Linq Multiple where based on different conditions

Question

In search page I have some options based on them search query must be different . I have wrote this :

int userId = Convert.ToInt32(HttpContext.User.Identity.GetUserId());

var followings = (from f in _context.Followers
                  where f.FollowersFollowerId == userId && f.FollowersIsAccept == true
                  select f.FollowersUserId).ToList();

int value;

if (spto.Page == 0)
{
    var post = _context.Posts.AsNoTracking().Where(p => (followings.Contains(p.PostsUserId) || p.PostsUser.UserIsPublic == true || p.PostsUserId == userId) && p.PostIsAccept == true).Select(p => p).AsEnumerable();

    if(spto.MinCost != null)
    {
        post = post.Where(p => int.TryParse(p.PostCost, out value) && Convert.ToInt32(p.PostCost) >= spto.MinCost).Select(p => p);
    }

    if (spto.MaxCost != null)
    {
        post = post.Where(p => int.TryParse(p.PostCost, out value) && Convert.ToInt32(p.PostCost) <= spto.MaxCost).Select(p => p);
    }

    if (spto.TypeId != null)
    {
        post = post.Where(p => p.PostTypeId == spto.TypeId).Select(p => p);
    }

    if (spto.CityId != null)
    {
        post = post.Where(p => p.PostCityId == spto.CityId).Select(p => p);
    }

    if (spto.IsImmidiate != null)
    {
        post = post.Where(p => p.PostIsImmediate == true).Select(p => p);
    }

    var posts = post.Select(p => new
     {
         p.Id,
         Image = p.PostsImages.Select(i => i.PostImagesImage.ImageAddress).FirstOrDefault(),
         p.PostCity.CityName,
         p.PostType.TypeName
     }).AsEnumerable().Take(15).Select(p => p).ToList();

    if (posts.Count != 0)
        return Ok(posts);

    return NotFound();

In this case I have 6 Query that take time and the performance is low and code is too long . Is there any better way for writing better code ?

Answer 1

I have solved my problem with ternary operator :

var post = _context.Posts.AsNoTracking().Where(p => 
(followings.Contains(p.PostsUserId) || p.PostsUser.UserIsPublic == true || p.PostsUserId == userId) && p.PostIsAccept == true
&& (spto.MinCost != null ? int.TryParse(p.PostCost, out value) && Convert.ToInt32(p.PostCost) >= spto.MinCost : 1 == 1)
&& (spto.MaxCost != null ? int.TryParse(p.PostCost, out value) && Convert.ToInt32(p.PostCost) <= spto.MaxCost : 1 == 1)
&& (spto.TypeId != null ? p.PostTypeId == spto.TypeId : 1 == 1)
&& (spto.CityId != null ? p.PostCityId == spto.CityId : 1 == 1)
&& (spto.IsImmidiate != null && spto.IsImmidiate == true ? p.PostIsImmediate == true : 1 == 1)).Select(p => new
{
    p.Id,
    Image = p.PostsImages.Select(i => i.PostImagesImage.ImageAddress).FirstOrDefault(),
    p.PostCity.CityName,
    p.PostType.TypeName
}).Skip(spto.Page * 15).Take(15).ToList();

EDIT (Better Code) :

Thanx to @ZoharPeled , @HaraldCoppoolse , @JonasH I have Changed the Code like this :

int value;

var post = _context.Posts.AsNoTracking().Where(p =>
    (followings.Contains(p.PostsUserId) || p.PostsUser.UserIsPublic == true || p.PostsUserId == userId) && p.PostIsAccept == true
    && (spto.MinCost == null || (int.TryParse(p.PostCost, out value) && Convert.ToInt32(p.PostCost) >= spto.MinCost))
    && (spto.MaxCost == null || (int.TryParse(p.PostCost, out value) && Convert.ToInt32(p.PostCost) <= spto.MaxCost))
    && (spto.TypeId == null || p.PostTypeId == spto.TypeId)
    && (spto.CityId == null || p.PostCityId == spto.CityId)
    && (spto.IsImmidiate == null || p.PostIsImmediate == true)).Select(p => new
    {
        p.Id,
        Image = p.PostsImages.Select(i => i.PostImagesImage.ImageAddress).FirstOrDefault(),
        p.PostCity.CityName,
        p.PostType.TypeName
    }).Skip(spto.Page * 15).Take(15).ToList();

Edit (Best Code) :

int userId = Convert.ToInt32(HttpContext.User.Identity.GetUserId());

var followings = _context.Followers
                        .Where(follower => follower.FollowersFollowerId == userId
                                        && follower.FollowersIsAccept)
                        .Select(follower => follower.FollowersUserId);

int value;

var post = _context.Posts.AsNoTracking().Where(p => p.PostIsAccept 
                     && (p.PostsUser.UserIsPublic || p.PostsUserId == userId 
                     || _context.Followers.Where(f => f.FollowersFollowerId == userId 
                     && f.FollowersIsAccept).Select(f => f.FollowersUserId).Any()));

if (spto.MinCost != null)
    post = post.Where(p => int.TryParse(p.PostCost, out value) && Convert.ToInt32(p.PostCost) >= spto.MinCost);

if (spto.MaxCost != null)
    post = post.Where(p => int.TryParse(p.PostCost, out value) && Convert.ToInt32(p.PostCost) <= spto.MaxCost);

if (spto.TypeId != null)
    post = post.Where(p => p.PostTypeId == spto.TypeId);

if (spto.CityId != null)
    post = post.Where(p => p.PostCityId == spto.CityId);

if (spto.IsImmidiate != null)
    post = post.Where(p => p.PostIsImmediate == true);

var posts = post.Select(p => new
{
    p.Id,
    Image = p.PostsImages.Select(i => i.PostImagesImage.ImageAddress).FirstOrDefault(),
    p.PostCity.CityName,
    p.PostType.TypeName
}).Skip(spto.Page).Take(15).ToList();

if (posts.Count != 0)
    return Ok(posts);

Answer 2

Some observations:

.AsEnumerable()

This is meant to hide where operators if you use a custom collection. It should not be needed in this case.

.Select(p => p)

I fail to see any purpose for this, remove it.

int.TryParse(p.PostCost, out value) && Convert.ToInt32(p.PostCost) >= spto.MinCost

Parsing can be expensive, so you want to do as little as possible, and this does it twice, of four times if you have both min and max. replace with direct compares with value, i.e. int.TryParse(p.PostCost, out value) && value >= spo.MinCost. I would also suggest have an explicit case when there is both a min and max cost to avoid parsing twice.

followings.Contains(p.PostsUserId) 

Followings is a list, so it will search thru all items. Use a HashSet to speed up performance. I.e. Replace .ToList() with ToHashSet() when creating the followings list. HashSet uses a hash table to make Contains() a constant time operation rather than a linear operation.

Query order

You would want to order the checks to eliminate as many items as early as possible, and make simple, fast, checks before slower checks.

Merge where operators

A single where operator is in general faster than multiple calls.

Use plain loop

If you really need as high performance as possible it might be better to use regular loops. Linq is great for writing compact code, but performance is usually better with plain loops.

Profile

Whenever you talk about performance it is important to point out the importance of profiling. The comments above are reasonable places to start, but there might be some completely different things that takes time. The only way to know is to profile. That should also give a good indication about the improvements.

Answer 3

Short answer: if you don't do the ToList and AsEnumerable until the end, then you will only execute one query on your dbContext.

So keep everything IQueryable<...>, until you create List<...> posts:

var posts = post.Select(p => new
 {
     p.Id,
     Image = p.PostsImages
              .Select(i => i.PostImagesImage.ImageAddress)
              .FirstOrDefault(),
     p.PostCity.CityName,
     p.PostType.TypeName,
 })
 .Take(15)
 .ToList();

IQueryable and IEnumerable

For the reason why skipping all the ToList / AsEnumerable would help to improve performance, you need to be aware about the difference between an IEnumerable<...> and an IQueryable<...>.

IEnumerable

A object of a class that implements IEnumerable<...> represents the potential to enumerate over a sequence that the object can produce.

The object holds everything to produce the sequence. Once you ask for the sequence, it is your local process that will execute the code to produce the sequence.

At low level, you produce the sequence by using GetEnumerator and repeatedly call MoveNext. As long as MoveNext returns true, there is a next element in the sequence. You can access this next element using property Current.

Enumerating the sequence is done like this:

IEnumerable<Customer> customers = ...
using (IEnumarator<Customer> customerEnumerator = customers.GetEnumerator())
{
    while (customerEnumerator.MoveNext())
    {
        // there is still a Customer in the sequence, fetch it and process it
        Customer customer = customerEnumerator.Current;
        ProcessCustomer(customer);
    }
}

Well this is a lot of code, so the creators of C# invented foreach, which will do most of the code:

foreach (Customer customer in customers)
    ProcessCustomer(customer);

Now that you know what code is behind the foreach, you might understand what happens in the first line of the foreach.

It is important to remember, that an IEnumerable<...> is meant to be processed by your local process. The IEnumerable<...> can call every method that your local process can call.

IQueryable

An object of a class that implements IQueryable<...> seems very much like an IEnumerable<...>, it also represents the potential to produce an enumerable sequence of similar object. The difference however, is that another process is supposed to provide the data.

For this, the IQueryable<...> object holds an Expression and a Provider. The Expression represents a formula to what data must be fetched in some generic format; the Provider knows who must provide the data (usually a database management system), and what language is used to communicate with this DBMS (usually SQL).

As long as you concatenate LINQ methods, or your own methods that only return IQueryable<...>, only the Expression is changed. No query is executed, the database is not contacted. Concatenating such statements is a fast method.

Only when you start enumerating, either at its lowest level using GetEnumerator / MoveNext / Current, or higher level using foreach, the Expression is sent to the Provider, who will translate it to SQL and fetch the data from the database. The returned data is represented as an enumerable sequence to the caller.

Be aware, that there are LINQ methods, that don't return IQueryable<TResult>, but a List<TResult>, TResult, a bool, or int, etc: ToList / FirstOrDefault / Any / Count / etc. Those methods will deep inside call GetEnumerator / MoveNext / Current`; so those are the methods that will fetch data from the database.

Back to your question

Database management systems are extremely optimized for handling data: fetching, ordering, filtering, etc. One of the slower parts of a database query is the transfer of the fetched data to your local process.

Hence it is wise to let the DBMS do as much database handling as possible, and only transfer the data to your local process that you actually plan to use.

So, try to avoid ToList, if your local process doesn't use the fetched data. In your case: you transfer the followings to your local process, only to transfer it back to the database in the IQueryable.Contains method.

Furthermore, (it depends a bit on the framework you are using), the AsEnumerable transfers data to your local process, so your local process has to do the filtering with the Where and the Contains.

Alas you forgot to give us a description of your requirements ("From all Posts, give me only those Posts that ..."), and it is a bit too much for me to analyze all your queries, but you gain most efficiency if you try to keep everything IQueryable<...> as long as possible.

There might be some problems with the Int.TryParse(...). Your provider probably will not know how to translate this into SQL. There are several solutions possible:

• Apparently PostCost represents a number. Consider to store it as a number. If it is an amount (price or something, something with a limited number of decimals), consider to store it as a decimal.
• If you really can't convince your project leaders that numbers should be stored as decimals, either search for a job where they make proper databases, or consider to create a stored procedure that converts the string that is in PostCost to a decimal / int.
• if you will only use fifteen elements, use the IQueryable.Take(15), not the IEnumerable.Take(15).

Further optimizations:

int userId = 

var followerUserIds = _context.Followers
    .Where(follower => follower.FollowersFollowerId == userId
                    && follower.FollowersIsAccept)
    .Select(follower => follower.FollowersUserId);

In words: make the following IQueryable, but don't execute it yet: "From all Followers, keep only those Followers that are Accepted and have a FollowersFollowerId equal to userId. From the remaining Followers, take the FollowersUserId".

It seems that you only plan to use it if page is zero. Why create this query also if page not zero?

By the way, never use statements like where a == true, or even worse: if (a == true) then b == true else b == false, this gives readers the impression that you have difficulty to grasp the idea of Booleans, just use: where a and b = a.

Next you decide to create a query that zero or more Posts, and thought it would be a good idea to give it a singular noun as identifier: post.

var post = _context.Posts
    .Where(post => (followings.Contains(post.PostsUserId) 
                           || post.PostsUser.UserIsPublic
                           || post.PostsUserId == userId)
                && post.PostIsAccept);

Contains will cause a Join with the Followers table. It will probably be more efficient if you only join Accepted posts with the followers table. So first check on PostIsAccept and the other predicates before you decide to join:

.Where(post => post.PostIsAccept
            && (post.PostsUser.UserIsPublic || post.PostsUserId == userId
                || followings.Contains(post.PostsUserId));

All non-accepted Posts won't have to be joined with the Followings; depending on whether your Provider is smart enough: it won't join all public users, or the one with userId, because it knows that it will already pass the filter.

Consider to use a Contains, instead of Any

It seems to me that you want the following:

I have a UserId; Give me all Accepted Posts, that are either from this user, or that are from a public user, or that have an accepted follower

var posts = dbContext.Posts
    .Were(post => post.IsAccepted
       && (post.PostsUser.UserIsPublic || post.PostsUserId == userId
           || dbContext.Followers
                       .Where(followers => ... // filter the followers as above)
                       .Any());

Be aware: I still haven't executed the query, I only have changed the Expression!

AFter this first definition of posts, you filter the posts further, depending on various values of spto. You could consider to make this one big query, but I think that won't speed up the process. It will only make it more unreadable.

Finally: why use:

.Select(post => post)

This doesn't do anything to your sequence, it will only make it slower.