To me, PetaPoco's Database.Fetch and Database.Query seem to be doing the same thing.
For example,
var db = new PetaPoco.Database("myDB");
ProductList products = db.Fetch<ProductList>("SELECT * FROM ProductList");
ProductList products = db.Query<ProductList>("SELECT * FROM ProductList");
Is there any significant difference between them?
According to the PetaPoco documentation, this is the answer:
Query vs Fetch
The Database class has two methods for retrieving records Query and Fetch. These are pretty much identical except Fetch returns a List<> of POCO's whereas Query uses yield return to iterate over the results without loading the whole set into memory.
Fetch and query behave differently if you use them inside a transaction. I had a use case where I needed to update several tables in one transaction but I needed to retrieve some data from a reference table in the middle of the sequence.
When I retrieved the data with Query, and subsequent Inserts or Updates failed with an InvalidOperationException: "There is already an open DataReader associated with this Command which must be closed first"
The solution was to replace the Query with a Fetch and I was able to complete the sequence.
The pseudocode for this is:
BeginTransaction
Update Tbl1
Update Tbl2
Query RefTblA ## Changed Query to Fetch to avoid: '...already an open DataReader..." exception
Update Tbl3 using data from RefTblA
CompleteTransaction
Related
I need data for a LINQ query that is not already saved to the database. Here is my code:
foreach (var item in BusProc)
{
var WorkEffortTypeXref = new WorkEffortTypeXref
{
SourceDataEntityTypeName = "BusProc",
SourceDataEntityTypeId = item.BusProcId,
};
_clDBContext.WorkEffortTypeXref.AddRange(WorkEffortTypeXref);
}
but I need this data in the SQL Database before I do a join LINQ query on the data. Although I don't really want to do a OnSave() after this function because I want to make the whole process transactional.
This is the LINQ I need to execute. What is the best way to do this?
var linqquery = from bupr in BusProc
join wrtx in WorkEffortTypeXref on bupr.BusProcId equals wrtx.SourceDataEntityTypeId
// where wrtx.SourceDataEntityTypeName == "BusProc"
select new
{
wrtx.TargetDataEntityTypeId
};
First, try to compile your code. This code won't compile and as-is isn't comprehensible. You are treating WorkEffortTypeXref as a class, a singular object and a list all in the first section of code, so we really can't know what it is supposed to be.
Now, as I understand your question, you want to query information that is being added to a table, (currently stored in memory as a collection of some sort) but you want to query it before it is added? What if the table has other rows that match your query? What if the insert violates a constraint and therefore fails? Linq can query an in-memory collection, but you have to choose, are you querying the collection that you have in memory (and isn't yet a row of your table) or the database (which has all of the rules/contstraints/etc that databases provide)? Until the records are saved to your table, they aren't the same thing.
I have some database ang now it contains a table with about 100 rows. But in future it will have not 100 but 1 000 000+ rows and I have to be careful with my web application I'm developing now.
Problem is next: at web page I need to create paged list what will show records to user. And here is a sample of code that I plan to use
public IQueryable<MyTable> GetRows(int from, int to)
{
var queryRes = (from row in SomeDataContext.MyTable
order by row.id
select row).AsQueriable();
return queryRes.Take(to).Skip(from);
}
It is only sample of code. I did not run it.
But question is what will go on in this case? I see tow scenarios
It will load all rows from database and at server side and records in range from 'from' to 'to' will be returned. Other will be ignored. In this case my application will have big troubles. Imagine load 1 000 000 rows from database every time. It will be disaster.
It will construct SQL request what will return only rows I need without loading others. That's exactly what I need.
I think that it will be 2 scenario but I'm not sure and can't check it. Am I correct?
As a side-note, you don't have to call AsQueryable. It is enough to do
var queryRes = SomeDataContext.MyTable.OrderBy(r => r.Id);
return queryRes.Take(to).Skip(from);
And to answer your question - scenario 2 will be executed. You can always check the generated SQL by using the SQL Server Profiler, but in case you are using Entity Framework, you can even do queryRes.ToString(). And as #Aron correctly pointed out - the query will be actually executed against the database only when enumerating the results (e.g. calling queryRes.ToList()).
These questions address the issue of looking up the SQL code in more detail:
How to view generated SQL from Entity Framework?
exact sql query executed by Entity Framework
Strictly speaking, neither 1 nor 2 is correct. Running the code DOES NOT hit the database. It constructs an expression tree. The calling code can still modify the expression tree further without hitting the database.
With the IQueryable interface no SQL is run. It is at the point when you call IEnumerable.GetEnumerator() that the underlying Linq Provider converts the WHOLE expression into a query. In this case a SQL query, and then run it.
So for example, with this code. You could have
void Main()
{
var foo = from x in GetRows(10, 10)
where x.Id > 1000
select x;
foreach(var f in foo)
{
//Stuff
}
}
The sql that is actually run will actually be closer to
SELECT a,b,c FROM
(SELECT a,b,c, ROW_NUMBER() OVER (ORDER BY ...) as row_number
FROM Table
WHERE id > 1000) t0
WHERE to.row_number BETWEEN 10 and 20;
To be honest you are going about this wrong. You don't need a GetRows method. I would directly call the Linq query when constructing the table itself. You should take a look at the IRepository pattern that MVC scaffolding uses.
Finally if this is meant to be called as a WebQuery for AJAX I would look at the two OData implementations in .net (WCF Data Services and WebAPI OData).
You are right.
The 2. scenario is what will happen. When the query is eventuallty exectuted.
I Would sugges to reverse the Take - Skip, so you start by Skip
queryRes.Skip(from).Take(to)
Debuggen this method will not make any calls to the database. It just returns the query - not the resualt.
If you want to test exactly what will happen, try download LinqPad - it is a great to for demystifying linq queries.
I have a table of 200,000 record where I am getting only the top 10 using .Take() but it is taking about 10 seconds to get the data.
My question is: does the .Take() method get all the data from the database and filter the top 10 on the client side?
Here is my code:
mylist = (from mytable in db.spdata().OrderByDescending(f => f.Weight)
group feed by mytable.id into g
select g.FirstOrDefault()).Take(10).ToList();
spdata() is a function Import from stored procedure.
Thanks
The stored procedure probably returns a lot of data to the client which is very slow. You cannot remote a query to an sproc. That would be possible using a view or a table-valued function.
There's no way to use an sproc in a query. You can only execute it by itself.
Your intention probably was to execute the Take(10) on the server. For that to work you need to switch to an inline query, a view or a TVF.
The extension method Take does not fetch all the results from the database. That is not how Takeworks.
However your db.spdata() call probably does fetch all rows.
I'm not 100% sure but as I remember you get an IEnumerable result when you call an SP using EF DataContext...
There are a couple of was to optimize the performance:
Pass the search criteria s as SP params and do the filtering in the stored procedure.
Or if you have a quite simple query in the SP where you are not declaring any variables and where you are just joining some tables then:
Create an indexed view where specify the query that you need and call the Take method on it.
What this will give you? You can map to the created view and EF will now be returning an IQueryable result and not an IEnumerable. This will optimize the sql command and rather the receiving all of the data and then taking the 10 elements that you need, a sql command that just retrieves the 10 elements will be formed.
I also advice you to see what is the deference between IEnumerable vs IQueryable.
It does, because you are sorting the data before grouping, which is not possible to do in SQL.
You should use an aggregate to get the highest weight from each group, then sort the weights to get the ten largest:
mylist = (
from mytable in db.spdata()
group feed by mytable.id into g
select g.Max(f => f.Weight)
).OrderByDescending(w => w).Take(10).ToList();
Using a SubSonic (2.2) SqlQuery object, I am querying a view that contains distinct rows from another table. The results of the query, however, contain multiple rows for certain rows in the view. It appears to be because of a join on a temporary table in the query generated to achieve paging. How can I avoid this duplication of rows?
Bonus points: I have to use the view because SubSonic can't do .Paged() and .Distinct() at the same time. Why not?
If I remember correctly you have to use distinct on the right position.
var query = DB.Select().From<Products>()
.Where(Products.CategoryColumn).IsEqualTo(5).Distinct();
var query = DB.Select().Distinct().From<Products>()
.Where(Products.CategoryColumn).IsEqualTo(5);
Both statements compile but the first generates invalid sql code. A good starting point for debugging SubSonic SqlQueries is to generate the output:
var sql = query.BuildSqlStatement();
Another solution could be to use Group instead of distinct so you can avoid the view in the first place.
I'm sure this is straight forward but I'm very new to entity queries and has probably been asked before.
What i need to to search for all business in my database where they have a category that exists in a collection of categories I have built up
IList<businessCategory> busCatList;
busCatList.Add(businessCategory.CreatebusinessCategory(1,"Tourism"));
busCatList.Add(businessCategory.CreatebusinessCategory(2,"Accomidation"));
busCatList.Add(businessCategory.CreatebusinessCategory(3,"Entertainment"));
busCatList.Add(businessCategory.CreatebusinessCategory(4,"Bar"));
busCatList.Add(businessCategory.CreatebusinessCategory(5,"Club"));
var items = Data.DBEntities.business.Where(b.businessCategory.Contains(busCatList) );
I know the syntax of the query is wrong but essentially what i what the query to do it pull out all the business where it has a category matching any of the categories in the busCatLsit
In my database one business can be linked to many categories
In SQL I would do
SELECT name FROM business
join businessCategoryRlnshp on businessCategoryRlnshp.businessID = business.ID
where categoryID in (1,2,3)
just trying to read your mind here ;)
var items = busCatList.Where(businessCategory => b.businessCategory.Contains(businessCategory));
I can't really imagine a nice solution in linq - I mean other than some kind of performing the query several times - one for each business category.
However in SQL Server 2008 there is a new feature - passing a table variable to stored procedure. This can be done from code by passing a DataSet with 1 DataTable as a parameter.
You can of course write an extension method for IEnumerable to convert it to a DataSet similar to ToList() or ToDictionary() methods.
Stored procedure returning entities can be used in EntityFramework 1.0 so this should theoretically make the puzzle click.
PS> There's also a solution using E-SQL and probably query builder methods.