CombinedProjection	Combines the things you need to turn a query into SQL and turn its results into `QueryResults`: SQL-related projection information (`DISTINCT`, columns, etc.) and a Datalog projector that turns SQL into structures.
ConstantProjector	A projector that produces a `QueryResult` containing fixed data. Takes a boxed function that should return an empty result set of the desired type.
QueryOutput
RelResult	The result you get from a 'rel' query, like:

Enums

QueryResults
SimpleAggregationOp

Traits

projected_column_for_var	Return the projected column -- that is, a value or SQL column and an associated name -- for a given variable. Also return the type. Callers are expected to determine whether to project a type tag as an additional SQL column.
query_projection	Compute a suitable SQL projection for an algebrized query. This takes into account a number of things: