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# QuillSQL Internals
Welcome to the technical documentation for QuillSQL.
This book documents QuillSQL as a SQL engine layered over Holt. It is intended for
developers and contributors who want to understand the parser, planner, optimizer,
Volcano executor, catalog, and transaction semantics without also maintaining a separate
page store.
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## Table of Contents
* [**Overall Architecture**](./architecture.md): A high-level overview of the entire system.
* **Core Modules**
* [**Transaction Manager**](./modules/transaction.md): Concurrency control with MVCC and 2PL.
* [**Query Plan**](./modules/plan.md): The journey from SQL to an executable plan.
* [**Query Optimizer**](./modules/optimizer.md): Rule-based plan transformations.
* [**Execution Engine**](./modules/execution.md): The Volcano (iterator) execution model.
* [**Catalog**](./modules/catalog.md): SQL metadata projection backed by Holt descriptors.