ClickHouse Parses SQL by Rewinding One Cursor: A 1,000,000-Backtrack Budget
ClickHouse’s SQL parser is a set of combinators that share one position, rewind it on failure, and budget every rewind.
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ClickHouse Resolves Identifiers While Parsing: The Table Qualifier Is Only a Guess
ClickHouse’s parser unquotes identifiers, splits compound names, guesses the table qualifier and builds parameter nodes before the analyzer runs.
ClickHouse's Aggregate State Lives in an Arena: One Padded Block per Group, Destroyed by Hand
ClickHouse aggregate functions own no data; their state lives in a bump-allocated Arena and is destroyed by hand, one contiguous block per group.
ClickHouse Builds Aggregate Functions from Name Suffixes: A Decorator Forwards Almost Everything
ClickHouse builds new aggregate functions by suffixing names: a factory strips the suffix, recurses into the base function, and wraps it in a decorator.
ClickHouse's topK Bounds Its Own Error: Filtered Space-Saving and an O(1) Alpha Map
ClickHouse’s topK uses Filtered Space-Saving: each candidate keeps a count and an error bound, and the least accurate counters are dropped when the table overflows.