oracle - 位图连接索引

Question

在事实表和维度之间创建 BJI 时

CREATE BITMAP INDEX IDX_BITJ ON fact(dimension.city) 
FROM fact, dimension 
WHERE fact.id_customer = dimension.id_customer;

然后查询：

select sum(a.count) from fact a, dimension b 
where a.id_customer = b.id_customer and b.city = 'London';

解释计划将始终对表格进行全面扫描......谁能解释我为什么？不应该使用位图索引吗？