clickhouse
https://github.com/yandex/clickhouse
C++
ClickHouse is a free analytic DBMS for big data.
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- Issues
- `optimize_extract_common_expressions` rejects valid `JOIN ON`: cast to `Nothing`
- Missing combinations of TopN dynamic filtering and the query condition cache for Parquet over object storage, data lakes and URL
- Parquet export with Snappy fails on rows over ~1.71 GiB: CANNOT_COMPRESS
- MergeTreeReaderWide recomputes stream file names (getStreamNameForColumn) on every read: 7-15% of time with small blocks
- Per-block overhead in MergeTreeReadersChain: getColumnsForPatches clones a Block without patch parts, NameSet and shrinkToFit per block
- MergeTreeSource::tryGenerate formats an OpenTelemetry span name per block even when tracing is disabled
- ExecutingGraph::updateNode costs 9-21% with small blocks: per-call heap allocations, map lookup and locks
- High per-block cost of function execution: default-implementation layers and type dispatch cost ~8 µs per Filter block
- Per-chunk bookkeeping: prepareAggregateInstructions, memory tracker check and progress reporting on every block
- Views and MVs ignore CTE column alias list, inlining body unaliased
- Docs
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