When quering over a base type why does the EF provider generate all those UNION All clauses

问题

If I have a model that looks like this:

and I do a Linq to Entities query like this:

var c = MyContext.Contact.Count();

I'll get a SQL query that is as big as all out doors!

SELECT 
[GroupBy1].[A1] AS [C1]
FROM ( SELECT 
    COUNT(1) AS [A1]
    FROM   [dbo].[Contacts] AS [Extent1]
    LEFT OUTER JOIN  (SELECT 
        [UnionAll1].[Id] AS [C1]
        FROM  (SELECT 
            [Extent2].[Id] AS [Id]
            FROM [dbo].[Companies] AS [Extent2]
        UNION ALL
            SELECT 
            [Extent3].[Id] AS [Id]
            FROM [dbo].[Employees] AS [Extent3]) AS [UnionAll1]
    UNION ALL
        SELECT 
        [Extent4].[Id] AS [Id]
        FROM [dbo].[Contractors] AS [Extent4]) AS [UnionAll2] ON [Extent1].[Id] = [UnionAll2].[C1]
    LEFT OUTER JOIN  (SELECT 
        [UnionAll3].[Id] AS [C1]
        FROM  (SELECT 
            [Extent5].[Id] AS [Id]
            FROM [dbo].[Suppliers] AS [Extent5]
        UNION ALL
            SELECT 
            [Extent6].[Id] AS [Id]
            FROM [dbo].[Customers] AS [Extent6]) AS [UnionAll3]
    UNION ALL
        SELECT 
        [Extent7].[Id] AS [Id]
        FROM [dbo].[People] AS [Extent7]) AS [UnionAll4] ON [Extent1].[Id] = [UnionAll4].[C1]
)  AS [GroupBy1]

It seems to me that this should be a very simple query that executes over the base type table (Contact in this case) The model that I've included here is a watered down sample of what I'm working with. As you can imagine with a hierarchy 6 levels deep rather than 3 the SQL queries on anything other than the most derived types are very expensive.

Is there any way to tweek the query, or the model definition to reduce the unnecessary complexity of this query.

回答1:

It's arguably a bug, and will be fixed RSN.

来源：https://stackoverflow.com/questions/4126668/when-quering-over-a-base-type-why-does-the-ef-provider-generate-all-those-union

标签

linq

linq-to-entities