Milvus vector search with scalar filter

[Roy-1224]

I know that Milvus supports vector query and scalar filter which enables us to do ANN on a specific subset of dataset.
I'm just wondering how does Milvus support this feature on the architecture level?
As far as I know, we can do post filter (search for a bunch of nearest neighbor and filter out top K that satisfy the scalar filter). However this method seems not efficient. I failed to Google the answer/article that help me understand it.
Can anyone share some brief intro/articles that can help me get a little deeper understanding on this?
Many thanks!