clickhouse
https://github.com/yandex/clickhouse
C++
ClickHouse is a free analytic DBMS for big data.
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- Issues
- uniqExact state preallocates 16 cells (~250 bytes) per group; dominates high-cardinality GROUP BY with tiny groups (ClickBench Q14 ~25%)
- Iceberg commit retries a failed metadata write as a lost race: `OPTIMIZE TABLE ... MANIFEST` ends in `LOGICAL_ERROR` after 100 attempts
- Fix `JSON_VALUE`, `JSON_QUERY` and `JSON_EXISTS` taking a JSONPath array index modulo 2^32
- In-order aggregation barely scales with max_threads: FinishAggregatingInOrderAlgorithm requests data from one input per round and blocks on it
- CI: warn instead of failing the build profile hook when the CI logs cluster does not answer
- `Float32` to `Decimal128`/`Int128` and wider saturates at the `Int64` range
- `UniqueKeyTxnCommit::insert` skips `max_temporary_table_size_bytes_*` check
- `ROWS` windows and 1-row partitions 10-19% slower: unused peer-group end search
- min/max/count from column statistics is disabled by any WHERE, even for parts that the index analysis proves fully match the filter
- Partial result on select without initimation when data part is corrupted.
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- C++ not yet supported