LINQ to Entities Any() and Contains() slow with small list

Question

I\'m using EF 6 to get products from a database. The product categories are mapped as a navigation property on products, and the data is from a ProductCategory pivot table. The

Answer 1

Try first filtering out the products and only then forming your models (Product_PL and Category_PL):

var filteredProducts = db.Products.Where(p => p.Categories.Any(c => currentCategoryIdAndChildren.Contains(c.ID)))
    .Select(p => new Product_PL
    {
        id = p.ID,
        name = p.Name,
        description = p.Description,
        categories = p.Categories
                    .Select(c => new Category_PL
                    {
                        categoryid = c.ID,
                    }),
    });

Answer 2

I tried many different things and finally found a solution.

I believe the main slowdown happened when EF was translating the Contains() into a SQL query. The most noticeable thing, however, was that it did not appear to cache the query. From what I can gather, this is because the list of category IDs (currentCategoryIdAndChildren) was generated outside of EF, so it assumed it would be different every time.

I was able to speed things up by using the PredicateBuilder in LINQKit. This allowed me to create the logic more explicitly:

var IsInCategory = PredicateBuilder.False<Product_PL>();

foreach (int categoryID in currentCategoryIdAndChildren)
{ IsInCategory = IsInCategory.Or(pl => pl.categories.Any(c => categoryID == c.categoryid)); }

products = products.Where(IsInCategory);

This got me a bit better performance with my initial query, and MUCH better performance with subsequent queries.