sessionization

Sessionization is the analytics primitive behind every 'how do users actually behave' question, and a favorite practical screen. The algorithm is easy; out-of-order events and duplicates are where the round is actually scored.

Sessionization is gaps-and-islands with a twist, and it shows up in both SQL screens and Spark rounds at Databricks. The LAG-plus-running-SUM pattern here solves a whole family of interview questions.