Field note · February 22, 2024
Pattern: Sessionisation
Turn a stream of timestamped events into sessions by flagging the gaps and cumulative-summing the flag.
The shape: Cumulative sum of a 0/1 "new group starts here" flag becomes a group id. — that is Sessionisation, and it comes up more than it should.
Turn a stream of timestamped events into sessions by flagging the gaps and cumulative-summing the flag.
What makes it a pattern rather than a tip is that the wrong version is the one you write naturally. It reads correctly, it runs, and it returns something. The failure is in the result, not in the execution — which means the only defence is recognising the shape before you are in it.
Two datasets on this site have the shape built in: ride-hail-trips and support-tickets. Both are small enough to run the broken version, see the number, then run the corrected one and see it change.
It lives under shaping because the fix is in how the rows are arranged, not in the arithmetic. Almost every "the number is wrong" report that turns out to be real lands here.
The long-form treatment is in the course (window-functions, sql-interview-patterns); the pattern page is the version to read at 4pm with a query open.
If you have a better formulation of this one, the repository takes issues. Several entries there are sharper than what we started with because someone pushed back.