to do it efficiently. After some investigation, it seems that the include (and theninclude) methods are the way to go, but I have the impression that the Where clause after the include is not executed at DB level but after all the data has been retrieved. This might work with small datasets, but I don't think it's a good idea for a production system with millions of rows.
List<Element> listOfElements = context.Elements.Include(x => x.SubElement).
Where(predicate).ToList();
I've seen some examples using EF+ library, but I'm looking for a solution using the nominal EF Core. Is there any clean/elegant way to do it?
Thank you.
There are a few scenarios when the data from DB is populating:
Deferred query execution: this is when you try to access your query results, for example, in the foreach statement.
Immediate Query Execution: this when you call ToList() (or conversions to other collections, like ToArray()).
I think the answer's to your question:
... but I have the impression that the Where clause after the include is not executed at DB level but after all the data has been retrieved.
is that your assumptions are wrong because you are calling ToList() at the end, not before the Where method.
For more information please also check here.
I've another suggestion also: to be sure about what is exactly executing at the DB level run SQL Server Profiler when executing your query.
Hope this will help ))
I have some legacy code that uses Entity Framework.
When I debug the code I can see that EF DbContext contains the whole table. It was passed by OData to the frontend, and then angular processed it.
So I tried to search, is it possible to get only a single record by EF?
Everywhere I see the SingleOrDefault method, or other IQueryable, but as I understood, these are parts of the collections.
Microsoft says: Sometimes the value of default(TSource) is not the default value that you want to use if the collection contains no elements.
Does that mean EF always get all the data from the table and I can use them later?
Or is there a way to force inner query to get only one, and only one row?
We are using postgresql.
With Entity Framework, you can use LINQ to run queries and get single records or limited sets. However, in your .NET project the controller should be parsing OData query parameters and filtering the dataset before returning results to the client application. Please check your Controller code against this tutorial to see if you might be missing something.
If you are somehow bypassing the built-in OData framework, what might help is understanding which queries execute immediately vs which ones are deferred. See this list to understand exactly which operations will force a trip to the database and try to hold off on anything with immediate execution until as late as possible.
No, EF will not SELECT the entire table into memory if you use it correctly. By correctly; I mean:
context.Table.First();
Will translate into a SQL query that only returns one row, that will then map to an object to be returned to the calling code. This is because the above code uses LINQ-to-Entities. If you did something like this instead:
context.Table.ToList().First();
Then the entire table is selected to make the ToList work, and LINQ-to-Objects handles the First. So as long as you do your queries with lazy enumeration (not realizing the result ahead of time), you'll be fine.
I was sure that issuing a FirstOrDefault() on entity framework will issue a query on the database and to hit cache I can use .Find().
But helping a colleague debug a problem in EF4.2 (query data didn't reflect the db data, and query wasn't in the sql profiler) got me to the solution that works. So use ".AsNoTracking()", and I later found other posts here that states:
" The following will sometimes pull from the EF cache" :
.FirstOrDefault();
And another post with same issue: here
If you go to msdn:
By default when an entity is returned in the results of a query, just before EF materializes it, the ObjectContext will check if an entity with the same key has already been loaded into its ObjectStateManager. If an entity with the same keys is already present EF will include it in the results of the query. Although EF will still issue the query against the database, this behavior can bypass much of the cost of materializing the entity multiple times.
.........
Unlike a regular query, the Find method in DbSet (APIs included for
the first time in EF 4.1) will perform a search in memory before even
issuing the query against the database.
And other SO posts: here and here seem to support msdn?
So how does EF object caching behave?
always hits DB except for .FirstOrDefault() when it works only sometimes (black box) so it's better to use .AsNoTracking().FirstOrDefault() just to be sure?
I should expect similar issues with other methods like: First(), Single(), SingleOrDefault()
Came across code where .Where(o => o.x == y) was changed to .Where(o => o.x.Equals(y)). I knew that == was parsed out by EF's SQL generator to execute on the server, but wasn't sure about .Equals(). Clearly this change was done as a matter of habit, perhaps someone coming out of a C++ background and not thinking about the fact that == would have been parsed as an expression, not executed as a function, and would be converted to SQL. This change compiles and runs but I was wondering if it's because EF is treating it as a Func instead of as an expression, and as such perhaps executing a generalized query and moving the filter to client-side, or something similarly ridiculous.
Linq-To-Entities, as of version 6 of EF, doesn't perform any kind of filtering in the client. If you try to execute any kind of not supported function (meaning it can't be translated to the DB provider) on an EF's IQueryable, it'll throw an exception.
So the answer is: no, it's not executing it locally.
PS: I've read somewhere that this feature is a planned addition to EF7, but this is unconfirmed and just speculation
Update: link to source here: http://blogs.msdn.com/b/adonet/archive/2014/10/27/ef7-v1-or-v7.aspx
Quoting the relevant part in case the link goes dead:
An example of this is how queries are processed. In EF6.x the entire LINQ query was translated into a single SQL query that was executed in the database. This meant your query could only contain things that EF knew how to translate to SQL and you would often get complex SQL that did not perform well.
In EF7 we are adopting a model where the provider gets to select which bits of the query to execute in the database, and how they are executed. This means that query now supports evaluating parts of the query on the client rather than database. It also means the providers can make use of queries with multiple results sets etc., rather than creating one single SELECT with everything in it.
I ran SQL Profiler. It generated "[table].[x] = 'y'" as was originally intended with '=='.
I'm noticed that some queries in my application are so slowly, so that's the reason why I want to know what is trying to accomplish my queries in LINQ TO SQL through entity framework.
In some sites, I realized that if you put your mouse over the IQueryable variable, you can see the T-SQL generated and at this moment, I can't see that.
I'd like to know if I'm doing a wrong configuration in my Entity Framework model
For Entity Framework you can see generated SQL query by inspecting your context Log property, or you can cast your IQueryable to System.Data.Objects.ObjectQuery and use method ToTraceString().
I want to suggest a different approach: Look at the real query in SQL Profiler. You can see all queries executed including parameter values. You can copy the query including parameter assignments to SSMS to debug it.