The Postgres extension `pg_graphql` reads the SQL schema in a database and exposes it as a GraphQL schema through a SQL function `graphql.resolve(...)`. The GraphQL interface is made available to any programming language without requiring additional servers, processes, or libraries. When a client sends a GraphQL request to `pg_graphql`, it gets parsed into an abstract syntax tree (AST) using the `graphql-parser` crate. The AST is then validated against the reflected GraphQL schema to ensure its correctness and executed by transpiling it into SQL. pg_graphql uses caching and memoization to optimize performance, and it provides a JSON response to the client after executing the query. Understanding how `pg_graphql` works internally can help developers make informed decisions about using GraphQL APIs and contribute to its development.