小编Tra*_*Juu的帖子

+----------+--------------+-------------------------+
| ticketid | ticketpostid |           date          |
+----------+--------------+-------------------------+
|  1387935 |      3147808 | 2012-09-17 13:33:01     |
|  1387935 |      3147812 | 2012-09-17 13:33:41     |
|  1387938 |      3147818 | 2012-09-17 13:35:01     |
|  1387938 |      3148068 | 2012-09-17 13:37:01     |
|  1387938 |      3148323 | 2012-09-17 14:47:01     |
|  1387939 |      3147820 | 2012-09-17 13:36:01     |
|  1387939 |      3147834 | 2012-09-17 13:36:25     |
|  1387939 |      3147851 | 2012-09-17 13:41:01     |
|  1387939 |      3147968 | 2012-09-17 13:59:06 …

我正在测试不同的查询,我很好奇数据库如何决定使用位图堆扫描和索引扫描.

在客户上创建索引customers_email_idx(电子邮件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:

从客户处选择*,其中包含'ITQ %@dell.com'等电子邮件; - >使用索引扫描查询

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:

从客户处选择*,其中包含'IT%@dell.com'等电子邮件; - >使用位图堆扫描查询

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 …