clickhouse
https://github.com/yandex/clickhouse
C++
ClickHouse is a free analytic DBMS for big data.
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- Issues
- Structure hint keeps the table alias: `INSERT ... SELECT f.id` stores zeros
- Join reordering puts 14-20M rows into hash tables in TPC-DS q4, q11 and q74: the UNION ALL filter materialize('s') = 's' is estimated as 1% selective
- Hint drops dependent `DEFAULT`s transitively: `w DEFAULT z + 10` stored as 0
- Skip preparing PREWHERE steps for Parquet files whose row groups are all pruned
- Isolate the filesystem of the server at startup
- MergeTree on a local disk reads 128 KiB at every isolated granule: end each read at the requested byte ranges (cold ClickBench Q19 -23%, SELECT * point lookup -20%)
- MergeTree PREWHERE steps filter the whole block before every next condition, even one that cannot throw; filtering only when needed makes ClickBench Q42 6-10% faster on a fully merged hits table
- Refuse a table function on the right of a `RIGHT JOIN` for parallel replicas when `additional_table_filters` is set
- Aggregate long runs of a single GROUP BY key through the constant-key path
- Unquoted alias `fetch` now fails with SYNTAX_ERROR
- Docs
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