go-mysql-server is the query engine for Dolt. It's a MySQL compatible parser, server, and query execution engine written in pure Go. As with Dolt, its goal is to be a 100% compatible drop-in replacement for MySQL.
go-mysql-server is storage engine agnostic, which means other projects can write their own storage engine plugins to query them via a MySQL connection. There are only two notable backend implementations so far:
In-memory database. This ships with go-mysql-server and is useful for testing the engine, or for using in tests for golang projects that want a fast, local MySQL database.
Dolt. In addition to the novel git-like storage engine, Dolt also adds a number of system tables, custom functions, and new connection semantics.
go-mysql-server defines a number of interfaces to allow integrators to communicate about their database's capabilities, what tables exist, what the schemas of tables are, and so on. Then another set of interfaces let integrators iterate over their table rows, update them with new values, delete them, and so on. For more information, see the project docs.
Broadly, the system has three main components:
The parser and server, which are mostly provided by Vitess. This component receives queries on the wire and parses them into an AST.
The query analyzer, which repeatedly transforms the AST to determine an optimal execution strategy, doing things like applying indexes or ordering tables for a join.
The execution engine, which iterates over table rows provided by integrators, evaluates expressions and functions, and produces the ultimate result set.
For much more technical details on how these pieces function, refer to the following blog articles: