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I am using ASP NET MVC 4.5 and EF6, code first migrations.
I have this code, which takes about 6 seconds.
var filtered = _repository.Requests.Where(r => some conditions); // this is fast, conditions match only 8 items
var list = filtered.ToList(); // this takes 6 seconds, has 8 items inside
I thought that this is because of relations, it must build them inside memory, but that is not the case, because even when I return 0 fields, it is still as slow.
var filtered = _repository.Requests.Where(r => some conditions).Select(e => new {}); // this is fast, conditions match only 8 items
var list = filtered.ToList(); // this takes still around 5-6 seconds, has 8 items inside
Now the Requests table is quite complex, lots of relations and has ~16k items. On the other hand, the filtered list should only contain proxies to 8 items.
Why is ToList() method so slow? I actually think the problem is not in ToList() method, but probably EF issue, or bad design problem.
Anyone has had experience with anything like this?
EDIT:
These are the conditions:
_repository.Requests.Where(r => ids.Any(a => a == r.Student.Id) && r.StartDate <= cycle.EndDate && r.EndDate >= cycle.StartDate)
So basically, I can checking if Student id is in my id list and checking if dates match.
Your filtered variable contains a query which is a question, and it doesn't contain the answer. If you request the answer by calling .ToList(), that is when the query is executed. And that is the reason why it is slow, because only when you call .ToList() is the query executed by your database.
It is called Deferred execution. A google might give you some more information about it.
If you show some of your conditions, we might be able to say why it is slow.
In addition to Maarten's answer I think the problem is about two different situation
some condition is complex and results in complex and heavy joins or query in your database
some condition is filtering on a column which does not have an index and this cause the full table scan and make your query slow.
I suggest start monitoring the query generated by Entity Framework, it's very simple, you just need to set Log function of your context and see the results,
using (var context = new MyContext())
{
context.Database.Log = Console.Write;
// Your code here...
}
if you see something strange in generated query try to make it better by breaking it in parts, some times Entity Framework generated queries are not so good.
if the query is okay then the problem lies in your database (assuming no network problem).
run your query with an SQL profiler and check what's wrong.
UPDATE
I suggest you to:
add index for StartDate and EndDate Column in your table (one for each, not one for both)
ToList executes the query against DB, while first line is not.
Can you show some conditions code here?
To increase the performance you need to optimize query/create indexes on the DB tables.
Your first line of code only returns an IQueryable. This is a representation of a query that you want to run not the result of the query. The query itself is only runs on the databse when you call .ToList() on your IQueryable, because its the first point that you have actually asked for data.
Your adjustment to add the .Select only adds to the existing IQueryable query definition. It doesnt change what conditions have to execute. You have essentially changed the following, where you get back 8 records:
select * from Requests where [some conditions];
to something like:
select '' from Requests where [some conditions];
You will still have to perform the full query with the conditions giving you 8 records, but for each one, you only asked for an empty string, so you get back 8 empty strings.
The long and the short of this is that any performance problem you are having is coming from your "some conditions". Without seeing them, its is difficult to know. But I have seen people in the past add .Where clauses inside a loop, before calling .ToList() and inadvertently creating a massively complicated query.
Jaanus. The most likely reason of this issue is complecity of generated SQL query by entity framework. I guess that your filter condition contains some check of other tables.
Try to check generated query by "SQL Server Profiler". And then copy this query to "Management Studio" and check "Estimated execution plan". As a rule "Management Studio" generatd index recomendation for your query try to follow these recomendations.
I've been working for the first time with the Entity Framework in .NET, and have been writing LINQ queries in order to get information from my model. I would like to program in good habits from the beginning, so I've been doing research on the best way to write these queries, and get their results. Unfortunately, in browsing Stack Exchange, I've seem to have come across two conflicting explanations in how deferred/immediate execution works with LINQ:
A foreach causes the query to be executed in each iteration of the loop:
Demonstrated in question Slow foreach() on a LINQ query - ToList() boosts performance immensely - why is this? , the implication is that "ToList()" needs to be called in order to evaluate the query immediately, as the foreach is evaluating the query on the data source repeatedly, slowing down the operation considerably.
Another example is the question Foreaching through grouped linq results is incredibly slow, any tips? , where the accepted answer also implies that calling "ToList()" on the query will improve performance.
A foreach causes a query to be executed once, and is safe to use with LINQ
Demonstrated in question Does foreach execute the query only once? , the implication is that the foreach causes one enumeration to be established, and will not query the datasource each time.
Continued browsing of the site has turned up many questions where "repeated execution during a foreach loop" is the culprit of the performance concern, and plenty of other answers stating that a foreach will appropriately grab a single query from a datasource, which means that both explanations seem to have validity. If the "ToList()" hypothesis is incorrect (as most of the current answers as of 2013-06-05 1:51 PM EST seem to imply), where does this misconception come from? Is there one of these explanations that is accurate and one that isn't, or are there different circumstances that could cause a LINQ query to evaluate differently?
Edit: In addition to the accepted answer below, I've turned up the following question over on Programmers that very much helped my understanding of query execution, particularly the the pitfalls that could result in multiple datasource hits during a loop, which I think will be helpful for others interested in this question: https://softwareengineering.stackexchange.com/questions/178218/for-vs-foreach-vs-linq
In general LINQ uses deferred execution. If you use methods like First() and FirstOrDefault() the query is executed immediately. When you do something like;
foreach(string s in MyObjects.Select(x => x.AStringProp))
The results are retrieved in a streaming manner, meaning one by one. Each time the iterator calls MoveNext the projection is applied to the next object. If you were to have a Where it would first apply the filter, then the projection.
If you do something like;
List<string> names = People.Select(x => x.Name).ToList();
foreach (string name in names)
Then I believe this is a wasteful operation. ToList() will force the query to be executed, enumerating the People list and applying the x => x.Name projection. Afterwards you will enumerate the list again. So unless you have a good reason to have the data in a list (rather than IEnumerale) you're just wasting CPU cycles.
Generally speaking using a LINQ query on the collection you're enumerating with a foreach will not have worse performance than any other similar and practical options.
Also it's worth noting that people implementing LINQ providers are encouraged to make the common methods work as they do in the Microsoft provided providers but they're not required to. If I were to go write a LINQ to HTML or LINQ to My Proprietary Data Format provider there would be no guarantee that it behaves in this manner. Perhaps the nature of the data would make immediate execution the only practical option.
Also, final edit; if you're interested in this Jon Skeet's C# In Depth is very informative and a great read. My answer summarizes a few pages of the book (hopefully with reasonable accuracy) but if you want more details on how LINQ works under the covers, it's a good place to look.
try this on LinqPad
void Main()
{
var testList = Enumerable.Range(1,10);
var query = testList.Where(x =>
{
Console.WriteLine(string.Format("Doing where on {0}", x));
return x % 2 == 0;
});
Console.WriteLine("First foreach starting");
foreach(var i in query)
{
Console.WriteLine(string.Format("Foreached where on {0}", i));
}
Console.WriteLine("First foreach ending");
Console.WriteLine("Second foreach starting");
foreach(var i in query)
{
Console.WriteLine(string.Format("Foreached where on {0} for the second time.", i));
}
Console.WriteLine("Second foreach ending");
}
Each time the where delegate is being run we shall see a console output, hence we can see the Linq query being run each time. Now by looking at the console output we see the second foreach loop still causes the "Doing where on" to print, thus showing that the second usage of foreach does in fact cause the where clause to run again...potentially causing a slow down.
First foreach starting
Doing where on 1
Doing where on 2
Foreached where on 2
Doing where on 3
Doing where on 4
Foreached where on 4
Doing where on 5
Doing where on 6
Foreached where on 6
Doing where on 7
Doing where on 8
Foreached where on 8
Doing where on 9
Doing where on 10
Foreached where on 10
First foreach ending
Second foreach starting
Doing where on 1
Doing where on 2
Foreached where on 2 for the second time.
Doing where on 3
Doing where on 4
Foreached where on 4 for the second time.
Doing where on 5
Doing where on 6
Foreached where on 6 for the second time.
Doing where on 7
Doing where on 8
Foreached where on 8 for the second time.
Doing where on 9
Doing where on 10
Foreached where on 10 for the second time.
Second foreach ending
It depends on how the Linq query is being used.
var q = {some linq query here}
while (true)
{
foreach(var item in q)
{
...
}
}
The code above will execute the Linq query multiple times. Not because of the foreach, but because the foreach is inside another loop, so the foreach itself is being executed multiple times.
If all consumers of a linq query use it "carefully" and avoid dumb mistakes such as the nested loops above, then a linq query should not be executed multiple times needlessly.
There are occasions when reducing a linq query to an in-memory result set using ToList() are warranted, but in my opinion ToList() is used far, far too often. ToList() almost always becomes a poison pill whenever large data is involved, because it forces the entire result set (potentially millions of rows) to be pulled into memory and cached, even if the outermost consumer/enumerator only needs 10 rows. Avoid ToList() unless you have a very specific justification and you know your data will never be large.
Sometimes it might be a good idea to "cache" a LINQ query using ToList() or ToArray(), if the query is being accessed multiple times in your code.
But keep in mind that "caching" it still calls a foreach in turn.
So the basic rule for me is:
if a query is simply used in one foreach (and thats it) - then I don't cache the query
if a query is used in a foreach and in some other places in the code - then I cache it in a var using ToList/ToArray
foreach, by itself, only runs through its data once. In fact, it specifically runs through it once. You can't look ahead or back, or alter the index the way you can with a for loop.
However, if you have multiple foreachs in your code, all operating on the same LINQ query, you may get the query executed multiple times. This is entirely dependent on the data, though. If you're iterating over an LINQ-based IEnumerable/IQueryable that represents a database query, it will run that query each time. If you're iterating over an List or other collection of objets, it will run through the list each time, but won't hit your database repeatedly.
In other words, this is a property of LINQ, not a property of foreach.
The difference is in the underlying type. As LINQ is built on top of IEnumerable (or IQueryable) the same LINQ operator may have completely different performance characteristics.
A List will always be quick to respond, but it takes an upfront effort to build a list.
An iterator is also IEnumerable and may employ any algorithm every time it fetches the "next" item. This will be faster if you don't actually need to go through the complete set of items.
You can turn any IEnumerable into a list by calling ToList() on it and storing the resulting list in a local variable. This is advisable if
You don't depend on deferred execution.
You have to access more total items than the whole set.
You can pay the upfront cost of retrieving and storing all items.
Using LINQ even without entities what you will get is that deferred execution is in effect.
It is only by forcing an iteration that the actual linq expression is evaluated.
In that sense each time you use the linq expression it is going to be evaluated.
Now with entities this is still the same, but there is just more functionality at work here.
When the entity framework sees the expression for the first time, it looks if he has executed this query already. If not, it will go to the database and fetch the data, setup its internal memory model and return the data to you. If the entity framework sees it already fetched the data beforehand, it is not going to go to the database and use the memory model that it setup earlier to return data to you.
This can make your life easier, but it can also be a pain. For instance if you request all records from a table by using a linq expression. The entity framework will load all data from the table. If later on you evaluate the same linq expression, even if in the time being records were deleted or added, you will get the same result.
The entity framework is a complicated thing. There are of course ways to make it reexecute the query, taking into account the changes it has in its own memory model and the like.
I suggest reading "programming entity framework" of Julia Lerman. It addresses lots of issues like the one you having right now.
It will execute the LINQ statement the same number of times no matter if you do .ToList() or not. I have an example here with colored output to the console:
What happens in the code (see code at the bottom):
Create a list of 100 ints (0-99).
Create a LINQ statement that prints every int from the list followed by two * to the console in red color, and then return the int if it's an even number.
Do a foreach on the query, printing out every even number in green color.
Do a foreach on the query.ToList(), printing out every even number in green color.
As you can see in the output below, the number of ints written to the console is the same, meaning the LINQ statement is executed the same number of times.
The difference is in when the statement is executed. As you can see, when you do a foreach on the query (that you have not invoked .ToList() on), the list and the IEnumerable object, returned from the LINQ statement, are enumerated at the same time.
When you cache the list first, they are enumerated separately, but still the same amount of times.
The difference is very important to understand, because if the list is modified after you have defined your LINQ statement, the LINQ statement will operate on the modified list when it is executed (e.g. by .ToList()). BUT if you force execution of the LINQ statement (.ToList()) and then modify the list afterwards, the LINQ statement will NOT work on the modified list.
Here's the output:
Here's my code:
// Main method:
static void Main(string[] args)
{
IEnumerable<int> ints = Enumerable.Range(0, 100);
var query = ints.Where(x =>
{
Console.ForegroundColor = ConsoleColor.Red;
Console.Write($"{x}**, ");
return x % 2 == 0;
});
DoForeach(query, "query");
DoForeach(query, "query.ToList()");
Console.ForegroundColor = ConsoleColor.White;
}
// DoForeach method:
private static void DoForeach(IEnumerable<int> collection, string collectionName)
{
Console.ForegroundColor = ConsoleColor.Yellow;
Console.WriteLine("\n--- {0} FOREACH BEGIN: ---", collectionName);
if (collectionName.Contains("query.ToList()"))
collection = collection.ToList();
foreach (var item in collection)
{
Console.ForegroundColor = ConsoleColor.Green;
Console.Write($"{item}, ");
}
Console.ForegroundColor = ConsoleColor.Yellow;
Console.WriteLine("\n--- {0} FOREACH END ---", collectionName);
}
Note about execution time: I did a few timing tests (not enough to post it here though) and I didn't find any consistency in either method being faster than the other (including the execution of .ToList() in the timing). On larger collections, caching the collection first and then iterating it seemed a bit faster, but there was no definitive conclusion from my test.
I am trying to create a method using LINQ that would take X ammount of products fron the DB, so I am using the .TAKE method for that.
The thing is, in situations I need to take all the products, so is there a wildcard I can give to .TAKE or some other method that would bring me all the products in the DB?
Also, what happens if I do a .TAKE (50) and there are only 10 products in the DB?
My code looks something like :
var ratingsToPick = context.RatingAndProducts
.ToList()
.OrderByDescending(c => c.WeightedRating)
.Take(pAmmount);
You could separate it to a separate call based on your flag:
IEnumerable<RatingAndProducts> ratingsToPick = context.RatingAndProducts
.OrderByDescending(c => c.WeightedRating);
if (!takeAll)
ratingsToPick = ratingsToPick.Take(pAmmount);
var results = ratingsToPick.ToList();
If you don't include the Take, then it will simply take everything.
Note that you may need to type your original query as IEnumerable<MyType> as OrderByDescending returns an IOrderedEnumerable and won't be reassignable from the Take call. (or you can simply work around this as appropriate based on your actual code)
Also, as #Rene147 pointed out, you should move your ToList to the end otherwise it will retrieve all items from the database every time and the OrderByDescending and Take are then actually operating on a List<> of objects in memory not performing it as a database query which I assume is unintended.
Regarding your second question if you perform a Take(50) but only 10 entries are available. That might depend on your database provider, but in my experience, they tend to be smart enough to not throw exceptions and will simply give you whatever number of items are available. (I would suggest you perform a quick test to make sure for your specific case)
Your current solution always takes all products from database. Because you are calling ToList(). After loading all products from database you are taking first N in memory. In order to conditionally load first N products, you need to build query
int? countToTake = 50;
var ratingsToPick = context.RatingAndProducts
.OrderByDescending(c => c.WeightedRating);
// conditionally take only first results
if (countToTake.HasValue)
ratingsToPick = ratingsToPick.Take(countToTake.Value);
var result = ratingsToPick.ToList(); // execute query
Trying to refactor some code that has gotten really slow recently and I came across a code block that is taking 5+ seconds to execute.
The code consists of 2 statements:
IEnumerable<int> StudentIds = _entities.Filters
.Where(x => x.TeacherId == Profile.TeacherId.Value && x.StudentId != null)
.Select(x => x.StudentId)
.Distinct<int>();
and
_entities.StudentClassrooms
.Include("ClassroomTerm.Classroom.School.District")
.Include("ClassroomTerm.Teacher.Profile")
.Include("Student")
.Where(x => StudentIds.Contains(x.StudentId)
&& x.ClassroomTerm.IsActive
&& x.ClassroomTerm.Classroom.IsActive
&& x.ClassroomTerm.Classroom.School.IsActive
&& x.ClassroomTerm.Classroom.School.District.IsActive).AsQueryable<StudentClassroom>();
So it's a bit messy but first I get a Distinct list of Id's from one Table (Filters), then I query another Table using it.
These are relatively small tables, but it's still 5+ seconds of query time.
I put this in LINQPad and it showed that it was doing the bottom query first then running 1000 "distinct" queries afterwards.
On a whim I changed the "StudentIds" code by just adding .ToArray() at the end. This improved the speed 1000x ... it now takes like 100ms to complete the same query.
What's the deal? What am I doing wrong?
This is one of the pitfalls of deferred execution in Linq: In your first approach StudentIds is really an IQueryable, not an in-memory collection. That means using it in the second query will run the query again on the database - each and every time.
Forcing execution of the first query by using ToArray() makes StudentIds an in-memory collection and the Contains part in your second query will run over this collection that contains a fixed sequence of items - This gets mapped to something equivalent to a SQL where StudentId in (1,2,3,4) query.
This query will of course, be much much faster since you determined this sequence once up-front, and not every time the Where clause is executed. Your second query without using ToArray() (I would think) would be mapped to a SQL query with an where exists (...) sub-query that gets evaluated for each row.
ToArray() Materializes the initial query to the server memory.
My guess would be the query provider is not able to parse the expression StudentIds.Contains(x.StudentId). Hence it probably thinks that the studentIds is an array already loaded to memory. So it's probably querying the database over and over again during the parsing phase. The only way to know for sure is to setup the profiler.
If you need to do this on the db server, use a join, instead of "contains". If you need to use contains to do what looks like a join problem, you are likely to be missing a surrogate primary key or a foreign key somewhere.
You could also declare studentIds as IQueryable instead of IEnumerable. This might give the query provider the hint it needs to interpret the studentIds as expression aka. data not already loaded to memory. I somehow doubt this but worth a try.
If all else fails, use ToArray(). This will load the initial studentIds to memory.
Say I have IQueryable<Car> cars.
I iterate it such as:
foreach (Car c in cars)
{
c.Doors = 2;
}
Why does c.Doors contain the original value after the foreach, instead of the changed value?
Thank you in advance
You indicated that IQueryable is the result of a linq to entities query which is not a correct statement. MyDatabaseContext.Cars.Where(x => x.Name == "Test") returns a IQueryable which on iteration perfoms a query on the database. (Iteration is when you perform a foreach over it). So it doesnt contain the result set yet, just the query.
looping twice over cars generates 2 identical queries to the database and returns 2 identical result sets. If you want to preserve the data. you need to call ToArray or ToList or after manipulation, perform a save of the changes before you iterate again.
Iterating over an IQueryable retrieves the result set from the database as it goes, as I'm sure you know. What I've observed in this and other situations is that this result is not cached, so you'll often find that iterating over the IQueryable again will actually run the query again, thus your modifications aren't preserved because you're getting a new result set which was never affected by that code.
The reason it works when you call ToList() first and iterate over the result of that is because ToList() retrieves and materialises the entire result set, which is then no longer linked to the database content and is really just a copy of what the database returned.
The only way to be sure your changes will stick is to operate on the local copy of the data, i.e. lift it out of IQueryable land. This can be as simple as saving an IEnumerable of the result set (i.e. the result of ToEnumerable()), which will then return the same result set each time you enumerate it, but unlike ToList() it won't cause evaluation immediately.