问题
We are implementing some EF data repositories, and we have some queries which would include TOP 1
I have read many posts suggesting to use .Take(1)
The code I'm reviewing uses .First()
I understand that both of these produce the same result for the object assignment, but do they both actually resolve to the same query? When the DB is queried, will it actually be with TOP 1 for both requests? Or will they execute the query in full into the enumerable, then simply take the first entry in the collection?
Furthermore, if we used .FirstOrDefault() then why should we expect any different behavior? I know that when using an IEnumerable, calling .First() on an empty collection will throw, but if this is actually only changing the query to include TOP 1 then I should expect absolutely no functional difference between .First() and .FirstOrDefault().... right?
Alternatively, is there some better method than these Enumerable extentions for making the query execute TOP 1?
回答1:
From LINQPad:
C#:
age_Centers.Select(c => c.Id).First();
age_Centers.Select(c => c.Id).FirstOrDefault();
age_Centers.Select(c => c.Id).Take(1).Dump();
SQL:
SELECT TOP (1) [t0].[Id]
FROM [age_Centers] AS [t0]
GO
SELECT TOP (1) [t0].[Id]
FROM [age_Centers] AS [t0]
GO
SELECT TOP (1) [t0].[Id]
FROM [age_Centers] AS [t0]
*Note that Take(1) enumerates and returns an IQueryable.
回答2:
Redirect the DataContext Log property to Console.Out or a TextFile and see what query each option produces.
回答3:
**First()** operates on a collection of any number of objects and returns the first object. **Take(1)** operates on a collection of any number of objects and returns a collection containing the first object.
You can also use Single Single() operates on a collection of exactly one object and simply returns the object.
回答4:
How .First() works:
If the collection is of type IList, then the first element is accessed by index position which is different depending on the collection implementation. Otherwise, an iterator returns the first element.
And .Take(int count) always iterate.
If there's any gain, it happens if the collection implements IList and the speed to access the first element by index is higher than that of returning an iterator. I don't believe it will be significant. ;)
Sources:
http://www.hookedonlinq.com/FirstOperator.ashx
http://www.hookedonlinq.com/TakeOperator.ashx
回答5:
First will query Take 1, so there is no difference in query. Calling FirstOrDefault will be one step statement, because Take returns IEnumerable do you will need to call First anyway.
First will throw exception so FirstOrDefault is always preferred.
And ofcourse people who wrote EF query converter are smart enough to call Take 1 instead executing entire result set and returning first item.
You can this verify using SQL profiler.
来源：https://stackoverflow.com/questions/10455414/with-entity-framework-is-it-better-to-use-first-or-take1-for-top-1