Understanding SQL Parser and Its Implementation in Apache ShardingSphere

SQL is the de‑facto standard for data processing in modern computing, and precise manipulation of SQL statements requires a robust SQL parser. Apache ShardingSphere, an open‑source distributed database middleware, provides its own independent parser engine to support rewriting, encryption, and other advanced operations.

The parser consists of a lexer that tokenizes the input SQL and a parser that builds an abstract syntax tree (AST). The lexer typically implements a deterministic finite automaton (DFA) to recognize identifiers, numbers, and operators, exposing a nextToken() interface that returns tokens sequentially.

Parsing relies on context‑free grammar (CFG) concepts: terminal and non‑terminal symbols, productions, start symbol, and derivations. An AST represents the hierarchical structure of a SQL statement, which can be transformed into a logical execution plan and then into a physical plan for execution by the storage engine.

Two classic parsing strategies are illustrated with a simple expression example:

LL(1) (top‑down) parsing scans the input left‑to‑right and expands productions left‑to‑right, selecting a rule based on the next token.

LR(1) (bottom‑up) parsing also scans left‑to‑right but reduces token sequences to non‑terminals, building the parse tree from leaves to the start symbol.

ShardingSphere’s parser is generated using ANTLR, which creates lexer and parser code from grammar definitions, offering high performance, extensibility, and easy maintenance. The MySQL parser module resides in shardingsphere‑sql‑parser‑mysql under src/main/antlr .

Key features of the ShardingSphere parser include:

Standalone SQL parsing engine.

Simple extension and modification of grammar rules.

SQL parameterization and formatting capabilities.

Support for multiple dialects (MySQL, PostgreSQL, SQLServer, Oracle, SQL92).

Caching mechanisms and customizable visitors.

Other common parsers are compared: MySQL parser (hand‑written lexer, Bison‑generated parser), PostgreSQL parser (Flex/Bison), TiDB parser (Goyacc), and various Java/JS parsers (JSqlParser, Druid, JQLParser). Differences in parsing algorithms (LL vs. LR, LL(*) vs. LALR) and their expressive power are discussed.

The AST is further used to generate logical and physical execution plans, perform SQL formatting, and apply transformations such as encryption or sharding rules.

References are provided for LL(*), LALR parsing, TiDB optimizer design, and MySQL optimizer design. The article concludes with author information and a recruitment notice for Apache ShardingSphere contributors.