PlanetScale, a database service built on top of Vitess, has demonstrated its ability to scale horizontally through sharding, achieving a goal of one million queries per second. The company ran benchmarks using the tpc-c sysbench workload and found that adding more shards increased throughput capacity, but eventually reached diminishing returns due to resource saturation. By doubling the number of shards, PlanetScale was able to exceed the target query volume, with 40 shards providing over 1 million QPS sustained for a 5-minute run. This showcases the scalability of PlanetScale's database service and highlights its ability to handle large query volumes predictably through horizontal scaling.