I am currently trying to find different areas where Linq is not sufficient and FromSqlRaw or ExecuteSqlRaw have to be used.
Some examples I have found are
Bulk updates https://learn.microsoft.com/en-us/ef/core/performance/efficient-updating
Executing stored procedures https://learn.microsoft.com/en-us/ef/core/querying/raw-sql
However I am looking for more areas where Linq does not perform good enough and even queries that cannot be generated from Linq in EF Core when it comes to database access.
My goal is to find poor performing Linq translations and examine the cause.
This is a bit of a solution looking for a problem. Given an application and looking for inefficiencies that might benefit from a different approach is something I would start off using a profiler and observing the database access in as close to a production capacity as I am allowed to get.
EF is like any tool, it can be leveraged to create works of art, and it can be abused and misused to create shanties. Even when done correctly, optimizations like indexes are something that are tuned based on looking at real-world options. There are many options that I would look at to address performance issues before considering direct to SQL. Typical culprits that can be easily identified via profiling:
Lazy loading. (Dozens to hundreds of queries following up a "main" query.)
Over-use of eager loading. (Queries involving a heck of a lot of joins)
Sloppy use of client-side evaluation. (Either enabling that feature in EF Core, or slapping a ToList somewhere when a query complains to "fix" it, AsSplitQuery can help here, Projection is a better solution in most cases)
Lack of pagination where more data is returned than necessary. (Similar to #3, having methods like "GetAll" and then applying filtering, pagination, etc.)
Giving users too much flexibility in querying that they don't need 99% of the time, but in that 1% someone does try it, grinds the system to a halt. (Giving users filters/sorts on ALL columns and performing things like string.Contains by default for text searches)
Giving users access to expensive, but necessary queries in real-time. (Big, justified queries, but being run against the production dataset and not "throttled" by something like a Queue to ensure too many of these monsters don't get run at once.)
Those are some of the top culprits that come to mind around performance, and none of them resort to going to SQL. Batch processing in your list is certainly one case that I believe does deserve looking outside of Linq, and potentially outside of EF all-together. Stored Procs I am mixed on. If there is business logic that is shared between an EF-supported application and another existing system and I want to share that business logic as-is. The trouble is that if I'm relying on the Sproc for business rules then there's little point to EF, and if I'm splitting business rules between C#/EF and Sprocs, then that's having to manage logic in two locations.
I recently upgraded EF 6.1.3 to 6.2.0 on one of our large projects, and it has broken a significant amount of our LINQ queries. Enabling MultipleActiveResultSets causes everything to work as normal again, but I'm struggling to understand the change. We have been using EF for years and gone through multiple major version changes without any issue. If I simply revert back to 6.1.3, everything works again as expected - in fact everything works even if I explicitly disable MARS in 6.1.3.
Let me give a few simplified examples. The first problem is with nested queries:
foreach(var row in dbSet.Where(<condition>))
foreach(var innerRow in otherDbSet.Where(_ => _.Property == row.Property))
This works fine in 6.1.3, but in 6.2.0 throws a "There is already an open DataReader..." exception. I understand the nature of the exception, and I can solve this by calling ToList() on the outer query to push the results into memory first - what I don't understand is why I didn't have to do this in 6.1.3 (even with MARS disabled). It isn't always desirable to simply load the whole outer set into memory.
This also seems to impact lazy-loaded properties. For example, we build ComboBoxes from simple queries like this:
return db.Collection
.Where(<condition>)
.AsEnumerable()
.Select(_ => new ListItem(_.Id, _.LazyNavigationProperty.Description))
.ToList();
This works fine in 6.1.3, but again in 6.2.0 throws the "There is already an open DataReader..." exception. The fix is I now have to eager-load the navigation property.
Ultimately I don't have an explicit question, I'm just trying to understand why a minor version update seemingly caused major breaking changes in how queries are handled.
Moving forward, this impacts far too many queries for us to refactor. When I was researching the problem, I saw vague warnings about enabling MARS, but nobody really gave anything concrete. Is there a compelling reason not to enable it?
you get this error because you're iterating through a result set while trying to open another result set (while the first one did not finish yet)-> sort of lazy loading (the first 'for each' iteration in your case) -> there are a lot of ways to solve this as you've already seen for yourself: using toList (drop to memory first), because it's no longer using the datareader to open the set.
it looks like it MIGHT be related to a bug fix in 6.2 (release notes: https://entityframework.net/ef-version-history) - looks like related to: "Bug: Retrying queries or SQL commands fails with "The SqlParameter is already contained by another SqlParameterCollection.")
Regarding enabling MARS:
you can find special warning here:
https://learn.microsoft.com/en-us/dotnet/framework/data/adonet/sql/enabling-multiple-active-result-sets
Entity Framework is supposed to deliver a tiny abstraction on your database model.
Such work requires performing multiple queries under the hood. The engine might also require more queries necessary when compared to the same workload encoded by hand.
This is a physiological evolution in order to be able to handle all possible user requests. Simply upgrading to a different Entity Framework version, can introduce differences on the database workload emitted under the hood.
MARS is required as EF changed the way object retrieval is performed (particularly, within loops combined with lazy loading). Unfortunately, most of the times, you are required to use MARS when using Entity Framework.
Nowadays, using async/await usually requires MARS too.
You can find additional information about how related entities are loaded on MSDN Loading Related Objects and Enabling Multiple Active Result Sets. This interesting blog goes a little more deeper.
I've got Entity Framework 4.1 with .NET 4.5 running on ASP.NET in Windows 2008R2. I'm using EF code-first to connect to SQL Server 2008R2, and executing a fairly complex LINQ query, but resulting in just a Count().
I've reproduced the problem on two different web servers but only one database (production of course). It recently started happening with no application, database structure, or server changes on the web or database side.
My problem is that executing the query under certain circumstances takes a ridiculous amount of time (close to 4 minutes). I can take the actual query, pulled from SQL Profiler, and execute in SSMS in about 1 second. This is consistent and reproducible for me, but if I change the value of one of the parameters (a "Date after 2015-01-22" parameter) to something earlier, like 2015-01-01, or later like 2015-02-01, it works fine in EF. But I put it back to 2015-01-22 and it's slow again. I can repeat this over and over again.
I can then run a similar but unrelated query in EF, then come back to the original, and it runs fine this time - same exact query as before. But if I open a new browser, the cycle starts over again. That part also makes no sense - we're not doing anything to retain the data context in a user session, so I have no clue whatsoever why that comes into play.
But this all tells me that the data itself is fine.
In Profiler, when the query runs properly, it takes about a second or two, and shows about 2,000,000 in reads and about 2,000 in CPU. When it runs slowly, it takes 3.5 minutes, and the values are 300,000,000 and 200,000 - so reads are about 150 times higher and CPU is 100 times higher. Again, for the identical SQL statement.
Any suggestions on what EF might be doing differently that wouldn't show up in the query text? Is there some kind of hidden connection property which might cause a different execution plan in certain circumstances?
EDIT
The query that EF builds is one of the ones where it builds a giant string with the parameter included in the text, not as a SQL parameter:
exec sp_executesql
N'SELECT [GroupBy1].[A1] AS [C1]
FROM (
SELECT COUNT(1) AS [A1]
...
AND ([Extent1].[Added_Time] >= convert(datetime2, ''2015-01-22 00:00:00.0000000'', 121))
...
) AS [GroupBy1]'
EDIT
I'm not adding this as an answer since it doesn't actually address the underlying issue, but this did end up getting resolved by rebuilding indexes and recomputing statistics. That hadn't been done in longer than usual, and it seems to have cleared up whatever caused the issue.
I'll keep reading up on some of the links here in case this happens again, but since it's all working now and unreproduceable, I don't know if I'll ever know for sure exactly what it was doing.
Thanks for all the ideas.
I recently had a very similar scenario, a query would run very fast executing it directly in the database, but had terrible performance using EF (version 5, in my case). It was not a network issue, the difference was from 4ms to 10 minutes.
The problem ended up being a mapping problem. I had a column mapped to NVARCHAR, while it was VARCHAR in the database. Seems inoffensive, but that resulted in an implicit conversion in the database, which totally ruined the performance.
I'm not entirely sure on why this happens, but from the tests I made, this resulted in the database doing an Index Scan instead of an Index Seek, and apparently they are very different performance-wise.
I blogged about this here (disclaimer: it is in Portuguese), but later I found that Jimmy Bogard described this exact problem in a post from 2012, I suggest you check it out.
Since you do have a convert in your query, I would say start from there. Double check all your column mappings and check for differences between your table's column and your entity's property. Avoid having implicit conversions in your query.
If you can, check your execution plan to find any inconsistencies, be aware of the yellow warning triangle that may indicate problems like this one about doing implicit conversion:
I hope this helps you somehow, it was a really difficult problem for us to find out, but made sense in the end.
Just to put this out there since it has not been addressed as a possibility:
Given that you are using Entity Framework (EF), if you are using Lazy Loading of entities, then EF requires Multiple Active Result Sets (MARS) to be enabled via the connection string. While it might seem entirely unrelated, MARS does sometimes produce this exact behavior of something running quickly in SSMS but horribly slow (seconds become several minutes) via EF.
One way to test this is to turn off Lazy Loading and either remove MultipleActiveResultSets=True; (the default is "false") or at least change it to be MultipleActiveResultSets=False;.
As far as I know, there is unfortunately no work-around or fix (currently) for this behavior.
Here is an instance of this issue: Same query with the same query plan takes ~10x longer when executed from ADO.NET vs. SMSS
There is an excellent article about Entity Framework performance consideration here.
I would like to draw your attention to the section on Cold vs. Warm Query Execution:
The very first time any query is made against a given model, the
Entity Framework does a lot of work behind the scenes to load and
validate the model. We frequently refer to this first query as a
"cold" query. Further queries against an already loaded model are
known as "warm" queries, and are much faster.
During LINQ query execution, the step "Metadata loading" has a high impact on performance for Cold query execution. However, once loaded metadata will be cached and future queries will run much faster. The metadata are cached outside of the DbContext and will be re-usable as long as the application pool lives.
In order to improve performance, consider the following actions:
use pre-generated views
use query plan caching
use no tracking queries (only if accessing for read-only)
create a native image of Entity Framework (only relevant if using EF 6 or later)
All those points are well documented in the link provided above. In addition, you can find additional information about creating a native image of Entity Framework here.
I don't have an specific answer as to WHY this is happening, but it certainly looks to be related with how the query is handled more than the query itself. If you say that you don't have any issues running the same generated query from SSMS, then it isn't the problem.
A workaround you can try: A stored procedure. EF can handle them very well, and it is the ideal way to deal with potentially complicated or expensive queries.
Realizing you are using Entity Framework 4.1, I would suggest you upgrade to Entity Framework 6.
There has been a lot of performance improvement and EF 6 is much faster than EF 4.1.
The MSDN article about Entity Framework performance consideration mentioned in my other response has also a comparison between EF 4.1 and EF 6.
There might be a bit of refactoring needed as a result, but the improvement in performance should be worth it (and that would reduce the technical debt at the same time).
I saw similar questions around the same topic, but i couldn't solve my issue.
I have asp.net web application with DB2 backend. And we have the entity framework model 3.5
So when i load the page for the very first time, it takes close to 15 seconds for executing the first query. And the query is very simple, selecting a row from one table and the where clauses are indexed. This is the query
protected Detail getProgramDetail(string id1,string id2, string id3)
{
Detail result = (from d in context.Detail
where d.id1.equals(id1) &&
d.id2.equals(id2) &&
d.id3.equals(id3)
select d).FirstorDefault();
return result;
}
I tried updating the statistics too, but it didnt help either.
After reading other performance tuning articles, i made this query as a compiled one, but still its taking close 15 seconds. But the subsequent calls are pretty fast in milliseconds. I think its taking time to establish the connection and run the query.
Is there a way to improve the initial performance?. I tried to generate views. But i am receiving the below error. Not sure how to handle this one.
The specified store provider cannot be found in the configuration, or
is not valid
My connection string is in machine.config.
Thanks in advance.
Srividhya
Is there a way to improve the initial performance?.
What about it is not ASp.NET / Entity Framework but IIS - STARTING THE APPLICATION, including doing the compilation needed ;)?
Make it hot - there is an extension that can call a page when IIS starts, so the applicatin is hot (compiled, loaded in memory).
http://weblogs.asp.net/scottgu/archive/2009/09/15/auto-start-asp-net-applications-vs-2010-and-net-4-0-series.aspx
There are multiple options which you look forward for solving this issue
1) Add the second level caching with the Entity framework .This article is very helpful
http://msdn.microsoft.com/hi-in/magazine/hh394143(en-us).aspx
2) Querying and ORM dont stick together very well ,Can you use a stored procedure and Datareader which will definitely optimize performance.
3)If you can use Optimize the first or default in your code
I am using Entity Framework to look up (and save) an entity in my SQL Server 2008 R2 database. My problem is with a simple ObjectContext.FirstOrDefault call (though it is abstracted via an IRepository pattern).
I am noticing really really poor performance. So attached a profiler and found that the first query I run is where most of the slow downs are.
So the first thing I thought was that I have a bad index. But running a lookup in SSMS is nearly instantaneous. (That is not the problem.) Also tried switching the first query I call and the performance hit stayed mostly with the first query.
There are two methods that EF is calling that take a high percentage of my run time. They are GetExecutionPlan and EnsureConnection.
Are these just overhead that I have to deal with if I want to use EF? Or is there a way to optimize these calls?
One thing I thought of is re-using my Entity Framework ObjectContext. I think if I did that then some of the slow downs would be overcome by caching. However, I have read bad things about reusing the ObjectContext (which is why I was making a new one with each of my service calls).