MVC4 Linq查询优化

Question

I have the below code which works but I do not feel this is the best way to achieve the result. I am looking at optimising my code. Any suggestions of a better option will be appreciated. sub is a subcategory which is nullable.

[AllowAnonymous]
    public ActionResult _relatedgrps(string cat, string sub)
    {
         if (!string.IsNullOrWhiteSpace(sub)){
            var pgs = db.Pages
            .Where(u=>u.MetaNoSearch==false)
            .Where(u => u.PaOk == true && u.Category.Name == cat &&   u.SubCategory.CatName == sub) 
            .OrderByDescending(u => u.PaCreatedOn);
        return PartialView(pgs.ToList());

         }else{
           var pgs = db.Pages
            .Where(u=>u.MetaNoSearch==false)
            .Where(u => u.PaOk == true && u.Category.Name == cat ) 
          .OrderByDescending(u => u.PaCreatedOn);   
        return PartialView(pgs.ToList());

    }}

Answer 1

Linq IEnumerables can be additive and the query will only be executed when enumerated for the first time (like calling .ToList() ). So you should be able to do something like this:

var pgs = db.Pages
    .Where(u => u.MetaNoSearch == false &&
           u.PaOk == true &&
           u.Category.Name == cat);

if (!string.IsNullOrWhiteSpace(sub))
{
    pgs = pgs.Where(u => u.SubCategory.CatName == sub);
}

return PartialView(pgs.OrderByDescending(u => u.PaCreatedOn).ToList());

Answer 2

@user3021830 - be careful with String.IsNullOrWhitespace, you cannot use that in a database query. You could do String.IsNullOrWhitespace(sub), but not String.IsNullOrWhitespace(u.*).

I'd avoid any conditionals in the query because that will likely result in a case statement in the SQL.

To produce the best SQL I'd do something like this:

var pgs = db.Pages.Where(u => u.MetaNoSearch == false)
                    .Where(u => u.PaOk == true)
                    .Where(u => u.Category.Name == cat);

if (!string.IsNullOrWhiteSpace(sub))
{
    pgs = pgs.Where(u => u.SubCategory.CatName == sub);   
}

var result = pgs.OrderByDescending(u => u.PaCreatedOn).ToList();

Answer 3

Create an object to query it. To improve it, you also could remove it boolean comparations because they are conditions.

var query = db.Pages.Where(u => !u.MetaNoSearch && u.PaOk && u.Category.Name == cat);

if (!string.IsNullOrWhiteSpace(sub))
    query = query.Where(u => u.SubCategory.CatName == sub);

query = query.OrderByDescending(u => u.PaCreatedOn);

return PartialView(query.ToList());