Why the first session matters so much?
First session is the foundation of cohorts, attribution, retention — everything. I’ve seen entire growth narratives collapse because the “first visit” was wrong.
Why session_start can’t be trusted? Because session_start doesn’t always fire!
- Mobile edge cases
- Background launches
- SDK behavior
If you rely on it, some users simply never “arrive” in your data.
What actually defines the first session?
The first session is not an event. It’s the earliest moment of user activity. Everything else is just an approximation.
How to rebuild it?
Try to look for the earliest event_timestamp per user and then attach the corresponding session.
This approach survives missing events and weird tracking — and works consistently.
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