Differential testing against Trino caught a distributed-scan bug the whole test suite missed: every projected query failed once a scan actually distributed. The first fix restored correctness by disabling projection pushdown. Then a Claude agent on the Fable model found the real bug, one line in the worker's streaming rewrite, and got the speed back: 3.1x overall, 10x on the worst query.
ReadTen sub-MRs in a day. The wire format, the fixture-driven debugging loop, and two bugs that the C++ reference encoder ships without telling you: WriteListWithDefault elision, and uninitialised bytes at NULL VARCHAR positions.
ReadTPC-DS q72 sat at 10 seconds while every other query ran in under 1.4. Five days of investigation chased scan parallelism, range-based NDV, iceberg-rust upgrades, and the RisingWave fork. None of those were the bug. The bug was a silently-skipped Err arm in our dynamic-filter evaluator that swallowed every Int32 vs Int64 type clash. Fixing it: 15.5s to 0.77s. q72 now beats Trino.
Read