I wrote a 70x faster SQL parser while barely looking at the code

The author undertakes the ambitious task of rewriting PostHog's SQL parser using AI assistance through multiple long-running Claude Code sessions, resulting in a significant performance boost and a new parser that is 454 times faster than its predecessor. The undertaking involves creating a "hand"-written recursive-descent parser with a Pratt expression core, achieving complete agreement with the existing C++ ANTLR-based parser for realistic SQL queries. The approach combines property-based testing with code-generation tools to create inputs, alongside prompt engineering to address AI's brittle fixes, ultimately leading to a robust parser that can withstand rigorous testing. The new parser was developed in Rust, leveraging AI capabilities to achieve a task that would have traditionally taken months, illustrating a potential shift in how parsers might be developed in the future, with AI-generated high-performance parsers complementing traditional parser generators like ANTLR.

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