project() isn't considered with index selection.
select $p from <graph:data> where $s $p $o; uses the GSPO index which requires a full external sort of the store to extract $p. If we use GPOS we get $p without the sort.
The problem is that project() call doesn't participate in index ordering within the query optimisation.
The solution is to introduce an Annotation that allows the project to specify a preferred order. Initially supporting this Annotation with StatementStoreResolution will be sufficient to satisfy the query above. A general solution will also require supporting it with Join and Append tuples.