c# - LINQ Join 的内存使用情况是多少？

Question

1. 如果两个 IEnumerable(s) 在内存中，连接它们会占用多少内存？假设选择所有列。是左表大小+右表大小+联表行数吗？

2. 如果两个 IEnumerable(s) 是由文件流定义的，如果它们太大而无法放入内存，Join 是否会抛出内存不足异常？或者它会一直加载直到接近内存不足并多次运行扫描(类似于数据库连接)吗？

Answer 1

在 Reflector 中查看 Enumerable.JoinIterator，似乎整个 inner(第二个)序列被加载到内存中(到 Lookup )，但外部(第一个)序列随后被流式传输(即不是全部加载到内存中)。

如果 inner 序列太大而无法放入内存，似乎没有什么聪明的事情发生。

Jon Skeets seems to agree on the first point :

The real Join operator uses the same behaviour as Except and Intersect when it comes to how the input sequences are consumed:

...

• When MoveNext is called on the result sequence for the first time, it immediately consumes the whole of the inner sequence, buffering it.

• The outer sequence is streamed - it's only read one element at a time. By the time the result sequence has started yielding results from the second element of outer, it's forgotten about the first element.