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Situation: Say we are executing a LINQ query that joins two in-memory lists (so no DbSets or SQL-query generation involved) and this query also has a where clause. This where only filters on properties included in the original set (the from part of the query).

Question: Does the linq query interpreter optimize this query in that it first executes the where before it performs the join, regardless of whether I write the where before or after the join? – so it does not have to perform a join on elements that are not included later anyways.

Example: For example, I have a categories list I want to join with a products list. However, I am just interested in the category with ID 1. Does the linq interpreter internally perform the exact same operations regardless of whether I write:

from category in categories

join prod in products on category.ID equals prod.CategoryID

where category.ID == 1 // <------ below join

select new { Category = category.Name, Product = prod.Name };

or

from category in categories

where category.ID == 1 // <------ above join

join prod in products on category.ID equals prod.CategoryID

select new { Category = category.Name, Product = prod.Name };

Previous research: I already saw this question but the OP author stated that his/her question is only targeting non-in-memory cases with generated SQL. I am explicitly interested with LINQ executing a join on two lists in-memory.

Update: This is not a dublicate of "Order execution of chain linq query" question as the referenced question clearly refers to a dbset and my question explicitly addressed a non-db scenario. (Moreover, although similar, I am not asking about inclusions based on navigational properties here but about "joins".)

Update2: Although very similar, this is also not a dublicate of "Is order of the predicate important when using LINQ?" as I am asking explicitly about in-memory situations and I cannot see the referenced question explicitly addressing this case. Moreover, the question is a bit old and I am actually interested in linq in the context of .NET Core (which didn't exist in 2012), so I updated the tag of this question to reflect this second point.

Please note: With this question I am aiming at whether the linq query interpreter somehow optimizes this query in the background and am hoping to get a reference to a piece of documentation or source code that shows how this is done by linq. I am not interested in answers such as "it does not matter because the performance of both queries is roughly the same".

The LINQ query syntax will be compiled to a method chain. For details, read e.g. in this question.

The first LINQ query will be compiled to the following method chain:

categories

    .Join(

        products,

        category => category.ID,

        prod => prod.CategoryID,

        (category, prod) => new { category, prod })

    .Where(t => t.category.ID == 1)

    .Select(t => new { Category = t.category.Name, Product = t.prod.Name });

The second one:

categories

    .Where(category => category.ID == 1)

    .Join(

        products,

        category => category.ID,

        prod => prod.CategoryID,

        (category, prod) => new { Category = category.Name, Product = prod.Name });

As you can see, the second query will cause less allocations (note only one anonymous type vs 2 in the first query, and note how many instances of those anonymous types will be created on performing the query).

Furthermore, it's clear that the first query will perform a join operation on lot more data than the second (already filtered) one.

There will be no additional query optimization in case of LINQ-to-objects queries.

So the second version is preferable.

For in memory lists (IEnumerables), no optimization is applied and query execution is made in chained order for in-memory lists.

I also tried result by first casting it to IQueryable then apply filtering but apparently casting time is pretty high for this big table.

I made a quick test for this case.

Console.WriteLine($"List Row Count = {list.Count()}"); 

Console.WriteLine($"JoinList Row Count = {joinList.Count()}"); 



var watch = Stopwatch.StartNew();

var result = list.Join(joinList, l => l.Prop3, i=> i.Prop3, (lst, inner) => new {lst, inner})

   .Where(t => t.inner.Prop3 == "Prop13")

   .Select(t => new { t.inner.Prop4, t.lst.Prop2}); 

result.Dump();

watch.Stop();



Console.WriteLine($"Result1 Elapsed = {watch.ElapsedTicks}");



watch.Restart();

var result2 = list

   .Where(t => t.Prop3 == "Prop13")

   .Join(joinList, l => l.Prop3, i=> i.Prop3, (lst, inner) => new {lst, inner})

   .Select(t => new { t.inner.Prop4, t.lst.Prop2});



result2.Dump();

watch.Stop();

Console.WriteLine($"Result2 Elapsed = {watch.ElapsedTicks}"); 



watch.Restart();

var result3 = list.AsQueryable().Join(joinList, l => l.Prop3, i=> i.Prop3, (lst, inner) => new {lst, inner})

   .Where(t => t.inner.Prop3 == "Prop13")

   .Select(t => new { t.inner.Prop4, t.lst.Prop2}); 

result3.Dump();

watch.Stop();

Console.WriteLine($"Result3 Elapsed = {watch.ElapsedTicks}"); 

Findings:

List Count = 100

JoinList Count = 10

Result1 Elapsed = 27

Result2 Elapsed = 17

Result3 Elapsed = 591



List Count = 1000

JoinList Count = 10

Result1 Elapsed = 20

Result2 Elapsed = 12

Result3 Elapsed = 586



List Count = 100000

JoinList Count = 10

Result1 Elapsed = 603

Result2 Elapsed = 19

Result3 Elapsed = 1277



List Count = 1000000

JoinList Count = 10

Result1 Elapsed = 1469

Result2 Elapsed = 88

Result3 Elapsed = 3219