clickhouse
https://github.com/yandex/clickhouse
C++
ClickHouse is a free analytic DBMS for big data.
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- Issues
- TimeSeries: order series by job, series kind and host in new tables: per-host, per-node and per-job PromQL selectors read fewer granules
- count() over file/url/S3 re-emits a cached row count as max_block_size chunks; one chunk per file makes ClickBench q1 on Parquet 13x faster
- Parquet ORDER BY x LIMIT n: with one sort column, row groups whose minimum equals the top-K threshold are still read; skipping them makes ORDER BY EventDate LIMIT 10 27% faster
- Object storage: a text search analyses the index of only max_threads parts at a time; building the next parts' index results in advance while it runs alone makes cold searches up to 50% faster
- Docs: publish nightly reference snapshot
- A `Bool` type hint over an integer column keeps the stored value: `x = true` misses rows that `x IN (true)` and `WHERE x` return
- 26.3: Propagate current roles to remote read nodes (backport #110867 + #121643)
- SETTINGS inside a UNION ALL leg or a subquery is ignored by query plan optimizations
- Do not return columns that only PREWHERE uses from file and url reads
- Parquet: one file is decoded by a single thread because each read stage gets 1/5 of the parsing threads (ClickBench q21 -48%, q24 -54%)
- Docs
- C++ not yet supported