clickhouse
https://github.com/yandex/clickhouse
C++
ClickHouse is a free analytic DBMS for big data.
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- Issues
- Codec MergeTree settings uppercase the whole expression: `Delta` unusable
- Sibling aggregate subqueries over the same tables are answered with one scan each instead of one scan total
- Check `CHECK` privilege before resolving the table
- Reject non-`Parquet` data format for DeltaLake writes
- A self-join that only needs the keys holding more than one distinct value materialises every pair: one grouped pass is 65% faster on TPC-DS query 95
- Fix a TSan report on `randomnessPid` in the vendored SQLite
- The set for an IN subquery is built once per query scope, so a CTE used by several UNION branches runs once per branch: TPC-DS query_23 -43%, query_14 -42%
- A correlated scalar min/max subquery reads its table a second time: computing it as a window aggregate over the rows already read is 39% faster on a TPC-H Q17-shaped query
- Do not substitute a join key into a predicate that can throw on the other side's rows
- Fix `escape_index_filenames` being reverted by an `ALTER` that also changes `auto_statistics_types`
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- C++ not yet supported