Does LINQ-to-SQL Support Composable Queries?

问题:

Speaking as a non-C# savvy programmer, I'm curious as to the evaluation semantics of LINQ queries like the following:

var people = from p in Person
             where p.age < 18
             select p

var otherPeople = from p in people
                  where p.firstName equals "Daniel"
                  select p

Assuming that Person is an ADO entity which defines the age and firstName fields, what would this do from a database standpoint? Specifically, would the people query be run to produce an in-memory structure, which would then be queried by the otherPeople query? Or would the construction of otherPeople merely pull the data regarding the query from people and then produce a new database-peered query? So, if I iterated over both of these queries, how many SQL statements would be executed?

回答1:

They are composable. This is possible because LINQ queries are actually expressions (code as data), which LINQ providers like LINQ-to-SQL can evaluate and generate corresponding SQL.

Because LINQ queries are lazily evaluated (e.g. won't get executed until you iterate over the elements), the code you showed won't actually touch the database. Not until you iterate over otherPeople or people will SQL get generated and executed.

回答2:

Yes, the resulting query is composed. It includes the full where clause. Turn on SQL profiling and try it to see for yourself.

Linq does this through expression trees. The first linq statement produces an expression tree; it doesn't execute the query. The second linq statement builds on the expression tree created by the first. The statement is only executed when you enumerate the resulting collection.

回答3:

var people = from p in Person
             where p.age < 18
             select p

Translates to:

SELECT [t0].[PersonId], [t0].[Age], [t0].[FirstName]
FROM [dbo].[Person] AS [t0]
WHERE [t0].[Age] < @p0

where @p0 gets sent through as 18

var otherPeople = from p in people
                  where p.firstName equals "Daniel"
                  select p

Translates to:

SELECT [t0].[PersonId], [t0].[Age], [t0].[FirstName]
FROM [dbo].[Person] AS [t0]
WHERE [t0].[FirstName] = @p0

where @p0 gets sent through as "Daniel"

var morePeople = from p1 in people
                 from p2 in otherPeople
                 where p1.PersonId == p2.PersonId
                 select p1;

Translates to:

SELECT [t0].[PersonId], [t0].[Age], [t0].[FirstName]
FROM [dbo].[Person] AS [t0], [dbo].[Person] AS [t1]
WHERE ([t0].[PersonId] = [t1].[PersonId]) AND ([t0].[Age] < @p0) AND ([t1].[FirstName] = @p1)

where @p0 is 18, @p1 is "Daniel"

When in doubt, call the ToString() on your IQueryable or give a TextWriter to the DataContext's Log property.

回答4:

people and otherPeople contain objects of type IQueryable<Person>.

If you iterate over both, separatly, it will run two queries. If you only iterate over otherPeople, it will run the expected query, with two where clauses.

If you do .ToList() on people and use the returned List<Person> in the second query instead of people, it becomes LINQ-to-Objects and no SQL is executed.

This behavior is referred to as deferred execution. Meaning no query is done until it is needed. Before execution they are just expression trees that get manipulated to formulate the final query.

回答5:

Both these queries will be executes when you'll try to access final results. You can try to view original SQL generated from DataContext object properties.