ClickHouse Parses SQL with a Backtracking Recursive-Descent Parser
ClickHouse’s SQL parser is a set of combinators that share one position, rewind it on failure, and budget every rewind.
Ast
ClickHouse Resolves Identifiers While It Parses
ClickHouse’s parser unquotes identifiers, splits compound names, guesses the table qualifier and builds parameter nodes before the analyzer runs.
ClickHouse Shares One AST Between Threads
The ClickHouse AST is a tree of 32-byte reference-counted nodes with a bit-packed flag word and an iterative destructor.