How the SQLite Virtual Machine Works

SQLite is a unique embedded database that not only has a transactional, b-tree storage layer but also includes a robust SQL execution engine. This article delves into the process of parsing, optimizing, and executing SQL queries in SQLite using an analogy of a sandwich-making machine. The first step involves tokenizing or lexing the input query to group characters together into meaningful tokens such as SELECT or FROM. Then, the parser builds a structure called an Abstract Syntax Tree (AST) that represents the parsed query. After parsing, SQLite uses statistics about its tables' contents to determine the optimal way to execute a query and avoid full table scans. Finally, SQLite executes the optimized plan using a virtual machine approach with domain-specific commands for database operations. Understanding this process can help improve application performance by reusing prepared statements and analyzing execution plans.