Every Millisecond Counts

A ClickHouse performance optimization project reduced an expensive query from 85+ seconds to sub-second latency over four months through incremental improvements. The main challenges stemmed from using ReplacingMergeTree engine to handle mutable events and an inefficient monthly partition key that spread data across many partitions, forcing slow single-threaded FINAL merges. Solutions involved simplifying the table schema and repartitioning strategy rather than implementing complex optimizations.