When does PostgreSQL automatically create a Bitmap index for a table?

Question

When does PostgreSQL automatically create a Bitmap index for a table?

I saw the following examples from PostgreSQL' documentation, and wonder why changing the value in WHERE could make a difference? Thanks.

EXPLAIN SELECT * FROM tenk1 WHERE unique1 < 7000;
                         QUERY PLAN
------------------------------------------------------------
 Seq Scan on tenk1  (cost=0.00..483.00 rows=7001 width=244)
   Filter: (unique1 < 7000)

and

EXPLAIN SELECT * FROM tenk1 WHERE unique1 < 100;
                                  QUERY PLAN
------------------------------------------------------------------------------
 Bitmap Heap Scan on tenk1  (cost=5.07..229.20 rows=101 width=244)
   Recheck Cond: (unique1 < 100)
   ->  Bitmap Index Scan on tenk1_unique1  (cost=0.00..5.04 rows=101
 width=0)
         Index Cond: (unique1 < 100)

Answer 1

Postgres does not have "Bitmap Indexes".

What you see there is an "index scan" that uses a bitmap while scanning the index.

Tom Lane's answer on the mailing list explains it quite well:

A plain Index Scan fetches one tuple-pointer at a time from the index, and immediately visits that tuple in the table. A bitmap scan fetches all the tuple-pointers from the index in one go, sorts them using an in-memory "bitmap" data structure, and then visits the table tuples in physical tuple-location order.

See also this question https://dba.stackexchange.com/questions/119386 for a more detailed explanation.