ClickHouse Runs Regular Expressions Four Different Ways
ClickHouse analyses a regexp before running it, and often never runs RE2 at all.
Internals
ClickHouse Shares a CPU Budget Between Queries with Fair CPU Slots
ClickHouse shares a global CPU-slot budget between queries with a max-min-fair scheduler, where each query’s slot ceiling is mutable and raised only when the pipeline finds real …
ClickHouse's Hash Join Stores the Right Table as 64-bit Row Words
The ClickHouse hash join keeps the right table as blocks and stores one tagged 64-bit row word per hash-table cell.
ClickHouse's Merge Join Walks Two Sorted Streams and Skips Whole Runs
ClickHouse’s merge join advances two sorted cursors in lockstep and uses a fused compare-and-run-search to skip whole equal-key runs.
ClickHouse's Native Protocol Is a Half-Duplex State Machine
ClickHouse’s TCP handler interleaves sending results with reading the next command on a timer, piggybacking logs and progress onto the output stream.