Your ledger grows past what a single Postgres can handle. Time to shard. The shard key choice determines what queries become CHEAP and what becomes EXPENSIVE. Shard by user_id: per-user queries (wallet balance, statement) are co-located in one shard, but cross-user queries (total revenue, top customers) fan out across all shards. Shard by date: time-window queries are local but per-user history fans out. Shard by account: per-account aggregates local but transactions touching multiple accounts hit multiple shards. The right answer depends on your QUERY PATTERN; lesson 41 (partitioning) is the time-shard adjacent. This lesson posts a user-sharded entry and discusses the trade-offs.