ClickHouse Turns an Out-of-Memory Into an Exception: Code 241, Not a Killed Process
When a ClickHouse query exceeds its memory budget, the allocation is refused with a MEMORY_LIMIT_EXCEEDED exception.
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ClickHouse Intercepts Every Allocation: It Charges the Size Class, Not the Request
ClickHouse replaces operator new and interposes the libc malloc symbols so every allocation is charged.
ClickHouse Batches Memory Charges Per Thread: 4 MiB Before It Touches the Tracker
ClickHouse does not charge the memory tracker on every allocation; each thread batches up to 4 MiB locally.
ClickHouse Shares One AST Between Threads: 32-Byte Nodes and an Iterative Destructor
The ClickHouse AST is a tree of 32-byte reference-counted nodes with a bit-packed flag word and an iterative destructor.
ClickHouse's Lexer Never Allocates: A Token Is Two Pointers Into the Query Buffer
ClickHouse tokenizes SQL with a pointer-arithmetic lexer that never allocates and knows nothing about keywords.