Simple Demo: OBSERVER execution

This is the Main Plan as ERA tree for the query:

This is the Main Plan for the query:

[ JOIN
   [ UNION
      [ UNION
         [ AUX-1 ] ( T1 )
         [ AUX-2 ] ( T2 )
          ( T1.Name T2.Name ) ( T1.Price T2.Price )
      ] ( T3 )
      [ AUX-3 ] ( T4 )
       ( T3.Name T4.Name ) ( T3.Price T4.Price )
   ] ( ALIAS6 )
   [ JOIN
      [ UNION
         [ UNION
            [ AUX-4] ( T1 )
            [ AUX-5 ] ( T2 )
             ( T1.Name T2.Name ) ( T1.Location T2.Location )
         ] ( T3 )
         [ AUX-6 ] ( T4 )
          ( T3.Name T4.Name ) ( T3.Location T4.Location )
      ] ( ALIAS4 )
      [ JOIN
         [ UNION
            [ UNION
               [ AUX-7] ( T1 )
               [ AUX-8] ( T2 )
                ( T1.Name T2.Name ) ( T1.Name T2.Name )
            ] ( T3 )
            [ AUX-9 ] ( T4 )
             ( T3.Name T4.Name ) ( T3.Name T4.Name )
         ] ( ALIAS2 )
         [ JOIN
            [ UNION
               [ UNION
                  [ AUX-10] ( T1 )
                  [ AUX-11] ( 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 )
               [ AUX-12 ] ( 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
                     [ AUX-13 ] ( T1 )
                     [ AUX-14 ] ( T2 )
                      ( T1.Name T2.Name ) ( T1.Price T2.Price )
                  ] ( T3 )
                  [ AUX-15 ] ( 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