NOT operator lose index

[klpx]

Hi! Is it bug? Or what i do wrong?

=> explain analyze select * from mytable where ( data @@ 'NOT attr = *');
                                                      QUERY PLAN                                                      
----------------------------------------------------------------------------------------------------------------------
 Seq Scan on mytable  (cost=0.00..101937.27 rows=506315 width=254) (actual time=0.080..920.274 rows=483749 loops=1)
   Filter: (data @@ '(NOT "attr" = *)'::jsquery)
   Rows Removed by Filter: 23073
 Planning time: 2.892 ms
 Execution time: 966.762 ms

=> explain analyze select * from mytable where ( data @@ 'attr = *');
                                                                QUERY PLAN                                                              
----------------------------------------------------------------------------------------------------------------------------------------
 Bitmap Heap Scan on mytable  (cost=31.93..1951.83 rows=507 width=254) (actual time=41.431..135.250 rows=23073 loops=1)
   Recheck Cond: (data @@ '"attr" = *'::jsquery)
   Heap Blocks: exact=13967
   ->  Bitmap Index Scan on mytable_data_idx1  (cost=0.00..31.80 rows=507 width=0) (actual time=38.447..38.447 rows=23073 loops=
         Index Cond: (data @@ '"attr" = *'::jsquery)
 Planning time: 2.073 ms
 Execution time: 138.120 ms
(7 rows)

[akorotkov]

In jsquery indexes have lossy representation of keys: hashes are stored instead of strings. That causes false positives which are eliminated during recheck. But in case of NOT, false positives may turn into false negatives which can't be handled. This is why NOT clauses are not optimized using index altogether.