clickhouse
https://github.com/yandex/clickhouse
C++
ClickHouse is a free analytic DBMS for big data.
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- Issues
- `NOT (coalesce(a, b) > c) OR ...` loses rows with combined skip indexes
- Under concurrent load GROUP BY loses the adaptive aggregator: one-thread plans are rejected (Q32 at max_threads=1 −61%, ClickBench 10-client QPS ×1.35)
- Parquet: a file whose row groups are all skipped by statistics still builds the PREWHERE steps; skipping that makes ClickBench Q43 over 100 files about 7% faster with 14% less CPU
- `SET profile` / `SETTINGS profile` / `?profile=` bypass `CONST` constraints for users defined in users.xml, contrary to the constraints docs
- Iceberg tables are much slower than MergeTree in the performance tests: subcolumns, sort order, join planning, position deletes
- Do not look up a stream file for the size hints of every granule of a Compact part
- Parquet ORDER BY <String> LIMIT with s != '' skips no row groups because every minimum is ''; bounding them by their dictionary makes ClickBench Q26 12-16% faster
- Fix ZNONODE race when reading StorageKafka2 topic partition locks
- parseDateTime resets month and day when a weekday follows the date
- Parquet: decoding dictionary indexes under a PREWHERE filter re-scans the filter for every run; one bitmask per call makes ClickBench Q31 and Q12 7-10% faster
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