If the GraphQL server is a separate component, then a GraphQL query needs to do its own query planning, then the resolvers turn that into SQL queries to the database, then the database does its own query planning and execution for each query.
Since one GraphQL query will often turn into multiple SQL queries, there's likely to be duplicated work on the database side across those queries since they relate to the same data.
By integrating the GraphQL server into the DBMS, it can do query planning once for the whole GraphQL query, which means that it can reuse parts of the plan and prevent duplicated work and N+1 queries.
Either way it's going to have to query the database to get all the data, so the essential work is the same. But this way you have opportunities to reduce wasted work to process the whole GraphQL query.
If the GraphQL server is a separate component, then a GraphQL query needs to do its own query planning, then the resolvers turn that into SQL queries to the database, then the database does its own query planning and execution for each query.
Since one GraphQL query will often turn into multiple SQL queries, there's likely to be duplicated work on the database side across those queries since they relate to the same data.
By integrating the GraphQL server into the DBMS, it can do query planning once for the whole GraphQL query, which means that it can reuse parts of the plan and prevent duplicated work and N+1 queries.
Either way it's going to have to query the database to get all the data, so the essential work is the same. But this way you have opportunities to reduce wasted work to process the whole GraphQL query.