Rewrite vs Redirect from ?p={ID}

Question

WordPress does a 301 from http://example.com/?p=100 to http://example.com/post-title/, but I thought a URL rewrite was different from a redirect and therefore this wouldn't result in a 301.

Can anyone clarify?

Answer 1

A rewrite enables http://example.com/post-title/ to translate internally to the proper query vars to load the requested post. The shortlink does a 301 redirect to the pretty URL because it should not be possible to access the same content from more than one URI. This is so a search engine won't index the same post under two URIs.