Index engine main scanning technique
Index scan
Bitmap scan
Index scan works fine when we deal with only a few values. However, as the number of retrieved rows increases, it is more likely to get back to the same table page several times. Therefore, the optimizer switches to the bitmap scan.
postgres=# explain (costs off) select * from t where a <= 100;
QUERY PLAN
------------------------------------
Bitmap Heap Scan on t
Recheck Cond: (a <= 100)
-> Bitmap Index Scan on t_a_idx
Index Cond: (a <= 100)
(4 rows)