数据库内核月报 - 2018 / 11

PgSQL · 应用案例 · Heap Only Tuple (降低UPDATE引入的索引写IO放大)

背景

PostgreSQL目前默认的存储引擎在更新记录时,会在堆内产生一条新版本,旧版本在不需要使用后VACUUM回收,回收旧版本前,需要先回收所有关联这个版本的所有索引POINT。

PG的索引的KEY为索引字段或表达式的值,VALUE为行号。

8.3以前,每个TUPLE版本(行号)都有对应的索引POINT,因此更新的放大比较大。

8.3开始,引入了HOT的概念,当更新记录时,如果能满足两个条件时,通过HEAP PAGE内部LINK来串起所有TUPLE版本,因此索引不变。

HOT必须满足如下两个条件:

Necessary Condition A: UPDATE does not change any of the index keys    
  
Necessary Condition B: The new version should fit in the same old block – HOT chains can not cross block boundary.   
  
1、索引字段的值不变。(其中任意一个索引字段的值发生了变化,则所有索引都需要新增版本)  
  
2、新的版本与旧的版本在同一个HEAP PAGE中。  

10以后,可以使用二级索引来解决更新引入的索引放大问题:

《PostgreSQL 10.0 preview 性能增强 - 间接索引(secondary index)》

HOT 实例解说

1、创建测试表,写入10条测试数据

postgres=# create table a(id int, c1 int, c2 int, c3 int);  
CREATE TABLE  
postgres=# insert into a select generate_series(1,10), random()*100, random()*100, random()*100;  
INSERT 0 10  

2、创建索引

postgres=# create index idx_a_1 on a (id);  
CREATE INDEX  
postgres=# create index idx_a_2 on a (c1);  
CREATE INDEX  
postgres=# create index idx_a_3 on a (c2);  
CREATE INDEX  

3、通过pageinspect插件观察索引页内容

postgres=# SELECT * FROM bt_metap('idx_a_1');  
 magic  | version | root | level | fastroot | fastlevel   
--------+---------+------+-------+----------+-----------  
 340322 |       2 |    1 |     0 |        1 |         0  
(1 row)  
  
postgres=# SELECT * FROM bt_metap('idx_a_2');  
 magic  | version | root | level | fastroot | fastlevel   
--------+---------+------+-------+----------+-----------  
 340322 |       2 |    1 |     0 |        1 |         0  
(1 row)  
  
postgres=# SELECT * FROM bt_metap('idx_a_3');  
 magic  | version | root | level | fastroot | fastlevel   
--------+---------+------+-------+----------+-----------  
 340322 |       2 |    1 |     0 |        1 |         0  
(1 row)  
postgres=# SELECT * FROM bt_page_items('idx_a_1',1);  
 itemoffset |  ctid  | itemlen | nulls | vars |          data             
------------+--------+---------+-------+------+-------------------------  
          1 | (0,1)  |      16 | f     | f    | 01 00 00 00 00 00 00 00  
          2 | (0,2)  |      16 | f     | f    | 02 00 00 00 00 00 00 00  
          3 | (0,3)  |      16 | f     | f    | 03 00 00 00 00 00 00 00  
          4 | (0,4)  |      16 | f     | f    | 04 00 00 00 00 00 00 00  
          5 | (0,5)  |      16 | f     | f    | 05 00 00 00 00 00 00 00  
          6 | (0,6)  |      16 | f     | f    | 06 00 00 00 00 00 00 00  
          7 | (0,7)  |      16 | f     | f    | 07 00 00 00 00 00 00 00  
          8 | (0,8)  |      16 | f     | f    | 08 00 00 00 00 00 00 00  
          9 | (0,9)  |      16 | f     | f    | 09 00 00 00 00 00 00 00  
         10 | (0,10) |      16 | f     | f    | 0a 00 00 00 00 00 00 00  
(10 rows)  
  
postgres=# SELECT * FROM bt_page_items('idx_a_2',1);  
 itemoffset |  ctid  | itemlen | nulls | vars |          data             
------------+--------+---------+-------+------+-------------------------  
          1 | (0,5)  |      16 | f     | f    | 05 00 00 00 00 00 00 00  
          2 | (0,7)  |      16 | f     | f    | 06 00 00 00 00 00 00 00  
          3 | (0,1)  |      16 | f     | f    | 0d 00 00 00 00 00 00 00  
          4 | (0,9)  |      16 | f     | f    | 0e 00 00 00 00 00 00 00  
          5 | (0,4)  |      16 | f     | f    | 20 00 00 00 00 00 00 00  
          6 | (0,3)  |      16 | f     | f    | 24 00 00 00 00 00 00 00  
          7 | (0,6)  |      16 | f     | f    | 28 00 00 00 00 00 00 00  
          8 | (0,10) |      16 | f     | f    | 2a 00 00 00 00 00 00 00  
          9 | (0,2)  |      16 | f     | f    | 3f 00 00 00 00 00 00 00  
         10 | (0,8)  |      16 | f     | f    | 55 00 00 00 00 00 00 00  
(10 rows)  
  
postgres=# SELECT * FROM bt_page_items('idx_a_3',1);  
 itemoffset |  ctid  | itemlen | nulls | vars |          data             
------------+--------+---------+-------+------+-------------------------  
          1 | (0,6)  |      16 | f     | f    | 09 00 00 00 00 00 00 00  
          2 | (0,5)  |      16 | f     | f    | 12 00 00 00 00 00 00 00  
          3 | (0,8)  |      16 | f     | f    | 13 00 00 00 00 00 00 00  
          4 | (0,1)  |      16 | f     | f    | 14 00 00 00 00 00 00 00  
          5 | (0,4)  |      16 | f     | f    | 22 00 00 00 00 00 00 00  
          6 | (0,10) |      16 | f     | f    | 2b 00 00 00 00 00 00 00  
          7 | (0,3)  |      16 | f     | f    | 30 00 00 00 00 00 00 00  
          8 | (0,9)  |      16 | f     | f    | 33 00 00 00 00 00 00 00  
          9 | (0,7)  |      16 | f     | f    | 46 00 00 00 00 00 00 00  
         10 | (0,2)  |      16 | f     | f    | 4b 00 00 00 00 00 00 00  
(10 rows)  

4、HOT更新,(更新的字段上没有索引,并且新的版本记录在同一个HEAP PAGE上)

postgres=# update a set c3=c3+1 where id=1;  
UPDATE 1  
postgres=# SELECT * FROM bt_page_items('idx_a_1',1);  
 itemoffset |  ctid  | itemlen | nulls | vars |          data             
------------+--------+---------+-------+------+-------------------------  
          1 | (0,1)  |      16 | f     | f    | 01 00 00 00 00 00 00 00  
          2 | (0,2)  |      16 | f     | f    | 02 00 00 00 00 00 00 00  
          3 | (0,3)  |      16 | f     | f    | 03 00 00 00 00 00 00 00  
          4 | (0,4)  |      16 | f     | f    | 04 00 00 00 00 00 00 00  
          5 | (0,5)  |      16 | f     | f    | 05 00 00 00 00 00 00 00  
          6 | (0,6)  |      16 | f     | f    | 06 00 00 00 00 00 00 00  
          7 | (0,7)  |      16 | f     | f    | 07 00 00 00 00 00 00 00  
          8 | (0,8)  |      16 | f     | f    | 08 00 00 00 00 00 00 00  
          9 | (0,9)  |      16 | f     | f    | 09 00 00 00 00 00 00 00  
         10 | (0,10) |      16 | f     | f    | 0a 00 00 00 00 00 00 00  
(10 rows)  
  
postgres=# SELECT * FROM bt_page_items('idx_a_2',1);  
 itemoffset |  ctid  | itemlen | nulls | vars |          data             
------------+--------+---------+-------+------+-------------------------  
          1 | (0,5)  |      16 | f     | f    | 05 00 00 00 00 00 00 00  
          2 | (0,7)  |      16 | f     | f    | 06 00 00 00 00 00 00 00  
          3 | (0,1)  |      16 | f     | f    | 0d 00 00 00 00 00 00 00  
          4 | (0,9)  |      16 | f     | f    | 0e 00 00 00 00 00 00 00  
          5 | (0,4)  |      16 | f     | f    | 20 00 00 00 00 00 00 00  
          6 | (0,3)  |      16 | f     | f    | 24 00 00 00 00 00 00 00  
          7 | (0,6)  |      16 | f     | f    | 28 00 00 00 00 00 00 00  
          8 | (0,10) |      16 | f     | f    | 2a 00 00 00 00 00 00 00  
          9 | (0,2)  |      16 | f     | f    | 3f 00 00 00 00 00 00 00  
         10 | (0,8)  |      16 | f     | f    | 55 00 00 00 00 00 00 00  
(10 rows)  
  
postgres=# SELECT * FROM bt_page_items('idx_a_3',1);  
 itemoffset |  ctid  | itemlen | nulls | vars |          data             
------------+--------+---------+-------+------+-------------------------  
          1 | (0,6)  |      16 | f     | f    | 09 00 00 00 00 00 00 00  
          2 | (0,5)  |      16 | f     | f    | 12 00 00 00 00 00 00 00  
          3 | (0,8)  |      16 | f     | f    | 13 00 00 00 00 00 00 00  
          4 | (0,1)  |      16 | f     | f    | 14 00 00 00 00 00 00 00  
          5 | (0,4)  |      16 | f     | f    | 22 00 00 00 00 00 00 00  
          6 | (0,10) |      16 | f     | f    | 2b 00 00 00 00 00 00 00  
          7 | (0,3)  |      16 | f     | f    | 30 00 00 00 00 00 00 00  
          8 | (0,9)  |      16 | f     | f    | 33 00 00 00 00 00 00 00  
          9 | (0,7)  |      16 | f     | f    | 46 00 00 00 00 00 00 00  
         10 | (0,2)  |      16 | f     | f    | 4b 00 00 00 00 00 00 00  
(10 rows)  
postgres=# update a set c3=c3 where id=1 returning ctid,*;  
  ctid  | id | c1 | c2 | c3   
--------+----+----+----+----  
 (0,13) |  1 | 13 | 20 | 15  
(1 row)  
  
UPDATE 1  
postgres=# SELECT * FROM bt_page_items('idx_a_1',1);  
 itemoffset |  ctid  | itemlen | nulls | vars |          data             
------------+--------+---------+-------+------+-------------------------  
          1 | (0,1)  |      16 | f     | f    | 01 00 00 00 00 00 00 00  
          2 | (0,2)  |      16 | f     | f    | 02 00 00 00 00 00 00 00  
          3 | (0,3)  |      16 | f     | f    | 03 00 00 00 00 00 00 00  
          4 | (0,4)  |      16 | f     | f    | 04 00 00 00 00 00 00 00  
          5 | (0,5)  |      16 | f     | f    | 05 00 00 00 00 00 00 00  
          6 | (0,6)  |      16 | f     | f    | 06 00 00 00 00 00 00 00  
          7 | (0,7)  |      16 | f     | f    | 07 00 00 00 00 00 00 00  
          8 | (0,8)  |      16 | f     | f    | 08 00 00 00 00 00 00 00  
          9 | (0,9)  |      16 | f     | f    | 09 00 00 00 00 00 00 00  
         10 | (0,10) |      16 | f     | f    | 0a 00 00 00 00 00 00 00  
(10 rows)  
  
postgres=# SELECT * FROM bt_page_items('idx_a_2',1);  
 itemoffset |  ctid  | itemlen | nulls | vars |          data             
------------+--------+---------+-------+------+-------------------------  
          1 | (0,5)  |      16 | f     | f    | 05 00 00 00 00 00 00 00  
          2 | (0,7)  |      16 | f     | f    | 06 00 00 00 00 00 00 00  
          3 | (0,1)  |      16 | f     | f    | 0d 00 00 00 00 00 00 00  
          4 | (0,9)  |      16 | f     | f    | 0e 00 00 00 00 00 00 00  
          5 | (0,4)  |      16 | f     | f    | 20 00 00 00 00 00 00 00  
          6 | (0,3)  |      16 | f     | f    | 24 00 00 00 00 00 00 00  
          7 | (0,6)  |      16 | f     | f    | 28 00 00 00 00 00 00 00  
          8 | (0,10) |      16 | f     | f    | 2a 00 00 00 00 00 00 00  
          9 | (0,2)  |      16 | f     | f    | 3f 00 00 00 00 00 00 00  
         10 | (0,8)  |      16 | f     | f    | 55 00 00 00 00 00 00 00  
(10 rows)  
  
postgres=# SELECT * FROM bt_page_items('idx_a_3',1);  
 itemoffset |  ctid  | itemlen | nulls | vars |          data             
------------+--------+---------+-------+------+-------------------------  
          1 | (0,6)  |      16 | f     | f    | 09 00 00 00 00 00 00 00  
          2 | (0,5)  |      16 | f     | f    | 12 00 00 00 00 00 00 00  
          3 | (0,8)  |      16 | f     | f    | 13 00 00 00 00 00 00 00  
          4 | (0,1)  |      16 | f     | f    | 14 00 00 00 00 00 00 00  
          5 | (0,4)  |      16 | f     | f    | 22 00 00 00 00 00 00 00  
          6 | (0,10) |      16 | f     | f    | 2b 00 00 00 00 00 00 00  
          7 | (0,3)  |      16 | f     | f    | 30 00 00 00 00 00 00 00  
          8 | (0,9)  |      16 | f     | f    | 33 00 00 00 00 00 00 00  
          9 | (0,7)  |      16 | f     | f    | 46 00 00 00 00 00 00 00  
         10 | (0,2)  |      16 | f     | f    | 4b 00 00 00 00 00 00 00  
(10 rows)  
postgres=# update a set c3=c3 where id=1;  
UPDATE 1  
postgres=# SELECT * FROM bt_page_items('idx_a_1',1);  
 itemoffset |  ctid  | itemlen | nulls | vars |          data             
------------+--------+---------+-------+------+-------------------------  
          1 | (0,1)  |      16 | f     | f    | 01 00 00 00 00 00 00 00  
          2 | (0,2)  |      16 | f     | f    | 02 00 00 00 00 00 00 00  
          3 | (0,3)  |      16 | f     | f    | 03 00 00 00 00 00 00 00  
          4 | (0,4)  |      16 | f     | f    | 04 00 00 00 00 00 00 00  
          5 | (0,5)  |      16 | f     | f    | 05 00 00 00 00 00 00 00  
          6 | (0,6)  |      16 | f     | f    | 06 00 00 00 00 00 00 00  
          7 | (0,7)  |      16 | f     | f    | 07 00 00 00 00 00 00 00  
          8 | (0,8)  |      16 | f     | f    | 08 00 00 00 00 00 00 00  
          9 | (0,9)  |      16 | f     | f    | 09 00 00 00 00 00 00 00  
         10 | (0,10) |      16 | f     | f    | 0a 00 00 00 00 00 00 00  
(10 rows)  
  
postgres=# SELECT * FROM bt_page_items('idx_a_2',1);  
 itemoffset |  ctid  | itemlen | nulls | vars |          data             
------------+--------+---------+-------+------+-------------------------  
          1 | (0,5)  |      16 | f     | f    | 05 00 00 00 00 00 00 00  
          2 | (0,7)  |      16 | f     | f    | 06 00 00 00 00 00 00 00  
          3 | (0,1)  |      16 | f     | f    | 0d 00 00 00 00 00 00 00  
          4 | (0,9)  |      16 | f     | f    | 0e 00 00 00 00 00 00 00  
          5 | (0,4)  |      16 | f     | f    | 20 00 00 00 00 00 00 00  
          6 | (0,3)  |      16 | f     | f    | 24 00 00 00 00 00 00 00  
          7 | (0,6)  |      16 | f     | f    | 28 00 00 00 00 00 00 00  
          8 | (0,10) |      16 | f     | f    | 2a 00 00 00 00 00 00 00  
          9 | (0,2)  |      16 | f     | f    | 3f 00 00 00 00 00 00 00  
         10 | (0,8)  |      16 | f     | f    | 55 00 00 00 00 00 00 00  
(10 rows)  
  
postgres=# SELECT * FROM bt_page_items('idx_a_3',1);  
 itemoffset |  ctid  | itemlen | nulls | vars |          data             
------------+--------+---------+-------+------+-------------------------  
          1 | (0,6)  |      16 | f     | f    | 09 00 00 00 00 00 00 00  
          2 | (0,5)  |      16 | f     | f    | 12 00 00 00 00 00 00 00  
          3 | (0,8)  |      16 | f     | f    | 13 00 00 00 00 00 00 00  
          4 | (0,1)  |      16 | f     | f    | 14 00 00 00 00 00 00 00  
          5 | (0,4)  |      16 | f     | f    | 22 00 00 00 00 00 00 00  
          6 | (0,10) |      16 | f     | f    | 2b 00 00 00 00 00 00 00  
          7 | (0,3)  |      16 | f     | f    | 30 00 00 00 00 00 00 00  
          8 | (0,9)  |      16 | f     | f    | 33 00 00 00 00 00 00 00  
          9 | (0,7)  |      16 | f     | f    | 46 00 00 00 00 00 00 00  
         10 | (0,2)  |      16 | f     | f    | 4b 00 00 00 00 00 00 00  
(10 rows)  

5、HOT更新,有索引的字段被更新,但是值不变。并且新版本在同一个PAGE里。

postgres=# update a set c2=c2 where id=1 returning ctid,*;  
  ctid  | id | c1 | c2 | c3   
--------+----+----+----+----  
 (0,14) |  1 | 13 | 20 | 15  
(1 row)  
  
UPDATE 1  
postgres=# SELECT * FROM bt_page_items('idx_a_1',1);  
 itemoffset |  ctid  | itemlen | nulls | vars |          data             
------------+--------+---------+-------+------+-------------------------  
          1 | (0,1)  |      16 | f     | f    | 01 00 00 00 00 00 00 00  
          2 | (0,2)  |      16 | f     | f    | 02 00 00 00 00 00 00 00  
          3 | (0,3)  |      16 | f     | f    | 03 00 00 00 00 00 00 00  
          4 | (0,4)  |      16 | f     | f    | 04 00 00 00 00 00 00 00  
          5 | (0,5)  |      16 | f     | f    | 05 00 00 00 00 00 00 00  
          6 | (0,6)  |      16 | f     | f    | 06 00 00 00 00 00 00 00  
          7 | (0,7)  |      16 | f     | f    | 07 00 00 00 00 00 00 00  
          8 | (0,8)  |      16 | f     | f    | 08 00 00 00 00 00 00 00  
          9 | (0,9)  |      16 | f     | f    | 09 00 00 00 00 00 00 00  
         10 | (0,10) |      16 | f     | f    | 0a 00 00 00 00 00 00 00  
(10 rows)  
  
postgres=# SELECT * FROM bt_page_items('idx_a_2',1);  
 itemoffset |  ctid  | itemlen | nulls | vars |          data             
------------+--------+---------+-------+------+-------------------------  
          1 | (0,5)  |      16 | f     | f    | 05 00 00 00 00 00 00 00  
          2 | (0,7)  |      16 | f     | f    | 06 00 00 00 00 00 00 00  
          3 | (0,1)  |      16 | f     | f    | 0d 00 00 00 00 00 00 00  
          4 | (0,9)  |      16 | f     | f    | 0e 00 00 00 00 00 00 00  
          5 | (0,4)  |      16 | f     | f    | 20 00 00 00 00 00 00 00  
          6 | (0,3)  |      16 | f     | f    | 24 00 00 00 00 00 00 00  
          7 | (0,6)  |      16 | f     | f    | 28 00 00 00 00 00 00 00  
          8 | (0,10) |      16 | f     | f    | 2a 00 00 00 00 00 00 00  
          9 | (0,2)  |      16 | f     | f    | 3f 00 00 00 00 00 00 00  
         10 | (0,8)  |      16 | f     | f    | 55 00 00 00 00 00 00 00  
(10 rows)  
  
postgres=# SELECT * FROM bt_page_items('idx_a_3',1);  
 itemoffset |  ctid  | itemlen | nulls | vars |          data             
------------+--------+---------+-------+------+-------------------------  
          1 | (0,6)  |      16 | f     | f    | 09 00 00 00 00 00 00 00  
          2 | (0,5)  |      16 | f     | f    | 12 00 00 00 00 00 00 00  
          3 | (0,8)  |      16 | f     | f    | 13 00 00 00 00 00 00 00  
          4 | (0,1)  |      16 | f     | f    | 14 00 00 00 00 00 00 00  
          5 | (0,4)  |      16 | f     | f    | 22 00 00 00 00 00 00 00  
          6 | (0,10) |      16 | f     | f    | 2b 00 00 00 00 00 00 00  
          7 | (0,3)  |      16 | f     | f    | 30 00 00 00 00 00 00 00  
          8 | (0,9)  |      16 | f     | f    | 33 00 00 00 00 00 00 00  
          9 | (0,7)  |      16 | f     | f    | 46 00 00 00 00 00 00 00  
         10 | (0,2)  |      16 | f     | f    | 4b 00 00 00 00 00 00 00  
(10 rows)  

以上HOT更新,所有索引都没有发生变化。

6、NON-HOT更新,更新了索引字段的值,所有索引都发生了变化,至少发生了3个索引IO。

postgres=# update a set c2=c2+1 where id=1 returning ctid,*;  
  ctid  | id | c1 | c2 | c3   
--------+----+----+----+----  
 (0,15) |  1 | 13 | 21 | 15  
(1 row)  
  
UPDATE 1  
postgres=# SELECT * FROM bt_page_items('idx_a_1',1);  
 itemoffset |  ctid  | itemlen | nulls | vars |          data             
------------+--------+---------+-------+------+-------------------------  
          1 | (0,15) |      16 | f     | f    | 01 00 00 00 00 00 00 00  
          2 | (0,1)  |      16 | f     | f    | 01 00 00 00 00 00 00 00  
          3 | (0,2)  |      16 | f     | f    | 02 00 00 00 00 00 00 00  
          4 | (0,3)  |      16 | f     | f    | 03 00 00 00 00 00 00 00  
          5 | (0,4)  |      16 | f     | f    | 04 00 00 00 00 00 00 00  
          6 | (0,5)  |      16 | f     | f    | 05 00 00 00 00 00 00 00  
          7 | (0,6)  |      16 | f     | f    | 06 00 00 00 00 00 00 00  
          8 | (0,7)  |      16 | f     | f    | 07 00 00 00 00 00 00 00  
          9 | (0,8)  |      16 | f     | f    | 08 00 00 00 00 00 00 00  
         10 | (0,9)  |      16 | f     | f    | 09 00 00 00 00 00 00 00  
         11 | (0,10) |      16 | f     | f    | 0a 00 00 00 00 00 00 00  
(11 rows)  
  
postgres=# SELECT * FROM bt_page_items('idx_a_2',1);  
 itemoffset |  ctid  | itemlen | nulls | vars |          data             
------------+--------+---------+-------+------+-------------------------  
          1 | (0,5)  |      16 | f     | f    | 05 00 00 00 00 00 00 00  
          2 | (0,7)  |      16 | f     | f    | 06 00 00 00 00 00 00 00  
          3 | (0,15) |      16 | f     | f    | 0d 00 00 00 00 00 00 00  
          4 | (0,1)  |      16 | f     | f    | 0d 00 00 00 00 00 00 00  
          5 | (0,9)  |      16 | f     | f    | 0e 00 00 00 00 00 00 00  
          6 | (0,4)  |      16 | f     | f    | 20 00 00 00 00 00 00 00  
          7 | (0,3)  |      16 | f     | f    | 24 00 00 00 00 00 00 00  
          8 | (0,6)  |      16 | f     | f    | 28 00 00 00 00 00 00 00  
          9 | (0,10) |      16 | f     | f    | 2a 00 00 00 00 00 00 00  
         10 | (0,2)  |      16 | f     | f    | 3f 00 00 00 00 00 00 00  
         11 | (0,8)  |      16 | f     | f    | 55 00 00 00 00 00 00 00  
(11 rows)  
  
postgres=# SELECT * FROM bt_page_items('idx_a_3',1);  
 itemoffset |  ctid  | itemlen | nulls | vars |          data             
------------+--------+---------+-------+------+-------------------------  
          1 | (0,6)  |      16 | f     | f    | 09 00 00 00 00 00 00 00  
          2 | (0,5)  |      16 | f     | f    | 12 00 00 00 00 00 00 00  
          3 | (0,8)  |      16 | f     | f    | 13 00 00 00 00 00 00 00  
          4 | (0,1)  |      16 | f     | f    | 14 00 00 00 00 00 00 00  
          5 | (0,15) |      16 | f     | f    | 15 00 00 00 00 00 00 00  
          6 | (0,4)  |      16 | f     | f    | 22 00 00 00 00 00 00 00  
          7 | (0,10) |      16 | f     | f    | 2b 00 00 00 00 00 00 00  
          8 | (0,3)  |      16 | f     | f    | 30 00 00 00 00 00 00 00  
          9 | (0,9)  |      16 | f     | f    | 33 00 00 00 00 00 00 00  
         10 | (0,7)  |      16 | f     | f    | 46 00 00 00 00 00 00 00  
         11 | (0,2)  |      16 | f     | f    | 4b 00 00 00 00 00 00 00  
(11 rows)  

7、垃圾回收,首先回收索引垃圾版本,最后回收表的垃圾版本。

postgres=# vacuum verbose a;  
INFO:  vacuuming "public.a"  
INFO:  scanned index "idx_a_1" to remove 1 row versions  
DETAIL:  CPU: user: 0.00 s, system: 0.00 s, elapsed: 0.00 s  
INFO:  scanned index "idx_a_2" to remove 1 row versions  
DETAIL:  CPU: user: 0.00 s, system: 0.00 s, elapsed: 0.00 s  
INFO:  scanned index "idx_a_3" to remove 1 row versions  
DETAIL:  CPU: user: 0.00 s, system: 0.00 s, elapsed: 0.00 s  
INFO:  "a": removed 1 row versions in 1 pages  
DETAIL:  CPU: user: 0.00 s, system: 0.00 s, elapsed: 0.00 s  
INFO:  index "idx_a_1" now contains 10 row versions in 2 pages  
DETAIL:  1 index row versions were removed.  
0 index pages have been deleted, 0 are currently reusable.  
CPU: user: 0.00 s, system: 0.00 s, elapsed: 0.00 s.  
INFO:  index "idx_a_2" now contains 10 row versions in 2 pages  
DETAIL:  1 index row versions were removed.  
0 index pages have been deleted, 0 are currently reusable.  
CPU: user: 0.00 s, system: 0.00 s, elapsed: 0.00 s.  
INFO:  index "idx_a_3" now contains 10 row versions in 2 pages  
DETAIL:  1 index row versions were removed.  
0 index pages have been deleted, 0 are currently reusable.  
CPU: user: 0.00 s, system: 0.00 s, elapsed: 0.00 s.  
INFO:  "a": found 5 removable, 10 nonremovable row versions in 1 out of 1 pages  
DETAIL:  0 dead row versions cannot be removed yet, oldest xmin: 716311280  
There were 4 unused item pointers.  
Skipped 0 pages due to buffer pins, 0 frozen pages.  
0 pages are entirely empty.  
CPU: user: 0.00 s, system: 0.00 s, elapsed: 0.00 s.  
VACUUM  

8、垃圾回收后,索引的垃圾版本被清除。

postgres=# SELECT * FROM bt_page_items('idx_a_1',1);  
 itemoffset |  ctid  | itemlen | nulls | vars |          data             
------------+--------+---------+-------+------+-------------------------  
          1 | (0,15) |      16 | f     | f    | 01 00 00 00 00 00 00 00  
          2 | (0,2)  |      16 | f     | f    | 02 00 00 00 00 00 00 00  
          3 | (0,3)  |      16 | f     | f    | 03 00 00 00 00 00 00 00  
          4 | (0,4)  |      16 | f     | f    | 04 00 00 00 00 00 00 00  
          5 | (0,5)  |      16 | f     | f    | 05 00 00 00 00 00 00 00  
          6 | (0,6)  |      16 | f     | f    | 06 00 00 00 00 00 00 00  
          7 | (0,7)  |      16 | f     | f    | 07 00 00 00 00 00 00 00  
          8 | (0,8)  |      16 | f     | f    | 08 00 00 00 00 00 00 00  
          9 | (0,9)  |      16 | f     | f    | 09 00 00 00 00 00 00 00  
         10 | (0,10) |      16 | f     | f    | 0a 00 00 00 00 00 00 00  
(10 rows)  
  
postgres=# SELECT * FROM bt_page_items('idx_a_2',1);  
 itemoffset |  ctid  | itemlen | nulls | vars |          data             
------------+--------+---------+-------+------+-------------------------  
          1 | (0,5)  |      16 | f     | f    | 05 00 00 00 00 00 00 00  
          2 | (0,7)  |      16 | f     | f    | 06 00 00 00 00 00 00 00  
          3 | (0,15) |      16 | f     | f    | 0d 00 00 00 00 00 00 00  
          4 | (0,9)  |      16 | f     | f    | 0e 00 00 00 00 00 00 00  
          5 | (0,4)  |      16 | f     | f    | 20 00 00 00 00 00 00 00  
          6 | (0,3)  |      16 | f     | f    | 24 00 00 00 00 00 00 00  
          7 | (0,6)  |      16 | f     | f    | 28 00 00 00 00 00 00 00  
          8 | (0,10) |      16 | f     | f    | 2a 00 00 00 00 00 00 00  
          9 | (0,2)  |      16 | f     | f    | 3f 00 00 00 00 00 00 00  
         10 | (0,8)  |      16 | f     | f    | 55 00 00 00 00 00 00 00  
(10 rows)  
  
postgres=# SELECT * FROM bt_page_items('idx_a_3',1);  
 itemoffset |  ctid  | itemlen | nulls | vars |          data             
------------+--------+---------+-------+------+-------------------------  
          1 | (0,6)  |      16 | f     | f    | 09 00 00 00 00 00 00 00  
          2 | (0,5)  |      16 | f     | f    | 12 00 00 00 00 00 00 00  
          3 | (0,8)  |      16 | f     | f    | 13 00 00 00 00 00 00 00  
          4 | (0,15) |      16 | f     | f    | 15 00 00 00 00 00 00 00  
          5 | (0,4)  |      16 | f     | f    | 22 00 00 00 00 00 00 00  
          6 | (0,10) |      16 | f     | f    | 2b 00 00 00 00 00 00 00  
          7 | (0,3)  |      16 | f     | f    | 30 00 00 00 00 00 00 00  
          8 | (0,9)  |      16 | f     | f    | 33 00 00 00 00 00 00 00  
          9 | (0,7)  |      16 | f     | f    | 46 00 00 00 00 00 00 00  
         10 | (0,2)  |      16 | f     | f    | 4b 00 00 00 00 00 00 00  
(10 rows)  

性能测试

1、创建测试表32个索引。

do language plpgsql $$  
declare  
  sql text;  
begin  
  sql := 'create table a (id int primary key,';  
  for i in 1..32 loop  
    sql := sql||'c'||i||' int default random()*1000,';  
  end loop;  
  sql := rtrim(sql,',');  
  sql := sql||') with (fillfactor=80)';   
  execute sql;  
  for i in 2..32 loop  
    execute 'create index idx_a_c'||i||' on a (c'||i||')';  
  end loop;  
end;  
$$;  

2、写入1000万记录

insert into a (id) select generate_series(1,10000000);  

3、non-hot更新

vi test_non_hot.sql  
\set id random(1,10000000)  
update a set c2=c2+random()*100-100 where id=:id;  

4、HOT更新

vi test_hot1.sql  
\set id random(1,10000000)  
update a set c1=c1+random()*100-100 where id=:id;  
  
vi test_hot2.sql  
\set id random(1,10000000)  
update a set c2=c2 where id=:id;  

5、性能对比

5.1、HOT

pgbench -M prepared -n -r -P 1 -f ./test_hot1.sql -c 28 -j 28 -T 120  
  
transaction type: ./test_hot1.sql  
scaling factor: 1  
query mode: prepared  
number of clients: 28  
number of threads: 28  
duration: 120 s  
number of transactions actually processed: 9139010  
latency average = 0.368 ms  
latency stddev = 0.187 ms  
tps = 76157.798606 (including connections establishing)  
tps = 76174.469712 (excluding connections establishing)  
script statistics:  
 - statement latencies in milliseconds:  
         0.001  \set id random(1,10000000)  
         0.366  update a set c1=c1+random()*100-100 where id=:id;  
  
Total DISK READ :       0.00 B/s | Total DISK WRITE :      13.14 M/s  
Actual DISK READ:       0.00 B/s | Actual DISK WRITE:      13.82 M/s  
  TID  PRIO  USER     DISK READ  DISK WRITE  SWAPIN     IO>    COMMAND                                                                                                                                                                         
45828 be/4 postgres    0.00 B/s   23.39 K/s  0.00 %  1.06 % postgres: postgres postgres 127.0.0.1(41326) UPDATE  
45810 be/4 postgres    0.00 B/s   23.39 K/s  0.00 %  0.99 % postgres: postgres postgres 127.0.0.1(41290) UPDATE  
45821 be/4 postgres    0.00 B/s   23.39 K/s  0.00 %  0.98 % postgres: postgres postgres 127.0.0.1(41312) idle    
45820 be/4 postgres    0.00 B/s    7.80 K/s  0.00 %  0.96 % postgres: postgres postgres 127.0.0.1(41310) UPDATE  
45822 be/4 postgres    0.00 B/s    0.00 B/s  0.00 %  0.96 % postgres: postgres postgres 127.0.0.1(41314) UPDATE  
45819 be/4 postgres    0.00 B/s    7.80 K/s  0.00 %  0.94 % postgres: postgres postgres 127.0.0.1(41308) UPDATE  
45806 be/4 postgres    0.00 B/s    7.80 K/s  0.00 %  0.92 % postgres: postgres postgres 127.0.0.1(41282) idle    
45824 be/4 postgres    0.00 B/s    7.80 K/s  0.00 %  0.90 % postgres: postgres postgres 127.0.0.1(41318) UPDATE  
45827 be/4 postgres    0.00 B/s   54.58 K/s  0.00 %  0.89 % postgres: postgres postgres 127.0.0.1(41324) UPDATE  
45814 be/4 postgres    0.00 B/s    0.00 B/s  0.00 %  0.89 % postgres: postgres postgres 127.0.0.1(41298) UPDATE  
45818 be/4 postgres    0.00 B/s    7.80 K/s  0.00 %  0.88 % postgres: postgres postgres 127.0.0.1(41306) idle    
45823 be/4 postgres    0.00 B/s   15.59 K/s  0.00 %  0.87 % postgres: postgres postgres 127.0.0.1(41316) UPDATE  
45805 be/4 postgres    0.00 B/s    0.00 B/s  0.00 %  0.87 % postgres: postgres postgres 127.0.0.1(41280) UPDATE  
45826 be/4 postgres    0.00 B/s    7.80 K/s  0.00 %  0.86 % postgres: postgres postgres 127.0.0.1(41322) UPDATE  
45809 be/4 postgres    0.00 B/s    0.00 B/s  0.00 %  0.86 % postgres: postgres postgres 127.0.0.1(41288) UPDATE  
45808 be/4 postgres    0.00 B/s   23.39 K/s  0.00 %  0.85 % postgres: postgres postgres 127.0.0.1(41286) UPDATE  
45825 be/4 postgres    0.00 B/s    0.00 B/s  0.00 %  0.85 % postgres: postgres postgres 127.0.0.1(41320) UPDATE  
45804 be/4 postgres    0.00 B/s   23.39 K/s  0.00 %  0.84 % postgres: postgres postgres 127.0.0.1(41278) UPDATE  
49040 be/4 postgres    0.00 B/s   12.85 M/s  0.00 %  0.84 % postgres: wal writer process  
45816 be/4 postgres    0.00 B/s    7.80 K/s  0.00 %  0.66 % postgres: postgres postgres 127.0.0.1(41302) idle    
45829 be/4 postgres    0.00 B/s    0.00 B/s  0.00 %  0.64 % postgres: postgres postgres 127.0.0.1(41328) UPDATE  
45803 be/4 postgres    0.00 B/s    0.00 B/s  0.00 %  0.62 % postgres: postgres postgres 127.0.0.1(41276) BIND    
45795 be/4 postgres    0.00 B/s    0.00 B/s  0.00 %  0.59 % postgres: postgres postgres 127.0.0.1(41274) UPDATE  
45807 be/4 postgres    0.00 B/s    7.80 K/s  0.00 %  0.58 % postgres: postgres postgres 127.0.0.1(41284) UPDATE  
45812 be/4 postgres    0.00 B/s   15.59 K/s  0.00 %  0.56 % postgres: postgres postgres 127.0.0.1(41294) UPDATE  
45811 be/4 postgres    0.00 B/s   15.59 K/s  0.00 %  0.55 % postgres: postgres postgres 127.0.0.1(41292) UPDATE  
45817 be/4 postgres    0.00 B/s    7.80 K/s  0.00 %  0.55 % postgres: postgres postgres 127.0.0.1(41304) UPDATE  
45815 be/4 postgres    0.00 B/s    7.80 K/s  0.00 %  0.55 % postgres: postgres postgres 127.0.0.1(41300) UPDATE  
45813 be/4 postgres    0.00 B/s    0.00 B/s  0.00 %  0.51 % postgres: postgres postgres 127.0.0.1(41296) UPDATE  

5.2、NON-HOT

pgbench -M prepared -n -r -P 1 -f ./test_non_hot.sql -c 28 -j 28 -T 120  
  
transaction type: ./test_non_hot.sql  
scaling factor: 1  
query mode: prepared  
number of clients: 28  
number of threads: 28  
duration: 120 s  
number of transactions actually processed: 6472445  
latency average = 0.519 ms  
latency stddev = 0.707 ms  
tps = 53922.273197 (including connections establishing)  
tps = 53933.908671 (excluding connections establishing)  
script statistics:  
 - statement latencies in milliseconds:  
         0.002  \set id random(1,10000000)  
         0.517  update a set c2=c2+random()*100-100 where id=:id;  
  
Total DISK READ :       0.00 B/s | Total DISK WRITE :     191.66 M/s  
Actual DISK READ:       0.00 B/s | Actual DISK WRITE:     142.11 M/s  
  TID  PRIO  USER     DISK READ  DISK WRITE  SWAPIN     IO>    COMMAND                                                                                                                                                                         
49040 be/4 postgres    0.00 B/s  136.56 M/s  0.00 %  7.17 % postgres: wal writer process  
45997 be/4 postgres    0.00 B/s 1918.73 K/s  0.00 %  1.60 % postgres: postgres postgres 127.0.0.1(41384) BIND    
45983 be/4 postgres    0.00 B/s 1903.25 K/s  0.00 %  1.56 % postgres: postgres postgres 127.0.0.1(41356) UPDATE  
45977 be/4 postgres    0.00 B/s 1829.75 K/s  0.00 %  1.54 % postgres: postgres postgres 127.0.0.1(41344) UPDATE  
45984 be/4 postgres    0.00 B/s 1918.73 K/s  0.00 %  1.53 % postgres: postgres postgres 127.0.0.1(41358) UPDATE  
45985 be/4 postgres    0.00 B/s 1616.99 K/s  0.00 %  1.50 % postgres: postgres postgres 127.0.0.1(41360) UPDATE  
45986 be/4 postgres    0.00 B/s 1748.52 K/s  0.00 %  1.49 % postgres: postgres postgres 127.0.0.1(41362) UPDATE  
45995 be/4 postgres    0.00 B/s 1616.99 K/s  0.00 %  1.47 % postgres: postgres postgres 127.0.0.1(41380) UPDATE  
45988 be/4 postgres    0.00 B/s 1910.99 K/s  0.00 %  1.46 % postgres: postgres postgres 127.0.0.1(41366) UPDATE  
45979 be/4 postgres    0.00 B/s 1763.99 K/s  0.00 %  1.46 % postgres: postgres postgres 127.0.0.1(41348) UPDATE  
45976 be/4 postgres    0.00 B/s 1918.73 K/s  0.00 %  1.45 % postgres: postgres postgres 127.0.0.1(41342) UPDATE  
45982 be/4 postgres    0.00 B/s 1887.78 K/s  0.00 %  1.43 % postgres: postgres postgres 127.0.0.1(41354) UPDATE  
45987 be/4 postgres    0.00 B/s 2019.31 K/s  0.00 %  1.42 % postgres: postgres postgres 127.0.0.1(41364) UPDATE  
45992 be/4 postgres    0.00 B/s 1616.99 K/s  0.00 %  1.40 % postgres: postgres postgres 127.0.0.1(41374) UPDATE  
45994 be/4 postgres    0.00 B/s 1941.94 K/s  0.00 %  1.38 % postgres: postgres postgres 127.0.0.1(41378) UPDATE  
45990 be/4 postgres    0.00 B/s 1794.94 K/s  0.00 %  1.36 % postgres: postgres postgres 127.0.0.1(41370) UPDATE  
45975 be/4 postgres    0.00 B/s 1934.20 K/s  0.00 %  1.35 % postgres: postgres postgres 127.0.0.1(41340) UPDATE  
45974 be/4 postgres    0.00 B/s 1910.99 K/s  0.00 %  1.35 % postgres: postgres postgres 127.0.0.1(41338) UPDATE  
45980 be/4 postgres    0.00 B/s 2003.83 K/s  0.00 %  1.28 % postgres: postgres postgres 127.0.0.1(41350) UPDATE  
45991 be/4 postgres    0.00 B/s 1748.52 K/s  0.00 %  1.13 % postgres: postgres postgres 127.0.0.1(41372) UPDATE  
45996 be/4 postgres    0.00 B/s 2003.83 K/s  0.00 %  1.04 % postgres: postgres postgres 127.0.0.1(41382) UPDATE  
45993 be/4 postgres    0.00 B/s 1918.73 K/s  0.00 %  1.02 % postgres: postgres postgres 127.0.0.1(41376) UPDATE  
45972 be/4 postgres    0.00 B/s 1903.25 K/s  0.00 %  1.02 % postgres: postgres postgres 127.0.0.1(41334) UPDATE  
45978 be/4 postgres    0.00 B/s 1740.78 K/s  0.00 %  1.02 % postgres: postgres postgres 127.0.0.1(41346) UPDATE  
45998 be/4 postgres    0.00 B/s 1841.36 K/s  0.00 %  1.02 % postgres: postgres postgres 127.0.0.1(41386) UPDATE  
45981 be/4 postgres    0.00 B/s 1818.15 K/s  0.00 %  1.01 % postgres: postgres postgres 127.0.0.1(41352) UPDATE  
45989 be/4 postgres    0.00 B/s 1895.52 K/s  0.00 %  1.01 % postgres: postgres postgres 127.0.0.1(41368) UPDATE  
45973 be/4 postgres    0.00 B/s 1941.94 K/s  0.00 %  0.99 % postgres: postgres postgres 127.0.0.1(41336) idle    
45961 be/4 postgres    0.00 B/s 1872.31 K/s  0.00 %  0.96 % postgres: postgres postgres 127.0.0.1(41332) UPDATE  
49039 be/4 postgres    0.00 B/s    4.37 M/s  0.00 %  0.00 % postgres: writer process  
49036 be/4 postgres    0.00 B/s    3.87 K/s  0.00 %  0.00 % postgres: logger process           

使用HOT技术,使得TPS从 53922 提升到了 76157 。IO资源消耗从 192MB/s 降低到了 14MB/s 。

小结

HOT、二级索引、zheap存储引擎,都可以解决更新引入的索引放大问题。

本文介绍了HOT,HOT必须满足如下两个条件:

Necessary Condition A: UPDATE does not change any of the index keys    
  
Necessary Condition B: The new version should fit in the same old block – HOT chains can not cross block boundary.   
  
1、索引字段的值不变。(其中任意一个索引字段的值发生了变化,则所有索引都需要新增版本)  
  
2、新的版本与旧的版本在同一个HEAP PAGE中。  

参考

《PostgreSQL 11 preview - Surjective indexes - 索引HOT增强(表达式)update评估》

HOT Inside - The Technical Architecture

《PostgreSQL 10.0 preview 性能增强 - 间接索引(secondary index)》

https://github.com/postgres/postgres/blob/master/src/backend/access/heap/README.HOT

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