Add Dataset#row_number_column to DRY up some adapter code and for fut…

…ure eager limit strategy use

lib/sequel/adapters/jdbc/as400.rb

@@ -88,13 +88,6 @@ def select_limit_sql(sql)
         def supports_window_functions?
           true
         end
-
-        private
-
-        # The alias to use for the row_number column when emulating LIMIT and OFFSET
-        def row_number_column
-          :x_sequel_row_number_x
-        end
       end
     end
   end

lib/sequel/adapters/shared/db2.rb

@@ -267,11 +267,6 @@ def literal_true
         BOOL_TRUE
       end
 
-      # Holds the ROW_NUMBER value, used in offset emulation.
-      def row_number_column
-        :x_sequel_row_number_x
-      end
-
       # Add a fallback table for empty from situation
       def select_from_sql(sql)
         @opts[:from] ? super : (sql << ' FROM "SYSIBM"."SYSDUMMY1"')

lib/sequel/adapters/shared/mssql.rb

@@ -519,11 +519,6 @@ def literal_true
         BOOL_TRUE
       end
 
-      # The alias to use for the row_number column when emulating OFFSET
-      def row_number_column
-        :x_sequel_row_number_x
-      end
-
       # MSSQL adds the limit before the columns
       def select_clause_methods
         SELECT_CLAUSE_METHODS

lib/sequel/dataset/misc.rb

@@ -140,6 +140,12 @@ def inspect
       "#<#{self.class}: #{sql.inspect}>"
     end
 
+    # The alias to use for the row_number column, used when emulating OFFSET
+    # support and for eager limit strategies
+    def row_number_column
+      :x_sequel_row_number_x
+    end
+
     # Splits a possible implicit alias in +c+, handling both SQL::AliasedExpressions
     # and Symbols.  Returns an array of two elements, with the first being the
     # main expression, and the second being the alias.