In Query#optimize! partition required from optional patterns so that optional patterns always come last.

Author: gkellogg

lib/rdf/query.rb

@@ -247,15 +247,22 @@ def optimize(**options)
     # Optimizes this query by reordering its constituent triple patterns
     # according to their cost estimates.
     #
+    # Optional patterns have greater cost than non-optional patterns so they will always come after non-optional patterns
+    #
     # @param  [Hash{Symbol => Object}] options
     #   any additional options for optimization
     # @return [self]
     # @see    RDF::Query::Pattern#cost
     # @since  0.3.0
     def optimize!(**options)
-      @patterns.sort! do |a, b|
+      optional, required = @patterns.partition(&:optional?)
+      required.sort! do |a, b|
+        (a.cost || 0) <=> (b.cost || 0)
+      end
+      optional.sort! do |a, b|
         (a.cost || 0) <=> (b.cost || 0)
       end
+      @patterns = required + optional
       self
     end
 

lib/rdf/query/variable.rb

@@ -65,7 +65,7 @@ class Variable
     #   the variable name
     # @param  [RDF::Term] value
     #   an optional variable value
-    # @param [Boolean] distinguished (true) Also interpreted by leading '??' or '$$' in name.
+    # @param [Boolean] distinguished (true) Also interpreted by leading '?' or '$' in name. If non-distinguished, '??' or '$$'.
     # @param [Boolean] existential (true) Also interpreted by leading '$' in name
     def initialize(name = nil, value = nil, distinguished: nil, existential: nil)
       name = (name || "g#{__id__.to_i.abs}").to_s