Understanding .AsEnumerable() in LINQ to SQL

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The reason for AsEnumerable is to

AsEnumerable(TSource)(IEnumerable(TSource))
can be used to choose between query
implementations when a sequence
implements IEnumerable(T) but also has
a different set of public query
methods available

So when you were calling the Where method before, you were calling a different Where method from the IEnumerable.Where. That Where statement was for LINQ to convert to SQL, the new Where is the IEnumerable one that takes an IEnumerable, enumerates it and yields the matching items. Which explains why you see the different SQL being generated. The table will be taken in full from the database before the Where extension will be applied in your second version of the code. This could create a serious bottle neck, because the entire table has to be in memory, or worse the entire table would have to travel between servers. Allow SQL server to execute the Where and do what it does best.

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