PostgreSQL何时自动为表创建位图索引?

PostgreSQL何时自动为表创建位图索引?

我从 PostgreSQL' documentation ,并且想知道为什么要更改在哪里可以产生影响的值?谢谢。

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)

和

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)