postgresql – 如何确定位图堆扫描和索引扫描？

create index customers_email_idx on customers(email
varchar_pattern_ops);

如您所见,有一个customers表(dellstore示例),我向email列添加索引.

第一个查询在这里：

select * from customers where email like ‘ITQ%@dell.com’; -> query
with Index Scan

解释分析查询在这里：

QUERY PLAN                                                            
---------------------------------------------------------------------------------------------------------------------------------
 Index Scan using customers_email_idx on customers  (cost=0.00..8.27 rows=2 width=268) (actual time=0.046..0.046 rows=0 loops=1)
   Index Cond: (((email)::text ~>=~ 'ITQ'::text) AND ((email)::text ~<~ 'ITR'::text))
   Filter: ((email)::text ~~ 'ITQ%@dell.com
 '::text)
 Total runtime: 0.113 ms

其他查询在这里：

select * from customers where email like ‘IT%@dell.com’; -> query
with Bitmap Heap Scan

解释分析查询在这里：

QUERY PLAN                                                          
------------------------------------------------------------------------------------------------------------------------------
 Bitmap Heap Scan on customers  (cost=4.54..106.77 rows=2 width=268) (actual time=0.206..0.206 rows=0 loops=1)
   Filter: ((email)::text ~~ 'IT%@dell.com
 '::text)
   ->  Bitmap Index Scan on customers_email_idx  (cost=0.00..4.54 rows=29 width=0) (actual time=0.084..0.084 rows=28 loops=1)
         Index Cond: (((email)::text ~>=~ 'IT'::text) AND ((email)::text ~<~ 'IU'::text))
 Total runtime: 0.273 ms

你能解释这个例子为什么在这里使用Bitmap和Index Scan吗？

谢谢..