What is the restful approach to compose a group by query

I don't think what you are asking for is possible.

If the client is expecting a collection of defects, then you can order by priority, but

a group of defects != a defect

So you can't return a collection of groups.

You could add a nasty hack to add attributes to the defect resource for priority and priority-group-size, but that seems bad to me.

I think the correct Restful ways to do it are:

• Design the service to return groups of defects

or

• Return defects ordered by priority, and let the client do the grouping and counting of the resources in each group.