Is it performant wise to use greedy LINQ operators such as ToList,ToLookUp,Distinct etc?
What would be a best practice(s) for LINQ query execution?
You often use for your objects List<> or making all your objects lists to IEnumerable<>. I know the latest gives more flexibility.
When working with memory (LINQ to Objects) it's ok to always use deffered loading, cause you can access it whenever you need without fear that tha data changed, added or inserted as the reference will execute the query as soon as you need access. But this changes with database LINQ queries such LINQ to EF.
Would like a StackOverflow users opinion.
Thank you!
What would be a best practice(s) for LINQ query execution?
A List may be accessed by index, a Lookup may be accessed by Key. These types are obviously serializable across a WCF boundary. A deferred IEnumerable doesn't do these things well.
For EF or LinqToSql, one must run their queries before the DataContext or whatever holds the SqlConnection gets disposed.
In my code, I use deferred IEnumerables only for method scoped variables when convenient. I use List for properties (sometimes the property constructs the List, but usually it's just backed by an instance) and method return types. Since I'm doing comparatively expensive things (like accessing the database or using WCF), the performance of eagerly executing in-memory Linq queries has never been an issue.
The final authority on any performance question is: how does it measure?
Related
When it comes to performance, should i use .ToList().Distinct() or .Distinct().ToList() ?
Both extension methods generate the same SQL query or not?
It seems that the second approach should perform better but is that true?
Are there any advantages or disadvantages of using one over another?
Short Answer: .Distinct().ToList()
Explain:
ToList: It converts an IEnumerable<T> to a List<T>, It's called Immediate execution. So you should filter all data in DB Server first instead of get all data then Distinct in "client-side"
It depends. If it is a query that is executed against a List<T> or a Dictionary<K,V> then the latter (Distinct().ToList()) would be preferrable.
The reason being, that if you do .ToList().Distinct(), Distinct() returns an IEnumerable that has to be executed again to get a real collection. In essence, you create two collections, but you would never use the first one.
There is a situation however where .ToList().Distinct() can be preferrable and that is if you are working with a Object-to-Relational mapper (see: EntityFramework) and you want to fetch all rows from a database table (maybe to populate a cache in the background or to use up less CPU on the database) and then do the .Distinct() operation locally.
Your mention of SQL suggests that your datasource is a DBContext of some kind.
In that situation, by definition, once you have done .ToList() all available data has been converted to objects in .NET Memory. Doing a .Distinct() after that can only run in .NET memory - it will run as if there is no database.
The SQL query for the above is definitely not the same as for .Distinct().ToList(), which will let the database do the DISTINCT operation.
To achieve the best performance, the best thing to do is .Distinct().ToList().
We must create and show at runtime (asp.net mvc) some complex reports from Oracle tables data with millions of records. The reports data must be obtained from groupings and little complex calculations.
So is it better for performance and maintainability of code that do these groupings and calculations via sql query (pl/sql) or via linq?
Thanks for your kindle reply
So is it better for performance and maintainability of code that do
these groupings and calculations via sql query (pl/sql) or via linq?
It depends on what you mean by via linq. If you mean that you fetch the complete table to local memory and then use linq statements to extract the result that you want, then of course SQL statements are faster.
However, if you mean that you use Entity Framework, or something similar, then the answer is not a easy to give.
If you use Entity Framework (or some clone), your tables will be represented by IQueryable<...> instead of IEnumerable<...>. An IQueryable has an Expression and a Provider. The Expression represents the query that must be performed. The Provider knows which system must execute the query (usually a Database Management System) and how to communicate with this system. When the query must be executed, it is the task of the Provider to translate the Expression into the language that the system knows (usually something SQL-like) and to execute the SQL-query.
There are two kinds of IQueryable LINQ statements: those that return an IQueryable<...> of something, and those that return a TResult. The ones that return IQueryable only change the Expression. They are functions that use deferred execution.
Function that do not return an IQueryable, are ToList(), FirstOrDefault(), Any(), Max(), etc. Internally they will call functions that will GetEnumerator() (usually via a foreach), which orders the Provider to translate the Expression and execute the query.
Back to your question
So which one is more efficient, entity framework or SQL? Efficiency is not only the time to perform the queries, it is also the development/testing time, for the first version and for future changes in the software.
If you use an entity-framework (-clone), the SQL-queries created from the Expressions are pretty efficient, depending on the framework manufacturer. If you look at the code, then sometimes the SQL query is not the optimal one, although you'll have to be a pretty good SQL-programmer to improve most queries.
The big advantage above using Entity Framework and LINQ queries above SQL statements is that development times will be shorter. The syntax of the LINQ statements is checked at compile time, SQL statements at run-time. Development and test periods will be shorter.
It is easy to reuse LINQ statements, while SQL statements almost always have to be written especially for the query you want to execute. LINQ statements can be tested without a database on any sequence of items that represent your tables.
My Advice
For most queries you won't notice any difference in execution time between the entity framework query or the SQL query.
If you expect complicated queries and future changes, I'd go for entity framework. With main argument the shorter development time, the better testing possibilities, and the better maintainability.
If you detect some queries where you notice that the execution time is too long, you can always decide to bypass entity framework by executing a SQL query instead of using LINQ.
If you've wrapped your DbContext in a proper repository, where you hide the use cases from their implementations, the users of your repository won't notice the difference.
I've seen a lot of people talking about IQueryable and I haven't quite picked up on what all the buzz is about. I always work with generic List's and find they are very rich in the way you can "query" them and work with them, even run LINQ queries against them.
I'm wondering if there is a good reason to start considering a different default collection in my projects.
The IQueryable interface allows you to define parts of a query against a remote LINQ provider (typically against a database, but doesn't have to be) in multiple steps, and with deferred execution.
E.g. your database layer could define some restriction (e.g. based on permissions, security - whatever) by adding a .Where(x => x.......) clause to your query. But this doesn't get executed just yet - e.g. you're not retrieving 150'000 rows that match that criteria.
Instead, you pass up the IQueryable interface to the next level, the business layer, where you might be adding additional requirements and where clauses to your query - again, nothing gets executed just yet, you're also not tossing out 80'000 of your 150'000 rows you retrieved - you're just defining additional query criteria.
And the UI layer might do the same thing, e.g. based on user input in a form or something.
The magic is that you're passing the IQueryable interface through all the layers, adding additional critieria to it - but it doesn't get executed / evaluated until you actually force it. This also means you're not needlessly selecting and retrieving tons of data which you end up discarding afterwards.
You can't really do that with a classic static list - you have to pick the data, possibly discarding a lot of it again later on in the process - you have a static list, after all.
IQueryable allows you to make queries using LINQ, just like the LINQ to Object queries, where the queries are actually "compiled" and run elsewhere.
The most common implementations work against databases. If you use List<T> and LINQ to Objects, you load the entire "table" of data into memory, then run your query against it.
By using IQueryable<T>, the LINQ provide can "translate" your LINQ statement into actual SQL code, and run it on the database. The results can be returned to you and enumerated.
This is much, much more efficient, especially if you're working in N-Tiered systems.
LINQ queries against IEnumerable<T> produce delegates (methods) which, when invoked, perform the described query.
LINQ queries against IQueryable<T> produce expression trees, a data structure which represents the code that produced the query. LINQ providers such as LINQ to SQL interpret these data structures, generating the same query on the target platform (T-SQL in this case).
For an example of how the compiler interprets the query syntax against IQueryable<T>, see my answer to this question:
Building Dynamic LINQ Queries based on Combobox Value
I am considering refactoring a repository I have to improve its flexibility and reduce the method count.
Where there are the following methods:
Collection GetAllUsersByRole(Role role)
User GetUserByuserName(string userName)
...I would like to have a single method taking a Linq expression:
ICollection GetUsers(Expression e)
{
//retrieve user collection from store
//apply expression to collection and return
}
Is this a reasonable approach? Presumably I'd lose some efficiency because the full users collection would need to be retrieved and filtered every time, rather than retrieving a subset of users according to some hard-coded criteria?
Edit: NHibernate provides ORM in my implementation.
You really want to take an Expression as the argument to that method.
As far as performance, it really comes down to how far you want to go with it. The simplest method is bringing all the objects into memory and then filtering with the predicate expression.
On the other hand, you mention some sort of criteria. I have no idea what your back end data system is, but you can take these passed filters and transform them into your criteria. This is essentially what Linq to SQL and Linq to Entities does, but hopefully the range of possibilities you need to support is significantly smaller. If not, it might make sense to switch to one of the ORM tools if you want to take this approach.
This is not a very reasonable approach, cos typically will cost you a lot of performance issues. If you use data access technology that accepts LINQ queries than you just can use this query (expression) with it. This can be IQueryable for LINQ to SQL or ObjectQuery for EntityFramework. It also can be ICriteria (without linq support) for nHibernate. All modern ORM tools have its own expression API, so you just need to use it. If you have custom Data Access layer you will need to write your own API for creating criterias, for example Query Object.
just wanted to gather different ideas and perspectives as to which layer should (and why) LINQ fall into?
LINQ = Language INtegrated Queries. This is the query extensions that allows you to query anything from databases to lists/collections to XML. The query language is useful in any layer.
However, a lot of people refer to LINQ to SQL as just "LINQ". In that context, a combined BLL/DAL makes sense when you're using L2S and that's where you do LINQ queries against your database. That does of course not exclude doing subsequent queries against the results from those same queries in new (Linq to objects) queries in higher layers...
it depends on what you want to do with linq. when using linq2sql i`d recommend the DAL, but Linq is more than just database access. you can use it to manipulate lists, ienumerables of business objects and so on... Linq itself can be useful everywhere in your application.
I consider your DataContext-derived object to your DAL layer itself, and LINQ is just a very flexible interface to it. Hence I use LINQ queries directly in the Business layer.
Both. DataContext is the DAL and, when using the designer, the auto-generated partial classes that map on to SQL objects (tables,views) can be considered part of your business layer. I implement partial classes that implement some of the partial methods to enforce validation and security as needed. Some business rules don't map directly on to DB objects and are handled via other classes.
I think if you are doing Linq to Sql, you should always do it in your DAL. However if you are doing Linq to Objects where you are just filtering, playing with different object you can do that is BL layer.
I think LINQ should be the very lower-level (DAL) and I think it should be wrapped into a BLL.
I know a lot of people like to use the partial accessibility of the models that LINQ to SQL generates but I think you should have clear separation of interests (see what I did there?). I think if you're going to have business logic it needs to be decoupled completely from your data access logic.
I think what makes it tricky is the fact that you can keep chaining those LINQ extension methods anywhere you have a using System.Linq line in your code. Again though I think LINQ belongs with the definition and should be at the lowest possible level. It also makes TDD/Unit Testing much, much easier when you wrap the usage of LINQ in a BLL.
I use linq in the traditional 'data access layer' or in 'data access objects'. This allows modularization of code, promotes data code in one place (vs cutting and pasting the same code a few different places) and allows a different front end to be developed with relative ease.
It depends on the architecture of your application, and it makes a huge difference how much the presentation model matches the data model. I agree with separating out business logic operations from the data objects and access methods created by LINQ. I also tend to wrap all data-level operations inside a manager class so I can make the data context an internal class.
I think the point of Linq is that it replaces your DAL.
The equivalent to your old DAL is all the auto-generated code behinf the DBML files + anything extra that Linq can't do added by you.