Performance
  
  * virtual pivot table vs aggregate view
  
  * select * from pivot_table where attribute_001 < X;
  
      X   rows  aggregate  virtual (first) virtual (second)
    ---   ---- ----------- -------------- ----------------
      0      0        0.5        0.5        0.1 seconds
      1      0        0.5        0.5        0.1
      2      2        0.4        0.5        0.1
      3      4        0.4        0.5        0.1
     10     26        0.4        0.6        0.3
     20     44        0.4        0.7        0.3
     30     67        0.4        0.8        0.4
     40     91        0.4        0.9        0.6
     50    116        0.4        1.0        0.7
    100    250        0.5        1.6        1.3
    200    493        0.5        2.8        2.4
    500   1199        0.5        5.8        5.5
  
  * virtual pivot table recalculates the distinct
    columns at the first query in a database session.
  * The virtual table is faster for _smaller_ results sets
    and slower for larger ones.
  * Similar results for larger tables (100,000 rows
    instead of 5,000 -- times increased by a factor of 20).