> If someone on HN knows how to do it, they will jump in and tell me exactly why it's "trivial,"

n.b. to you and your parent comment's commenter: you're the only two who used the word trivial in the entire comments section modulo another comment not in this thread that contains "SQL in theory makes this trivial...". The HNer claiming to build a streaming median function in a weekend fantasy might not come to fruition :(