A Deep Dive into PostgreSQL: Exploring the Parser and Key Components

Discovering PostgreSQL: Examining the Parser and Key Structural Elements


3 min read

Understanding the internals of PostgreSQL, especially the parser, provides deep insights into how this powerful database management system processes and executes SQL queries. PostgreSQL's architecture is modular and consists of several components that interact closely to handle various aspects of database operations. Here, I will focus on the overall architecture and then dive specifically into how the PostgreSQL parser is implemented.

Components of PostgreSQL Internals

PostgreSQL's architecture can be broadly divided into several key components:

  1. Parser: Responsible for parsing SQL queries into parse trees.

  2. Rewriter: Transforms parse trees by applying rewrite rules for views, rules, and subqueries.

  3. Planner/Optimizer: Converts parse trees into query plans which detail the specific steps to execute the queries, including which indexes to use and how to join tables.

  4. Executor: Executes the query plan using the runtime engine, fetching data from the storage system and performing operations like joins, sorts, and aggregates.

  5. Transaction Manager: Manages transaction processing, ensuring ACID properties.

  6. Lock Manager: Handles locking to maintain consistency and concurrency.

  7. WAL (Write Ahead Logging) System: Ensures data integrity and supports crash recovery.

  8. Background Writer: Reduces disk I/O contention and improves system performance.

  9. Statistics Collector: Collects data about database activity, which the planner uses to make more informed decisions.

PostgreSQL Parser Implementation

The PostgreSQL parser is a crucial part of the query processing pipeline. It takes SQL text input and transforms it into a parse tree that represents the structure of the SQL statement. Here’s how the parser is implemented:

1. Lexical Analysis (Scanner)

The parser starts with the lexical analysis performed by a scanner, generated using a tool like flex. This scanner reads the SQL input and breaks it down into tokens (lexical units such as keywords, identifiers, operators, etc.).

2. Syntax Analysis (Parser)

The output from the scanner is fed into the parser, which is responsible for syntax analysis. PostgreSQL uses a bottom-up parser generated by Bison, which works by recognizing the SQL grammar's productions. If the input tokens form a valid SQL statement according to PostgreSQL’s SQL grammar, the parser constructs a parse tree representing the statement's structure.

  • Parse Tree: Each node of the parse tree represents a construct from the SQL query, such as SELECT, INSERT, expressions, function calls, etc.

3. Grammar and Syntax Rules

The parser is governed by a set of grammar rules defined in a Bison grammar file (gram.y). These rules determine how tokens should be combined to form valid SQL constructs. Adjusting these rules can extend or modify the SQL dialect that PostgreSQL understands.

4. Error Handling

The parser also includes error handling mechanisms to catch and report syntax errors, offering hints and position information to help users correct their SQL queries.

5. Extending the Parser

To "hack" or modify the PostgreSQL parser:

  • Modify the Grammar: You can add new grammar rules or modify existing ones in the gram.y file.

  • Regenerate the Parser: After changes, the parser needs to be regenerated using Bison.

  • Recompile PostgreSQL: The entire PostgreSQL server must be recompiled for changes to take effect.


Understanding the PostgreSQL parser is key for developers looking to contribute to PostgreSQL or develop advanced features that require deep integration. The parser not only ensures that SQL queries are syntactically correct but also plays a pivotal role in the effective execution of queries by producing well-structured parse trees that subsequent components can efficiently process.

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