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.
Internals
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.
ClickHouse's Thread Pool Wakes the Newest Idle Thread and Counts Events Per CPU
ClickHouse’s thread pool wakes the most recently idle thread, and its counters are sharded per CPU so that counting never costs a cache-line ping-pong.
ClickHouse's Token-Bucket Throttler Sleeps Without Burning a CPU Slot
ClickHouse’s throttler is a token bucket that sleeps for the token deficit and parks the caller’s CPU lease so the wait need not consume concurrency budget.