Simple Demo: OBSERVER execution

This is the mapping as ERA tree for the query:

This is the mapping information for the query:

[ JOIN
   [ UNION
      [ UNION
         [ PROJECTION
            [ F-TRANS
               dbHotels.hotels
               [  Price ] 
                 [ observer.transformFunctions.Currency.dolar2euro ]
            ]
            [  dbHotels.hotels.name dbHotels.hotels.price ]
         ] ( T1 )
         [ PROJECTION
            [ F-TRANS
               XMLHotels.hotels
               [  Price ] 
                 [ observer.transformFunctions.Currency.dolar2pound ]
            ]
            [  XMLHotels.hotels.name XMLHotels.hotels.price ]
         ] ( T2 )
          ( T1.Name T2.Name ) ( T1.Price T2.Price )
      ] ( T3 )
      [ PROJECTION
         [ F-TRANS
            WebSearchHotels.hotels
            [  Price ] 
              [ observer.transformFunctions.Currency.dolar2dkcrown ]
         ]
         [  WebSearchHotels.hotels.name WebSearchHotels.hotels.price ]
      ] ( T4 )
       ( T3.Name T4.Name ) ( T3.Price T4.Price )
   ] ( ALIAS6 )
   [ JOIN
      [ UNION
         [ UNION
            [ PROJECTION
               dbHotels.hotels
               [  dbHotels.hotels.name dbHotels.hotels.location ]
            ] ( T1 )
            [ PROJECTION
               XMLHotels.hotels
               [  XMLHotels.hotels.name XMLHotels.hotels.location ]
            ] ( T2 )
             ( T1.Name T2.Name ) ( T1.Location T2.Location )
         ] ( T3 )
         [ PROJECTION
            WebSearchHotels.hotels
            [  WebSearchHotels.hotels.name WebSearchHotels.hotels.location ]
         ] ( T4 )
          ( T3.Name T4.Name ) ( T3.Location T4.Location )
      ] ( ALIAS4 )
      [ JOIN
         [ UNION
            [ UNION
               [ PROJECTION
                  dbHotels.hotels
                  [  dbHotels.hotels.name dbHotels.hotels.name ]
               ] ( T1 )
               [ PROJECTION
                  XMLHotels.hotels
                  [  XMLHotels.hotels.name XMLHotels.hotels.name ]
               ] ( T2 )
                ( T1.Name T2.Name ) ( T1.Name T2.Name )
            ] ( T3 )
            [ PROJECTION
               WebSearchHotels.hotels
               [  WebSearchHotels.hotels.name WebSearchHotels.hotels.name ]
            ] ( T4 )
             ( T3.Name T4.Name ) ( T3.Name T4.Name )
         ] ( ALIAS2 )
         [ JOIN
            [ UNION
               [ UNION
                  [ SELECTION
                     dbHotels.hotels
                     [ AND [ >= dbHotels.hotels.units 1 ] [ = dbHotels.hotels.building "collective" ] ]
                  ] ( T1 )
                  [ SELECTION
                     XMLHotels.hotels
                     [ AND [ >= XMLHotels.hotels.units 1 ] [ = XMLHotels.hotels.building "collective" ] ]
                  ] ( T2 )
                   ( T1.Name T2.Name ) ( T1.Location T2.Location ) ( T1.Meals T2.Meals ) ( T1.Building T2.Building ) ( T1.Facilities T2.Facilities ) ( T1.Units T2.Units ) ( T1.Price T2.Price ) ( T1.Stars T2.Stars )
               ] ( T3 )
               [ SELECTION
                  WebSearchHotels.hotels
                  [ AND [ >= WebSearchHotels.hotels.units 1 ] [ = WebSearchHotels.hotels.building "collective" ] ]
               ] ( T4 )
                ( T3.Name T4.Name ) ( T3.Location T4.Location ) ( T3.Meals T4.Meals ) ( T3.Building T4.Building ) ( T3.Facilities T4.Facilities ) ( T3.Units T4.Units ) ( T3.Price T4.Price ) ( T3.stars T4.stars )
            ] ( ALIAS0 )
            [ SELECTION
               [ UNION
                  [ UNION
                     [ PROJECTION
                        [ F-TRANS
                           dbHotels.hotels
                           [  price ] 
                             [ observer.transformFunctions.Currency.dolar2euro ]
                        ]
                        [  dbHotels.hotels.name dbHotels.hotels.price ]
                     ] ( T1 )
                     [ PROJECTION
                        [ F-TRANS
                           XMLHotels.hotels
                           [  price ] 
                             [ observer.transformFunctions.Currency.dolar2pound ]
                        ]
                        [  XMLHotels.hotels.name XMLHotels.hotels.price ]
                     ] ( T2 )
                      ( T1.Name T2.Name ) ( T1.Price T2.Price )
                  ] ( T3 )
                  [ PROJECTION
                     [ F-TRANS
                        WebSearchHotels.hotels
                        [  price ] 
                          [ observer.transformFunctions.Currency.dolar2dkcrown ]
                     ]
                     [  WebSearchHotels.hotels.name WebSearchHotels.hotels.price ]
                  ] ( T4 )
                   ( T3.Name T4.Name ) ( T3.Price T4.Price )
               ]
               [ = dbHotels.hotels.price 200 ]
            ] ( ALIAS1 )
            [ = ALIAS0.name ALIAS1.name ]
         ] ( ALIAS3 )
         [ = ALIAS2.name ALIAS3.name ]
      ] ( ALIAS5 )
      [ = ALIAS4.name ALIAS5.name ]
   ] ( ALIAS7 )
   [ = ALIAS6.name ALIAS7.name ]
]
	f: NULO