PostgreSQL index scan,bitmap index scan区别

1. 建表

postgres=# create table tb_index_test(id serial primary key,name character varying);
CREATE TABLE 
postgres=# 
postgres=# \d tb_index_test;
                              Table "public.tb_index_test"
 Column |       Type        |                         Modifiers                          
--------+-------------------+------------------------------------------------------------
 id     | integer           | not null default nextval('tb_index_test_id_seq'::regclass)
 name   | character varying | 
Indexes:
    "tb_index_test_pkey" PRIMARY KEY,btree (id)

2. 插入测试数据：

 postgres=# insert into tb_index_test values(generate_series(1,10000),'john');
INSERT 0 10000

3. 区别一、启动成本
> index sacn: 因为需要扫描索引块，节点才能输出，索引启动成本大于0

postgres=# explain(analyze,verbose,buffers)select count(0) from tb_index_test where id<400;
                                                                     QUERY PLAN                                                                     
----------------------------------------------------------------------------------------------------------------------------------------------------
 Aggregate  (cost=22.29..22.30 rows=1 width=0) (actual time=0.108..0.108 rows=1 loops=1)
   Output: count(0)
   Buffers: shared hit=6
   ->  Index Scan using tb_index_test_pkey on public.tb_index_test  (<span style="color:#ff0000;">cost=0.29</span>..21.29 rows=400 width=0) (actual time=0.016..0.074 rows=399 loops=1)
         Output: id,name
         Index Cond: (tb_index_test.id < 400)
         Buffers: shared hit=6
 Total runtime: 0.140 ms
(8 rows)


Time: 0.620 ms

> bitmap index scan: 因为需要扫描索引块，节点才能放输出，索引启动成本也大于0，但是bitmap index scan的启动成本却是0，可以解释为bitmapIndex Scan节点和Bitmap Heap Scan是一对的,启动成本算在Bitmap Heap Scan上.

postgres=# explain(analyze,buffers)select count(0) from tb_index_test where id<400;
                                                              QUERY PLAN                                                              
--------------------------------------------------------------------------------------------------------------------------------------
 Aggregate  (cost=72.39..72.40 rows=1 width=0) (actual time=0.178..0.178 rows=1 loops=1)
   Output: count(0)
   Buffers: shared hit=6
   ->  Bitmap Heap Scan on public.tb_index_test  (cost=11.38..71.39 rows=400 width=0) (actual time=0.062..0.120 rows=399 loops=1)
         Output: id,name
         Recheck Cond: (tb_index_test.id < 400)
         Buffers: shared hit=6
         ->  Bitmap Index Scan on tb_index_test_pkey  (c<span style="color:#ff0000;">ost=0.00</span>..11.29 rows=400 width=0) (actual time=0.047..0.047 rows=399 loops=1)
               Index Cond: (tb_index_test.id < 400)
               Buffers: shared hit=3
 Total runtime: 0.217 ms
(11 rows)


Time: 0.741 ms

4. 区别二、节点返回的类型
index scan: 输出的是tuple，它先扫描索引块，然后得到rowid扫描数据块得到目标记录。
bitmap index scan; 输出的是索引条目，并不是行的数据，输出索引条目后，交给上一个几点 bitmap heap scan(之间可能将索引条目根据物理排列顺序进行排序)。

以上参考自德哥的教程

5. 区别三、扫描索引块和数据块的区别。
index scan：一次只读一条索引项，那么一个 PAGE面有可能被多次访问；
bitmap index scan ：一次性将满足条件的索引项全部取出，然后交给bitmap heap scan节点，并在内存中进行排序,根据取出的索引项访问表数据。
参考自：http://francs3.blog.163.com/blog/static/405767272011665227181/

PostgreSQL index scan,bitmap index scan区别

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