ClickHouse Aggregates In Order and Stops Reading Early
If your table is sorted by the GROUP BY key, ClickHouse can skip the hash table entirely: it counts each contiguous run of equal keys and emits it.
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ClickHouse Decides Which Parts to Merge: A Cost Model for Write Amplification
ClickHouse picks merge candidates with a tiny, explicit cost model: score each contiguous range as sum_size / (count - 1.9), pick the cheapest admissible one, and only ever…
ClickHouse Estimates Distinct Counts by Thinning a Hash Table
uniq() is a hash table that deliberately throws data away. When it fills up, it halves its sample rate, keeps the survivors, and multiplies the count back up by the…
ClickHouse Evaluates IN with Hash Sets
IN is not a comparison, it is a hash set. ClickHouse picks one of thirteen specialisations based on the shape of the right-hand side, packs integer keys into the hash…
ClickHouse JIT-Compiles Scalar Expressions with LLVM
ClickHouse compiles hot scalar expressions to native code with LLVM.