问LINQ Join运算符是否使用嵌套循环、Merge或HashSet Join？

EN

public static IEnumerable<TResult> Join<TOuter, TInner, TKey, TResult>
    (this IEnumerable<TOuter> outer,
     IEnumerable<TInner> inner,
     Func<TOuter, TKey> outerKeySelector,
     Func<TInner, TKey> innerKeySelector,
     Func<TOuter, TInner, TResult> resultSelector)
{
    Lookup<TKey, TInner> lookup = inner.ToLookup(innerKeySelector);
    foreach (TOuter outerItem in outer)
    {
        TKey key = outerKeySelector(outerItem);
        foreach (TInner innerItem in lookup[key])
        {
            yield return resultSelector(outerItem, innerItem);
        }
    }
}