PostGraphile compiles GraphQL directly to SQL. The SQL is "custom" in that it's specific to the GraphQL operation, though naturally it does follow rules. For example, Hasura does the same thing, and among the rules that it follows is that it uses `LEFT LATERAL JOIN` between tables (at least, on PostgreSQL). Full disclosure, I work for Hasura, so I'm not super familiar with the style of SQL PostGraphile generates but one thing you can do is just have PostGraphile report back the generated SQL for inspection:
https://www.graphile.org/postgraphile/debugging/#via-postgra...