> [!NOTE] Bitmap Index Scan > <table> > <tr> > <td width="25%"><img src="assets/ex_bitmapindexscan.png"></td> > <td>Traverses an index to find matching rows but instead of fetching them immediately, it populates a bitmap in memory marking which pages in the table contain matches. This bitmap can then be combined with others (AND/OR) before the heap is visited.</td> > </tr> > </table> > > ```sql > -- Querying a medium-selectivity range > EXPLAIN (ANALYZE, COSTS, BUFFERS, VERBOSE) > SELECT * FROM animals WHERE species_id = 1; > ``` > >  > > ```text > Bitmap Heap Scan on public.animals (cost=27.79..126.78 rows=2000 width=27) (actual time=0.059..0.353 rows=2000 loops=1) > Output: id, name, species_id, created_at > Recheck Cond: (animals.species_id = 1) > Heap Blocks: exact=74 > Buffers: shared hit=77 > -> Bitmap Index Scan on idx_animals_species_id (cost=0.00..27.29 rows=2000 width=0) (actual time=0.045..0.045 rows=2000 loops=1) > Index Cond: (animals.species_id = 1) > Buffers: shared hit=3 > Planning: > Buffers: shared hit=86 > Planning Time: 0.416 ms > Execution Time: 0.442 ms > ``` > >  > > <table> > <tr> > <td rowspan="2" width="25%"><img src="assets/ex_bmp_index.svg"></td> > <td><b>Performance</b></td><td>Efficient for medium-selectivity predicates; bitmap combination delays heap access until matching pages are known.</td> > </tr> > <tr><td><b>Cost</b></td><td><code>index traversal + bitmap construction + matching heap pages</code></td></tr> > </table>