Add the ga_sessions table and ga_events.session_id.
Sessions are server-derived per-visitor visits (30-min sliding inactivity + acquisition-change boundary). ga_sessions is a mutable, NON-partitioned OLTP row UPDATEd in place on every event, unlike append-only ga_events.
Summary
Functions
Rolls back this version's migration by executing the down SQL file.
Applies this version's migration by executing the up SQL file.