[Solved] Why does LINQ-to-EF render certain \"Contains\" as two separate JOINs?

EverSQL Database Performance Knowledge Base

Why does LINQ-to-EF render certain \"Contains\" as two separate JOINs?

Possible Duplicate:
EF4.2 extra left outer join to same table

I have three entities in a Parent -> Child -> Grandchild relationship: Order -> OrderCustomer -> OrderItem

Each child entity has a non-nullable FK to its parent (i.e. OrderItem.OrderCustomerID and OrderCustomer.OrderID). And I want to query all OrderItems (grandchildren) for a group of Orders. In SQL I would write:

select
    oc.OrderID,
    oi.Quantity,
    oi.SKU
from OrderItems oi
    join OrderCustomers oc on oi.OrderCustomerID = oc.OrderCustomerID
where
    oc.OrderID in (1, 2, 3, 4, 5)

...so this is what I wrote in LINQ...

OrderItems
    .Where(oi => new[] { 1, 2, 3, 4, 5 }.Contains(oi.OrderCustomer.OrderID))
    .Select(oi => new
    {
        oi.OrderCustomer.OrderID,
        oi.Quantity,
        oi.SKU
    })

...but this is the SQL generated by EF...

SELECT 
    [Extent1].[OrderCustomerID] AS [OrderCustomerID], 
    [Extent3].[OrderID] AS [OrderID], 
    [Extent1].[Quantity] AS [Quantity], 
    [Extent1].[SKU] AS [SKU]
FROM [dbo].[OrderItems] AS [Extent1]
    INNER JOIN [dbo].[OrderCustomers] AS [Extent2] ON [Extent1].[OrderCustomerID] = [Extent2].[OrderCustomerID]
    LEFT OUTER JOIN [dbo].[OrderCustomers] AS [Extent3] ON [Extent1].[OrderCustomerID] = [Extent3].[OrderCustomerID]
WHERE
    [Extent2].[OrderID] = 1 OR [Extent3].[OrderID] IN (2,3,4,5)

Why both an INNER JOIN and an OUTER JOIN?

And why split up the WHERE clause?

On a database with millions of records this query is horribly slow.

But wait, if I change the LINQ to do the Select before the Where...

OrderItems
    .Select(oi => new
    {
        oi.OrderCustomer.OrderID,
        oi.Quantity,
        oi.SKU
    })
    .Where(x => new[] { 1, 2, 3, 4, 5 }.Contains(x.OrderID))

...I get the SQL I wanted...

SELECT 
    [Extent1].[OrderCustomerID] AS [OrderCustomerID], 
    [Extent2].[OrderID] AS [OrderID], 
    [Extent1].[Quantity] AS [Quantity], 
    [Extent1].[SKU] AS [SKU]
FROM [dbo].[OrderItems] AS [Extent1]
    INNER JOIN [dbo].[OrderCustomers] AS [Extent2] ON [Extent1].[OrderCustomerID] = [Extent2].[OrderCustomerID]
WHERE
    [Extent2].[OrderID] IN (1,2,3,4,5)

So, I guess I can help EF by doing the LINQ Select first, but does anyone know what is up with the first query?

How to optimize this SQL query?

The following recommendations will help you in your SQL tuning process.
You'll find 3 sections below:

  1. Description of the steps you can take to speed up the query.
  2. The optimal indexes for this query, which you can copy and create in your database.
  3. An automatically re-written query you can copy and execute in your database.
The optimization process and recommendations:
  1. Create Optimal Indexes (modified query below): The recommended indexes are an integral part of this optimization effort and should be created before testing the execution duration of the optimized query.
Optimal indexes for this query:
ALTER TABLE `OrderCustomers` ADD INDEX `ordercustomers_idx_orderid` (`OrderID`);
ALTER TABLE `OrderItems` ADD INDEX `orderitems_idx_ordercustomerid` (`OrderCustomerID`);
The optimized query:
SELECT
        oc.OrderID,
        oi.Quantity,
        oi.SKU 
    FROM
        OrderItems oi 
    JOIN
        OrderCustomers oc 
            ON oi.OrderCustomerID = oc.OrderCustomerID 
    WHERE
        oc.OrderID IN (
            1, 2, 3, 4, 5
        )

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* original question posted on StackOverflow here.