ClickHouse's SLRU Cache Gives Hot Entries a Second Queue
ClickHouse’s default cache policy is SLRU, not LRU: entries hit twice are promoted to a protected queue that a sequential scan cannot flush.
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How ClickHouse Batches Async Inserts in a Per-Key Queue
ClickHouse hashes each async insert into a per-key buffer and flushes it on size, count, or an adaptive deadline.
How ClickHouse Maps an INSERT onto the Table's Schema
ClickHouse resolves an INSERT’s columns by name into a sample block, then squashes, dedup-stamps and fans the data out to the sinks.
How ClickHouse Picks a Projection to Read
ClickHouse scores each projection by the marks it must read, including the parts it cannot serve, and picks the cheapest.
How ClickHouse Recompresses a Part When Its TTL Expires
A RECOMPRESS TTL rewrites a whole part with a new codec once all of its rows have expired.