Modern table formats such as Apache Iceberg compute and store metadata-commit timestamps, record counts, and column-level statistics such as null counts and value bounds at write time as part of file writing. These statistics serve query planning, yet they overlap substantially with data quality (DQ) monitoring needs. We describe a metadata-first approach that repurposes write-time statistics for continuous DQ observability: anomaly detection, drift monitoring, null-rate tracking; without scanning any data. Deployed at LinkedIn across 200,000+ Iceberg tables (800+ PB), this approach satisfies approximately 60% of user-defined DQ rules at zero marginal compute cost and reduces profiling resource consumption by around 50%. Extending manifest statistics with lightweight counters (sum, zero-value counts, boolean counts) and incrementally mergeable sketches; Theta sketches for distinct counts, KLL sketches for quantiles; can further raise metadata-satisfiable coverage to close to 90% of production DQ rules. We validate sketch accuracy, mergeability, and storage overhead on production data and propose that table formats should store per-file sketches in Puffin sidecar files, following the same store-then-aggregate pattern used for existing manifest statistics.
