Linq Joins - Performance

Why is LINQ JOIN so much faster than linking with WHERE?

1. Your first approach (SQL query in the DB) is quite efficient because the DB knows how to perform a join. But it doesn't really make sense to compare it with the other approaches, since they work directly in memory (Linq to DataSet)

2. The query with multiple tables and a Where condition actually performs a cartesian product of all the tables, then filters the rows that satisfy the condition. This means the Where condition is evaluated for each combination of rows (n1 * n2 * n3 * n4)

3. The Join operator takes the rows from the first tables, then takes only the rows with a matching key from the second table, then only the rows with a matching key from the third table, and so on. This is much more efficient, because it doesn't need to perform as many operations

Improve LINQ query performance. A query with joins or small queries?

This should combine the queries into one. I used lambda syntax since you started with that, though I think in this case fluent syntax may be easier to follow:

var result = (context.Table3.Join(context.Table1, t3 => t3.Id_User, t1 => t1.id1, (t3, t1) => new { t3, t1 })
                            .Join(context.Table2, t3t1 => t3t1.t3.Id_Branch, t2 => t2.id2, (t3t1, t2) => new { t3t1.t3, t3t1.t1, t2 })
                            .Where(t3t1t2 => t3t1t2.t1.UserName == user && t3t1t2.t2.BranchName == branch)
             )
             .Any();

Here is the query syntax equivalent:

var res2 = (from t3 in context.Table3
            join t1 in context.Table1 on t3.Id_User equals t1.id1
            join t2 in context.Table2 on t3.Id_Branch equals t2.id2
            where t1.UserName == user && t2.BranchName == branch
            select t3
           )
           .Any();

LINQ Joins - Performance

Linq to SQL does not send join hints to the server. Thus the performance of a join using Linq to SQL will be identical to the performance of the same join sent "directly" to the server (i.e. using pure ADO or SQL Server Management Studio) without any hints specified.

Linq to SQL also doesn't allow you to use join hints (as far as I know). So if you want to force a specific type of join, you'll have to do it using a stored procedure or the Execute[Command|Query] method. But unless you specify a join type by writing INNER [HASH|LOOP|MERGE] JOIN, then SQL Server always picks the type of join it thinks will be most efficient - it doesn't matter where the query came from.

Other Linq query providers - such as Entity Framework and NHibernate Linq - will do exactly the same thing as Linq to SQL. None of these have any direct knowledge of how you've indexed your database and so none of them send join hints.

Linq to Objects is a little different - it will (almost?) always perform a "hash join" in SQL Server parlance. That is because it lacks the indexes necessary to do a merge join, and hash joins are usually more efficient than nested loops, unless the number of elements is very small. But determining the number of elements in an IEnumerable<T> might require a full iteration in the first place, so in most cases it's faster just to assume the worst and use a hashing algorithm.

LINQ JOIN performance optimization

a simpler solution would be to sort the data first and then group it based on the id. This should be more performant than joining the data.

    var searchResults = new List<SearchResult>
    {
        new SearchResult { Id = 1 },
        new SearchResult { Id = 2 }, // yes
        new SearchResult { Id = 3 }, // yes
        new SearchResult { Id = 4 }, // yes
        new SearchResult { Id = 5 },
        
        new SearchResult { Id = 1, ContactId = 1 }, // yes
        new SearchResult { Id = 5, ContactId = 3 }, // yes
        
        new SearchResult { Id = 1, ContactId = 1 },
        new SearchResult { Id = 8, ContactId = 4 }, // yes
        
        new SearchResult { Id = 1 }, 
        new SearchResult { Id = 2 }, 
        new SearchResult { Id = 10 }, // yes
        new SearchResult { Id = 11 }, // yes
        new SearchResult { Id = 12 }, // yes
    };
    
    var result = searchResults
                 .OrderBy(x => x.Id)
                 .ThenByDescending(x => x.ContactId)
                 .GroupBy(p => p.Id)
                 .Select(x => x.First());
    
    foreach(var sr in result){         
        Console.WriteLine(sr.Id + " " + sr.ContactId);
    }  

How to improve performance when joining List and Linq object

Try the following code. Instead of GroupJoin, which is not needed here I have used Join. Also moved filters up in query.

var lsDuplicateEmail = 
    from imp in lsData
    where !string.IsNullOrEmpty(imp.Email)
    join cust in lsCustomer
       on ImportHelpers.GetPerfectStringWithoutSpace(imp.Email) equals ImportHelpers.GetPerfectStringWithoutSpace(cust.Email)
    where !ImportHelpers.CompareString(imp.Code, cust.Code)
    select new
    {
        ImportItem = imp,
        CustomerItem = cust,
    };

Also show GetPerfectStringWithoutSpace implementation, maybe it is slow.

Another possible solution is to swap lsData and lsCustomer in query, maybe lookup search is not so fast.

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