sql – max()与ORDER BY DESC LIMIT 1的性能

我今天正在对一些慢速SQL查询进行故障排除,并且不太了解下面的性能差异：

当尝试根据某些条件从数据表中提取max(时间戳)时,如果存在匹配的行,则使用MAX()比ORDER BY时间戳LIMIT 1慢,但如果找不到匹配的行,则速度要快得多.

SELECT timestamp
FROM data JOIN sensors ON ( sensors.id = data.sensor_id )
WHERE sensor.station_id = 4
ORDER BY timestamp DESC
LIMIT 1;
(0 rows)  
Time: 1314.544 ms

SELECT timestamp
FROM data JOIN sensors ON ( sensors.id = data.sensor_id )
WHERE sensor.station_id = 5
ORDER BY timestamp DESC
LIMIT 1;
(1 row)  
Time: 10.890 ms

SELECT MAX(timestamp)
FROM data JOIN sensors ON ( sensors.id = data.sensor_id )
WHERE sensor.station_id = 4;
(0 rows)
Time: 0.869 ms

SELECT MAX(timestamp)
FROM data JOIN sensors ON ( sensors.id = data.sensor_id )
WHERE sensor.station_id = 5;
(1 row)
Time: 84.087 ms

(timestamp)和(sensor_id,timestamp)上有索引,我注意到Postgres对这两种情况使用了非常不同的查询计划和索引：

QUERY PLAN (ORDER BY)                                              
--------------------------------------------------------------------------------------------------------
Limit  (cost=0.43..9.47 rows=1 width=8)
    ->  Nested Loop  (cost=0.43..396254.63 rows=43823 width=8)
          Join Filter: (data.sensor_id = sensors.id)
          ->  Index Scan using timestamp_ind on data  (cost=0.43..254918.66 rows=4710976 width=12)
          ->  Materialize  (cost=0.00..6.70 rows=2 width=4)
              ->  Seq Scan on sensors  (cost=0.00..6.69 rows=2 width=4)
                  Filter: (station_id = 4)
(7 rows)

QUERY PLAN (MAX)                                               
----------------------------------------------------------------------------------------------------------
Aggregate  (cost=3680.59..3680.60 rows=1 width=8)
    ->  Nested Loop  (cost=0.43..3571.03 rows=43823 width=8)
        ->  Seq Scan on sensors  (cost=0.00..6.69 rows=2 width=4)
              Filter: (station_id = 4)
        ->  Index Only Scan using sensor_ind_timestamp on data  (cost=0.43..1389.59 rows=39258 width=12)
              Index Cond: (sensor_id = sensors.id)
(6 rows)

所以我的两个问题是：

>这种性能差异来自哪里？我已经在MIN/MAX vs ORDER BY and LIMIT看到了接受的答案,但这似乎并不适用于此.任何好的资源将不胜感激.
>有没有比添加EXISTS检查更好的方法来提高所有情况下的性能(匹配行与没有匹配的行)？

编辑以解决以下评论中的问题.我保留了上面的初始查询计划以供将来参考：

表定义：

Table "public.sensors"
        Column        |          Type          |                            Modifiers                            
----------------------+------------------------+-----------------------------------------------------------------
id                    | integer                | not null default nextval('sensors_id_seq'::regclass)
station_id            | integer                | not null
....

Indexes:
    "sensor_primary" PRIMARY KEY,btree (id)
    "ind_station_id" btree (station_id,id)
    "ind_station" btree (station_id)

                                  Table "public.data"
  Column   |           Type           |                            Modifiers                             
-----------+--------------------------+------------------------------------------------------------------
 id        | integer                  | not null default nextval('data_id_seq'::regclass)
 timestamp | timestamp with time zone | not null
 sensor_id | integer                  | not null
 avg       | integer                  |

Indexes:
    "timestamp_ind" btree ("timestamp" DESC)
    "sensor_ind" btree (sensor_id)
    "sensor_ind_timestamp" btree (sensor_id,"timestamp")
    "sensor_ind_timestamp_desc" btree (sensor_id,"timestamp" DESC)

请注意,在下面的@Erwin建议之后,我刚刚在传感器上添加了ind_station_id.时间没有真正改变,在ORDER BY DESC LIMIT 1情况下仍然> 1200ms,在MAX情况下仍然是~0.9ms.

查询计划：

QUERY PLAN (ORDER BY)
----------------------------------------------------------------------------------------------------------
Limit  (cost=0.58..9.62 rows=1 width=8) (actual time=2161.054..2161.054 rows=0 loops=1)
  Buffers: shared hit=3418066 read=47326
  ->  Nested Loop  (cost=0.58..396382.45 rows=43823 width=8) (actual time=2161.053..2161.053 rows=0 loops=1)
        Join Filter: (data.sensor_id = sensors.id)
        Buffers: shared hit=3418066 read=47326
        ->  Index Scan using timestamp_ind on data  (cost=0.43..255048.99 rows=4710976 width=12) (actual time=0.047..1410.715 rows=4710976 loops=1)
              Buffers: shared hit=3418065 read=47326
        ->  Materialize  (cost=0.14..4.19 rows=2 width=4) (actual time=0.000..0.000 rows=0 loops=4710976)
              Buffers: shared hit=1
              ->  Index Only Scan using ind_station_id on sensors  (cost=0.14..4.18 rows=2 width=4) (actual time=0.004..0.004 rows=0 loops=1)
                    Index Cond: (station_id = 4)
                    Heap Fetches: 0
                    Buffers: shared hit=1
Planning time: 0.478 ms
Execution time: 2161.090 ms
(15 rows)

QUERY (MAX)
----------------------------------------------------------------------------------------------------------
Aggregate  (cost=3678.08..3678.09 rows=1 width=8) (actual time=0.009..0.009 rows=1 loops=1)
   Buffers: shared hit=1
   ->  Nested Loop  (cost=0.58..3568.52 rows=43823 width=8) (actual time=0.006..0.006 rows=0 loops=1)
         Buffers: shared hit=1
         ->  Index Only Scan using ind_station_id on sensors  (cost=0.14..4.18 rows=2 width=4) (actual time=0.005..0.005 rows=0 loops=1)
               Index Cond: (station_id = 4)
               Heap Fetches: 0
               Buffers: shared hit=1
         ->  Index Only Scan using sensor_ind_timestamp on data  (cost=0.43..1389.59 rows=39258 width=12) (never executed)
               Index Cond: (sensor_id = sensors.id)
               Heap Fetches: 0
 Planning time: 0.435 ms
 Execution time: 0.048 ms
 (13 rows)

就像前面解释的那样,ORDER BY使用timestamp_in对数据执行扫描,这在MAX情况下没有完成.

Postgres版本：
来自Ubuntu repos的Postgres：x86_64-unknown-linux-gnu上的Postgresql 9.4.5,由gcc编译(Ubuntu 5.2.1-21ubuntu2)5.2.1 20151003,64位

请注意,存在NOT NULL约束,因此ORDER BY不必对空行进行排序.

另请注意,我对差异的来源非常感兴趣.虽然不理想,但我可以使用EXISTS(<1ms)然后SELECT(~11ms)相对快速地检索数据.

解决方法

在sensor.station_id上似乎没有索引,这在这里很重要.

max()和ORDER BY DESC LIMIT之间存在实际差异1.很多人似乎都错过了. NULL值按降序排序顺序排序.因此,ORDER BY时间戳DESC LIMIT 1返回时间戳为IS NULL的行(如果存在),而聚合函数max()忽略NULL值并返回最新的非空时间戳.

对于您的情况,由于您的列d.timestamp被定义为NOT NULL(如您的更新所示),因此没有有效的区别.具有DESC NULLS LAST的索引和LIMIT查询的ORDER BY中的相同子句应该仍然是最好的.我建议这些索引(我的查询建立在第二个上)：

sensor(station_id,id)
data(sensor_id,timestamp DESC NULLS LAST)

您可以删除其他索引变量sensor_ind_timestamp和sensor_ind_timestamp_desc,除非您有其他仍需要它们的查询(不太可能,但可能).

更重要的是,还有另一个困难：第一个表传感器上的过滤器返回很少,但仍然(可能)多行. Postgres希望在你添加的EXPLAIN输出中找到2行(rows = 2).
完美的技术将是第二个表数据的松散索引扫描 – 目前在Postgres 9.4(或Postgres 9.5)中没有实现.您可以通过各种方式重写查询以解决此限制.细节：

> Optimize GROUP BY query to retrieve latest record per user

最好的应该是：

SELECT d.timestamp
FROM   sensors s
CROSS  JOIN LATERAL  (
   SELECT timestamp
   FROM   data
   WHERE  sensor_id = s.id
   ORDER  BY timestamp DESC NULLS LAST
   LIMIT  1
   ) d
WHERE  s.station_id = 4
ORDER  BY d.timestamp DESC NULLS LAST
LIMIT  1;

由于外部查询的样式大多不相关,您还可以：

SELECT max(d.timestamp) AS timestamp
FROM   sensors s
CROSS  JOIN LATERAL  (
   SELECT timestamp
   FROM   data
   WHERE  sensor_id = s.id
   ORDER  BY timestamp DESC NULLS LAST
   LIMIT  1
   ) d
WHERE  s.station_id = 4;

并且max()变体现在应该执行速度快：

SELECT max(d.timestamp) AS timestamp
FROM   sensors s
CROSS  JOIN LATERAL  (
   SELECT max(timestamp) AS timestamp
   FROM   data
   WHERE  sensor_id = s.id
   ) d
WHERE  s.station_id = 4;

甚至,最短的：

SELECT max((SELECT max(timestamp) FROM data WHERE sensor_id = s.id)) AS timestamp
FROM   sensors s
WHERE  station_id = 4;

请注意双括号！

LIMATER在LATERAL连接中的另一个优点是,您可以检索所选行的任意列,而不仅仅是最新的时间戳(一列).

有关：

> Why do NULL values come first when ordering DESC in a PostgreSQL query?
> What is the difference between LATERAL and a subquery in PostgreSQL?
> Select first row in each GROUP BY group?
> Optimize groupwise maximum query

原文链接：https://www.f2er.com/mssql/74808.html

sql – max()与ORDER BY DESC LIMIT 1的性能

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