This question already has an answer here:
In explicit LINQ-to-SQL (C#) does order matter?
(1 answer)
Closed 8 years ago.
I have this linq Query in a C# project
var query = from p in db.Posts
join a in db.Author on p.Author equals a
join u in db.Users on p.PostedBy equals u
where p.IsActive.Equals(true)
orderby p.PostedDate descending
select new ViewModel
{
...
};
If I move where p.IsActive.Equals(true) near from p in db.Posts, like
var query = from p in db.Posts
where p.IsActive.Equals(true) //Moved
join a in db.Author on p.Author equals a
join u in db.Users on p.PostedBy equals u
orderby p.PostedDate descending
select new ViewModel
{
...
};
will it make any difference to the performance of the query?
I'm not sure that it will work if you'll do it, but if you are using it against a SQL database it should not make any difference. Both queries will be translated to the SQL like this one:
SELECT ...
FROM Posts INNER JOIN Author ON ... INNER JOIN Users ON ...
WHERE Posts.IsActive = true
...
Concerning performance, I would strongly suggest to put
the joins before any where.
The reason is, that the joins already make a smaller set
(the p is selected from Posts and then only
the matching rows from Author and Users are taken.
In opposite, if you have a where near the beginning
a caresian (big) product is calculated and then filtered.
(in your special case, only one table is affected, but
the query may be altered sometime and nobody knows then
why it is slow).
Please read this SO Why is LINQ join so much faster than LINQ where
And more on your special case: as this is affecting a database,
the resulting SQL statements should be the same in both queries.
Have a look at it!
Related
We are writing entity lambda expression query like this. But when we checked in profile. There were almost all the tables which were used in join returning entire table to the .net linq queries.
We have few transaction tables which has thousands of records. which is causing performance issue.
Please let us know if we can avoid table returning entire rows to .net
var result = (from f in f
join a in this.Context.a on f.primeryKey equals a.primeryKey
join d in this.Context.d on f.secondid equals d.secondid
join t in this.Context.t on d.thirdId equals t.thirdId
where t.isfoo && pfIds.Contains(a.fourthId.HasValue ? a.fourthId.Value : -1)
select f).Distinct().ToList();
Well, no real answer, for that I don't have enough info, but a few remarks to improve your query.
First remark: Don't do Contains and HasValue, because Linq won't SQL-ize these operations. I'm also not quite sure about the this.Context. stuff.
Second: NULL won't join in smart joins.
Third: Instead of selecting f, you'd typically select only a few fields of f that you need.
You'll need to rewrite your query. EF really needs to get all lines to utilize operator ? in order to evaluate value in a.fourthId column. I believe that
var result = (from f in f
join a in this.Context.a on f.primeryKey equals a.primeryKey
join d in this.Context.d on f.secondid equals d.secondid
join t in this.Context.t on d.thirdId equals t.thirdId
where t.isfoo && pfIds.Contains(a.fourthId)
select f).Distinct().ToList();
would meet your needs without necessary overhead, that evaluation seems to be superfluous.
I have two classes: Property and PropertyValue. A property has several values where each value is a new revision.
When retrieving a set of properties I want to include the latest revision of the value for each property.
in T-SQL this can very efficiently be done like this:
SELECT
p.Id,
pv1.StringValue,
pv1.Revision
FROM dbo.PropertyValues pv1
LEFT JOIN dbo.PropertyValues pv2 ON pv1.Property_Id = pv2.Property_Id AND pv1.Revision < pv2.Revision
JOIN dbo.Properties p ON p.Id = pv1.Property_Id
WHERE pv2.Id IS NULL
ORDER BY p.Id
The "magic" in this query is to join on the lesser than condition and look for rows without a result forced by the LEFT JOIN.
How can I accomplish something similar using LINQ to EF?
The best thing I could come up with was:
from pv in context.PropertyValues
group pv by pv.Property into g
select g.OrderByDescending(p => p.Revision).FirstOrDefault()
It does produce the correct result but is about 10 times slower than the other.
Maybe this can help. Where db is the database context:
(
from pv1 in db.PropertyValues
from pv2 in db.PropertyValues.Where(a=>a.Property_Id==pv1.Property_Id && pv1.Revision<pv2.Revision).DefaultIfEmpty()
join p in db.Properties
on pv1.Property_Id equals p.Id
where pv2.Id==null
orderby p.Id
select new
{
p.Id,
pv1.StringValue,
pv1.Revision
}
);
Next to optimizing a query in Linq To Entities, you also have to be aware of the work it takes for the Entity Framework to translate your query to SQL and then map the results back to your objects.
Comparing a Linq To Entities query directly to a SQL query will always result in lower performance because the Entity Framework does a lot more work for you.
So it's also important to look at optimizing the steps the Entity Framework takes.
Things that could help:
Precompile your query
Pre-generate views
Decide for yourself when to open the database connection
Disable tracking (if appropriate)
Here you can find some documentation with performance strategies.
if you want to use multiple conditions (less than expression) in join you can do this like
from pv1 in db.PropertyValues
join pv2 in db.PropertyValues on new{pv1.Property_ID, Condition = pv1.Revision < pv2.Revision} equals new {pv2.Property_ID , Condition = true} into temp
from t in temp.DefaultIfEmpty()
join p in db.Properties
on pv1.Property_Id equals p.Id
where t.Id==null
orderby p.Id
select new
{
p.Id,
pv1.StringValue,
pv1.Revision
}
I need compute some join operations on my data, on the result of these operations I need to use a WHERE statement.
At the moment I use this syntax below, no errors, but result is not as expected.
For sure I need Group the result for my join and apply a query on it.
Could you please provide me an example of code? Unfortunately I'm pretty new at Linq and I'm not able to do it.
var myImagesForUser = from i in context.CmsImagesContents
join c in context.CmsContents on i.ContentId equals c.ContentId
join a in context.CmsAuthors on c.AuthorId equals a.AuthorId
join u in context.aspnet_Users on a.UserId equals u.UserId
where u.UserId == (Guid)myLoggedInUser.ProviderUserKey
select i;
Probably related data for current logged in user is not present in any one or more tables (Authors, Contents, ImagesContents) and you are not getting any result due to inner join on these tables. you can try outer join instead. Look at this article for left outer join in Linq queries
Hmm. I see two things which I would do different.
1.) You forgot to call .ToList() at the end in order to execute the query. Otherwise it can be, that you misuse the LINQ Statement in the processing afterwords.
var myImagesForUser = (YOUR LINQ STATEMENT).ToList();
2.) It's strange that you need to convert the UserKey to GUID. If really needed I would do it prior the LINQ Statement and check against a local variable.
var userId = (Guid)myLoggedInUser.ProviderUserKey;
var myImagesForUser = (YOUR LINQ STATEMENT ... where u.UserId == userId ).ToList();
Perhaps this helps you to get the expected result.
where a.UserId == (Guid)myLoggedInUser.ProviderUserKey.
I want to achieve the following in Linq to Entities:
Get all Enquires that have no Application or the Application has a status != 4 (Completed)
select e.*
from Enquiry enq
left outer join Application app
on enq.enquiryid = app.enquiryid
where app.Status <> 4 or app.enquiryid is null
Has anyone done this before without using DefaultIfEmpty(), which is not supported by Linq to Entities?
I'm trying to add a filter to an IQueryable query like this:
IQueryable<Enquiry> query = Context.EnquirySet;
query = (from e in query
where e.Applications.DefaultIfEmpty()
.Where(app=>app.Status != 4).Count() >= 1
select e);
Thanks
Mark
In EF 4.0+, LEFT JOIN syntax is a little different and presents a crazy quirk:
var query = from c1 in db.Category
join c2 in db.Category on c1.CategoryID equals c2.ParentCategoryID
into ChildCategory
from cc in ChildCategory.DefaultIfEmpty()
select new CategoryObject
{
CategoryID = c1.CategoryID,
ChildName = cc.CategoryName
}
If you capture the execution of this query in SQL Server Profiler, you will see that it does indeed perform a LEFT OUTER JOIN. HOWEVER, if you have multiple LEFT JOIN ("Group Join") clauses in your Linq-to-Entity query, I have found that the self-join clause MAY actually execute as in INNER JOIN - EVEN IF THE ABOVE SYNTAX IS USED!
The resolution to that? As crazy and, according to MS, wrong as it sounds, I resolved this by changing the order of the join clauses. If the self-referencing LEFT JOIN clause was the 1st Linq Group Join, SQL Profiler reported an INNER JOIN. If the self-referencing LEFT JOIN clause was the LAST Linq Group Join, SQL Profiler reported an LEFT JOIN.
Do this:
IQueryable<Enquiry> query = Context.EnquirySet;
query = (from e in query
where (!e.Applications.Any())
|| e.Applications.Any(app => app.Status != 4)
select e);
I don't find LINQ's handling of the problem of what would be an "outer join" in SQL "goofy" at all. The key to understanding it is to think in terms of an object graph with nullable properties rather than a tabular result set.
Any() maps to EXISTS in SQL, so it's far more efficient than Count() in some cases.
Thanks guys for your help. I went for this option in the end but your solutions have helped broaden my knowledge.
IQueryable<Enquiry> query = Context.EnquirySet;
query = query.Except(from e in query
from a in e.Applications
where a.Status == 4
select e);
Because of Linq's goofy (read non-standard) way of handling outers, you have to use DefaultIfEmpty().
What you'll do is run your Linq-To-Entities query into two IEnumerables, then LEFT Join them using DefaultIfEmpty(). It may look something like:
IQueryable enq = Enquiry.Select();
IQueryable app = Application.Select();
var x = from e in enq
join a in app on e.enquiryid equals a.enquiryid
into ae
where e.Status != 4
from appEnq in ae.DefaultIfEmpty()
select e.*;
Just because you can't do it with Linq-To-Entities doesn't mean you can't do it with raw Linq.
(Note: before anyone downvotes me ... yes, I know there are more elegant ways to do this. I'm just trying to make it understandable. It's the concept that's important, right?)
Another thing to consider, if you directly reference any properties in your where clause from a left-joined group (using the into syntax) without checking for null, Entity Framework will still convert your LEFT JOIN into an INNER JOIN.
To avoid this, filter on the "from x in leftJoinedExtent" part of your query like so:
var y = from parent in thing
join child in subthing on parent.ID equals child.ParentID into childTemp
from childLJ in childTemp.Where(c => c.Visible == true).DefaultIfEmpty()
where parent.ID == 123
select new {
ParentID = parent.ID,
ChildID = childLJ.ID
};
ChildID in the anonymous type will be a nullable type and the query this generates will be a LEFT JOIN.
This question follows on from a question I asked yesterday about why using the join query on my Entities produced horrendously complicated SQL. It seemed that performing a query like this:
var query = from ev in genesisContext.Events
join pe in genesisContext.People_Event_Link
on ev equals pe.Event
where pe.P_ID == key
select ev;
Produced the horrible SQL that took 18 seconds to run on the database, whereas joining the entities through a where clause (sort of like pre-ANSI SQL syntax) took less than a second to run and produced the same result
var query = from pe in genesisContext.People_Event_Link
from ev in genesisContext.Events
where pe.P_ID == key && pe.Event == ev
select ev;
I've googled all over but still don't understand why the second is produces different SQL to the first. Can someone please explain the difference to me? When should I use the join keyword
This is the SQL that was produced when I used Join in my query and took 18 seconds to run:
SELECT
1 AS [C1],
[Extent1].[E_ID] AS [E_ID],
[Extent1].[E_START_DATE] AS [E_START_DATE],
[Extent1].[E_END_DATE] AS [E_END_DATE],
[Extent1].[E_COMMENTS] AS [E_COMMENTS],
[Extent1].[E_DATE_ADDED] AS [E_DATE_ADDED],
[Extent1].[E_RECORDED_BY] AS [E_RECORDED_BY],
[Extent1].[E_DATE_UPDATED] AS [E_DATE_UPDATED],
[Extent1].[E_UPDATED_BY] AS [E_UPDATED_BY],
[Extent1].[ET_ID] AS [ET_ID],
[Extent1].[L_ID] AS [L_ID]
FROM [dbo].[Events] AS [Extent1]
INNER JOIN [dbo].[People_Event_Link] AS [Extent2] ON EXISTS (SELECT
1 AS [C1]
FROM ( SELECT 1 AS X ) AS [SingleRowTable1]
LEFT OUTER JOIN (SELECT
[Extent3].[E_ID] AS [E_ID]
FROM [dbo].[Events] AS [Extent3]
WHERE [Extent2].[E_ID] = [Extent3].[E_ID] ) AS [Project1] ON 1 = 1
LEFT OUTER JOIN (SELECT
[Extent4].[E_ID] AS [E_ID]
FROM [dbo].[Events] AS [Extent4]
WHERE [Extent2].[E_ID] = [Extent4].[E_ID] ) AS [Project2] ON 1 = 1
WHERE ([Extent1].[E_ID] = [Project1].[E_ID]) OR (([Extent1].[E_ID] IS NULL) AND ([Project2].[E_ID] IS NULL))
)
WHERE [Extent2].[P_ID] = 291
This is the SQL that was produce using the ANSI Style syntax (and is fairly close to what I would write if I were writing the SQL myself):
SELECT * FROM Events AS E INNER JOIN People_Event_Link AS PE ON E.E_ID=PE.E_ID INNER JOIN PEOPLE AS P ON P.P_ID=PE.P_ID
WHERE P.P_ID = 291
Neither of the above queries are entirely "correct." In EF, it is generally correct to use the relationship properties in lieu of either of the above. For example, if you had a Person object with a one to many relationship to PhoneNumbers in a property called Person.PhoneNumbers, you could write:
var q = from p in Context.Person
from pn in p.PhoneNumbers
select pn;
The EF will build the join for you.
In terms of the question above, the reason the generated SQL is different is because the expression trees are different, even though they produce equivalent results. Expression trees are mapped to SQL, and you of course know that you can write different SQL which produces the same results but with different performance. The mapping is designed to produce decent SQL when you write a farily "conventional" EF query.
But the mapping is not so smart as to take a very unconventional query and optimize it. In your first query, you state that the objects must be equivalent. In the second, you state that the ID property must be equivalent. My sample query above says "just get the details for this one record." The EF is designed to work with the way I show, principally, but also handles scalar equivalence well.