ExtSQL is an open-source database server designed to bring the power of SQL databases to file-based storage systems. It is developed using Go language and provides a way to query and manage structured data stored in plain text files.

Some key features of ExtSQL include:

1. SQL compatibility: ExtSQL supports a subset of the SQL language, allowing users to write queries and perform operations on the data stored in the files.

2. File-based storage: Instead of using traditional database management systems, ExtSQL leverages the file system to store and organize data. This makes it easy to work with plain text files and eliminates the need for a dedicated server or complex setup.

3. Speed and efficiency: ExtSQL is designed to be fast and efficient, providing quick query execution and data retrieval. It achieves this by using indexes, caching, and other performance optimization techniques.

4. Simple setup: Setting up an ExtSQL server is straightforward, as it does not require any complex installation or configuration. Users only need to provide the necessary files and define the schema before they can start querying the data.

5. Scalability: ExtSQL supports horizontal scalability, allowing users to distribute the data across multiple servers to handle larger loads and improve performance.

Being an open-source project, ExtSQL encourages contributions from the community, giving developers the opportunity to extend its functionality and customize it according to their specific requirements.

Overall, ExtSQL provides a lightweight and flexible solution for working with structured data in file-based storage systems, making it a viable choice for various use cases where traditional SQL databases may not be suitable.