Four schemes measured: monthly share, day-15 share, one-sensor fan-out
Four schemes, two clocks, one query
monthly totals hide the instantaneous hotspot — measure both, then count fan-out
scheme
monthly max share
day-15 max share
one-sensor fan-out
ts-range
13.5%
100% today
8 shards
sensor:ts
47.2%
47.0% today
1 shard
hash(sensor)
48.5%
48.1% today
1 + full scan
hash(full)
12.6%
13.1% today
8 shards
teaching numbers: 120k writes, 51 sensors, sensor-0 at 40%, 8 shards, fixed seed
pink cell: even monthly totals, yet 100% of one day on one shard — the hot-shard trap
legend: even writes cost fan-out; locality costs skew — pick the cheap side for your queries