阿里云RDS PostgreSQL时序数据的优化

快来打我* 2022-05-23 12:39 205阅读 0赞

# 阿里云RDS PostgreSQL时序数据的优化 #

*  2008年6月8日
 *  版权声明：本文为博主chszs的原创文章，未经博主允许不得转载。

PostgreSQL是最流行的开源数据库之一。PostgreSQL的一个优势是它可以通过多种方式进行优化，例如数据合并和数据清理。而数据合并和数据清理在一些应用场景下是必需的。

例如：

*  当数据表的具体内容被更新（插入、更新、删除）时，我们需要先合并，然后迅速得到每个主键的最新值。
 *  当我们有大量连续不断地报告数据的传感器时，我们需要及时收集每个传感器的最新读数。
 *  我们可以使用窗口查询进行这种操作，但我们需要快速检索批量数据。

通常有四种优化时序数据的方法：

1.  当只有很少的唯一值和一个未知的范围时，我们可以使用递归（recursion）。
2.  当只有很少的唯一值并且它们的范围已经确定，我们可以使用子查询（subquery）。
3.  当有许多唯一值时，窗口查询（Window query）比上述方法更合适。
4.  流计算（Stream computing）是所有场景中最好的。

本文只会比较前三种方法。流计算不需要进行比较，因为它是所有场景中最强大的方法。

## 递归 vs. 子查询 vs. 窗口查询 ##

在比较中，将使用一个包含500万条唯一值的数据库作为数据源，并在以下情况下比较这些方法。

### 递归 ###

#### 情景1、有大量有效的唯一值（100万个唯一值） ####

`第1步：创建一个表`

\timing  
    drop table test;  
    create unlogged table test(id int , info text, crt_time timestamp);

`第2步：构建数据`

insert into test select ceil(random()*1000000), md5(random()::text), clock_timestamp() from generate_series(1,5000000);

`第3步：创建一个索引`

create index idx_test_1 on test (id, crt_time desc);

`第4步：递归查询效率`

explain (analyze,verbose,timing,costs,buffers) with recursive skip as (    
      (    
        select test as v from test where id in (select id from test where id is not null order by id,crt_time desc limit 1) limit 1  
      )    
      union all    
      (    
        select (  
          select t as v from test t where t.id>(s.v).id and t.id is not null order by id,crt_time desc limit 1  
        ) from skip s where (s.v).id is not null  
      )      -- The "where (s.v).id is not null" must be included. Else you will be stuck in an infinite loop.   
    )     
    select (t.v).id, (t.v).info, (t.v).crt_time from skip t where t.* is not null;   
                                                                                          QUERY PLAN                                                                                        
    --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------  
     CTE Scan on skip t  (cost=54.35..56.37 rows=100 width=44) (actual time=0.042..6626.084 rows=993288 loops=1)  
       Output: (t.v).id, (t.v).info, (t.v).crt_time  
       Filter: (t.* IS NOT NULL)  
       Rows Removed by Filter: 1  
       Buffers: shared hit=3976934  
       CTE skip  
         ->  Recursive Union  (cost=0.91..54.35 rows=101 width=69) (actual time=0.034..6006.615 rows=993289 loops=1)  
               Buffers: shared hit=3976934  
               ->  Limit  (cost=0.91..0.93 rows=1 width=69) (actual time=0.033..0.033 rows=1 loops=1)  
                     Output: test.*  
                     Buffers: shared hit=8  
                     ->  Nested Loop  (cost=0.91..10.19 rows=500 width=69) (actual time=0.032..0.032 rows=1 loops=1)  
                           Output: test.*  
                           Buffers: shared hit=8  
                           ->  HashAggregate  (cost=0.48..0.49 rows=1 width=4) (actual time=0.021..0.021 rows=1 loops=1)  
                                 Output: test_1.id  
                                 Group Key: test_1.id  
                                 Buffers: shared hit=4  
                                 ->  Limit  (cost=0.43..0.47 rows=1 width=12) (actual time=0.016..0.016 rows=1 loops=1)  
                                       Output: test_1.id, test_1.crt_time  
                                       Buffers: shared hit=4  
                                       ->  Index Only Scan using idx_test_1 on public.test test_1  (cost=0.43..173279.36 rows=5000002 width=12) (actual time=0.015..0.015 rows=1 loops=1)  
                                             Output: test_1.id, test_1.crt_time  
                                             Index Cond: (test_1.id IS NOT NULL)  
                                             Heap Fetches: 1  
                                             Buffers: shared hit=4  
                           ->  Index Scan using idx_test_1 on public.test  (cost=0.43..9.64 rows=6 width=73) (actual time=0.009..0.009 rows=1 loops=1)  
                                 Output: test.*, test.id  
                                 Index Cond: (test.id = test_1.id)  
                                 Buffers: shared hit=4  
               ->  WorkTable Scan on skip s  (cost=0.00..5.14 rows=10 width=32) (actual time=0.006..0.006 rows=1 loops=993289)  
                     Output: (SubPlan 1)  
                     Filter: ((s.v).id IS NOT NULL)  
                     Rows Removed by Filter: 0  
                     Buffers: shared hit=3976926  
                     SubPlan 1  
                       ->  Limit  (cost=0.43..0.49 rows=1 width=81) (actual time=0.005..0.005 rows=1 loops=993288)  
                             Output: t_1.*, t_1.id, t_1.crt_time  
                             Buffers: shared hit=3976926  
                             ->  Index Scan using idx_test_1 on public.test t_1  (cost=0.43..102425.17 rows=1666667 width=81) (actual time=0.005..0.005 rows=1 loops=993288)  
                                   Output: t_1.*, t_1.id, t_1.crt_time  
                                   Index Cond: ((t_1.id > (s.v).id) AND (t_1.id IS NOT NULL))  
                                   Buffers: shared hit=3976926  
     Planning time: 0.354 ms  
     Execution time: 6706.105 ms  
    (45 rows)

#### 情景二、只有很少有效的唯一值（1,000个唯一值） ####

`第1步：创建一个表`

\timing  
    drop table test;  
    create unlogged table test(id int , info text, crt_time timestamp);

`第2步：构建数据`

insert into test select ceil(random()*1000), md5(random()::text), clock_timestamp() from generate_series(1,5000000);

`第3步：创建一个索引`

create index idx_test_1 on test (id, crt_time desc);

`第4步：递归查询效率`

Query statement stays unchanged  
                                                                                          QUERY PLAN                                                                                        
    --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------  
     CTE Scan on skip t  (cost=55.09..57.11 rows=100 width=44) (actual time=0.046..8.859 rows=1000 loops=1)  
       Output: (t.v).id, (t.v).info, (t.v).crt_time  
       Filter: (t.* IS NOT NULL)  
       Rows Removed by Filter: 1  
       Buffers: shared hit=4007  
       CTE skip  
         ->  Recursive Union  (cost=0.91..55.09 rows=101 width=69) (actual time=0.039..8.203 rows=1001 loops=1)  
               Buffers: shared hit=4007  
               ->  Limit  (cost=0.91..1.67 rows=1 width=69) (actual time=0.038..0.038 rows=1 loops=1)  
                     Output: test.*  
                     Buffers: shared hit=8  
                     ->  Nested Loop  (cost=0.91..6335.47 rows=8333 width=69) (actual time=0.038..0.038 rows=1 loops=1)  
                           Output: test.*  
                           Buffers: shared hit=8  
                           ->  HashAggregate  (cost=0.48..0.49 rows=1 width=4) (actual time=0.021..0.021 rows=1 loops=1)  
                                 Output: test_1.id  
                                 Group Key: test_1.id  
                                 Buffers: shared hit=4  
                                 ->  Limit  (cost=0.43..0.47 rows=1 width=12) (actual time=0.016..0.017 rows=1 loops=1)  
                                       Output: test_1.id, test_1.crt_time  
                                       Buffers: shared hit=4  
                                       ->  Index Only Scan using idx_test_1 on public.test test_1  (cost=0.43..173279.55 rows=5000002 width=12) (actual time=0.015..0.015 rows=1 loops=1)  
                                             Output: test_1.id, test_1.crt_time  
                                             Index Cond: (test_1.id IS NOT NULL)  
                                             Heap Fetches: 1  
                                             Buffers: shared hit=4  
                           ->  Index Scan using idx_test_1 on public.test  (cost=0.43..6284.98 rows=5000 width=73) (actual time=0.015..0.015 rows=1 loops=1)  
                                 Output: test.*, test.id  
                                 Index Cond: (test.id = test_1.id)  
                                 Buffers: shared hit=4  
               ->  WorkTable Scan on skip s  (cost=0.00..5.14 rows=10 width=32) (actual time=0.008..0.008 rows=1 loops=1001)  
                     Output: (SubPlan 1)  
                     Filter: ((s.v).id IS NOT NULL)  
                     Rows Removed by Filter: 0  
                     Buffers: shared hit=3999  
                     SubPlan 1  
                       ->  Limit  (cost=0.43..0.49 rows=1 width=81) (actual time=0.007..0.007 rows=1 loops=1000)  
                             Output: t_1.*, t_1.id, t_1.crt_time  
                             Buffers: shared hit=3999  
                             ->  Index Scan using idx_test_1 on public.test t_1  (cost=0.43..102425.80 rows=1666667 width=81) (actual time=0.007..0.007 rows=1 loops=1000)  
                                   Output: t_1.*, t_1.id, t_1.crt_time  
                                   Index Cond: ((t_1.id > (s.v).id) AND (t_1.id IS NOT NULL))  
                                   Buffers: shared hit=3999  
     Planning time: 0.353 ms  
     Execution time: 8.980 ms  
    (45 rows)

### 子查询 ###

#### 情景1、有大量有效的唯一值（100万个唯一值） ####

`第1步：子查询查询效率`

如果ID的值范围过宽，子查询效率会很低下。

需要维护一个唯一的ID表。这里我们使用generate\_series作为测试的替代。

explain (analyze,verbose,timing,costs,buffers) select (select test from test where id=t.id order by crt_time desc limit 1) from generate_series(1,1000000) t(id);  
                                                                     QUERY PLAN                                                                    
    ---------------------------------------------------------------------------------------------------------------------------------------------  
     Function Scan on pg_catalog.generate_series t  (cost=0.00..1976.65 rows=1000 width=32) (actual time=70.682..2835.109 rows=1000000 loops=1)  
       Output: (SubPlan 1)  
       Function Call: generate_series(1, 1000000)  
       Buffers: shared hit=3997082  
       SubPlan 1  
         ->  Limit  (cost=0.43..1.97 rows=1 width=77) (actual time=0.002..0.002 rows=1 loops=1000000)  
               Output: test.*, test.crt_time  
               Buffers: shared hit=3997082  
               ->  Index Scan using idx_test_1 on public.test  (cost=0.43..9.64 rows=6 width=77) (actual time=0.002..0.002 rows=1 loops=1000000)  
                     Output: test.*, test.crt_time  
                     Index Cond: (test.id = t.id)  
                     Buffers: shared hit=3997082  
     Planning time: 0.119 ms  
     Execution time: 2892.712 ms  
    (14 rows)

#### 情景二、只有很少有效的唯一值（1,000个唯一值） ####

`第1步：子查询查询效率`

查询语句更改为

explain (analyze,verbose,timing,costs,buffers) select (select test from test where id=t.id order by crt_time desc limit 1) from generate_series(1,1000) t(id);  
                                                                       QUERY PLAN                                                                     
    ------------------------------------------------------------------------------------------------------------------------------------------------  
     Function Scan on pg_catalog.generate_series t  (cost=0.00..1699.41 rows=1000 width=32) (actual time=0.107..7.041 rows=1000 loops=1)  
       Output: (SubPlan 1)  
       Function Call: generate_series(1, 1000)  
       Buffers: shared hit=4000  
       SubPlan 1  
         ->  Limit  (cost=0.43..1.69 rows=1 width=77) (actual time=0.006..0.007 rows=1 loops=1000)  
               Output: test.*, test.crt_time  
               Buffers: shared hit=4000  
               ->  Index Scan using idx_test_1 on public.test  (cost=0.43..6284.98 rows=5000 width=77) (actual time=0.006..0.006 rows=1 loops=1000)  
                     Output: test.*, test.crt_time  
                     Index Cond: (test.id = t.id)  
                     Buffers: shared hit=4000  
     Planning time: 0.131 ms  
     Execution time: 7.126 ms  
    (14 rows)

### 窗口查询 ###

#### 情景1、有大量有效的唯一值（100万个唯一值） ####

`第1步：窗口查询效率`

explain (analyze,verbose,timing,costs,buffers) select id,info,crt_time from (select row_number() over (partition by id order by crt_time desc) as rn, * from test) t where rn=1;  
    postgres=# explain (analyze,verbose,timing,costs,buffers) select id,info,crt_time from (select row_number() over (partition by id order by crt_time desc) as rn, * from test) t where rn=1;  
                                                                           QUERY PLAN                                                                          
    ---------------------------------------------------------------------------------------------------------------------------------------------------------  
     Subquery Scan on t  (cost=0.43..310779.41 rows=25000 width=45) (actual time=0.027..6398.308 rows=993288 loops=1)  
       Output: t.id, t.info, t.crt_time  
       Filter: (t.rn = 1)  
       Rows Removed by Filter: 4006712  
       Buffers: shared hit=5018864  
       ->  WindowAgg  (cost=0.43..248279.39 rows=5000002 width=53) (actual time=0.026..5973.497 rows=5000000 loops=1)  
             Output: row_number() OVER (?), test.id, test.info, test.crt_time  
             Buffers: shared hit=5018864  
             ->  Index Scan using idx_test_1 on public.test  (cost=0.43..160779.35 rows=5000002 width=45) (actual time=0.019..4058.476 rows=5000000 loops=1)  
                   Output: test.id, test.info, test.crt_time  
                   Buffers: shared hit=5018864  
     Planning time: 0.121 ms  
     Execution time: 6446.901 ms  
    (13 rows)

#### 情景二、只有很少有效的唯一值（1,000个唯一值） ####

`第1步：窗口查询效率`

查询语句保持不变

QUERY PLAN                                                                          
    ---------------------------------------------------------------------------------------------------------------------------------------------------------  
     Subquery Scan on t  (cost=0.43..310779.61 rows=25000 width=45) (actual time=0.027..6176.801 rows=1000 loops=1)  
       Output: t.id, t.info, t.crt_time  
       Filter: (t.rn = 1)  
       Rows Removed by Filter: 4999000  
       Buffers: shared hit=4744850 read=18157  
       ->  WindowAgg  (cost=0.43..248279.58 rows=5000002 width=53) (actual time=0.026..5822.576 rows=5000000 loops=1)  
             Output: row_number() OVER (?), test.id, test.info, test.crt_time  
             Buffers: shared hit=4744850 read=18157  
             ->  Index Scan using idx_test_1 on public.test  (cost=0.43..160779.55 rows=5000002 width=45) (actual time=0.020..4175.082 rows=5000000 loops=1)  
                   Output: test.id, test.info, test.crt_time  
                   Buffers: shared hit=4744850 read=18157  
     Planning time: 0.108 ms  
     Execution time: 6176.924 ms  
    (13    rows)

### 横向效率比较图 ###

![这里写图片描述][70]

## 结论 ##

随着物联网的兴起，越来越多的业务会使用时序数据，在必须根据这些数据提供服务的情况下，计算数据中的最新值和滑动窗口中的值是至关重要的。

PostgreSQL是开源数据库的最佳选择，因为它为相同的问题提供了几种解决方案。参考数据优化方法，我们可以得出结论：

1.  递归适用于只有少量唯一值但范围未知的情况。
2.  子查询适用于唯一值很少且范围已确定的情况。
3.  当有大量唯一值时，窗口查询比子查询更合适。
4.  流计算是所有场景的最佳选择。

[70]: /images/20220523/d9b0318be06341158517099ba825f0f7.png