Query DAG: Single query (syntactically), multiple queries (in practice)

[rbalicki2]

There are several seemingly unrelated features that can be solved with the same feature:

• Supporting queries with selections from multiple backends ({ sql_foo graphql_bar }), which compiles into multiple queries
• The ability to "jump" to the root, e.g. (field User.DisplayName { __root { viewer { name } } name }, e.g. to distinguish between "this user is the viewer" and vice versa), which also compiles into multiple queries (at least if the path to the User includes a nullable field)
• The ability to thread results from one field to another, e.g. (with made-up syntax) { viewer { height @name(variableName: "height") } jealousOf: usersTallerThan(height: $height) { name } }, which also compiles into multiple queries (in GraphQL)
• Perhaps, @loadable fields (which are a simple version of the above.)

The common feature is that these appear to be a single query (syntactically), but compile into a DAG of queries.