高效的查询，以从大表中获取每个组的最大价值

给定表：

    Column    |            Type             
 id           | integer                     
 latitude     | numeric(9,6)                
 longitude    | numeric(9,6)                
 speed        | integer                     
 equipment_id | integer                     
 created_at   | timestamp without time zone
Indexes:
    "geoposition_records_pkey" PRIMARY KEY, btree (id)

该表有2000万条记录，相对而言，这不是一个很大的记录。但这会使顺序扫描变慢。

如何获得max(created_at)每个记录的最后一条记录（）equipment_id？

我已经尝试了以下两个查询，并阅读了有关该主题的许多答案的几种变体：

select max(created_at),equipment_id from geoposition_records group by equipment_id;

select distinct on (equipment_id) equipment_id,created_at 
  from geoposition_records order by equipment_id, created_at desc;

我也尝试过为它创建btree索引，equipment_id,created_at但是Postgres发现使用seqscan更快。强制enable_seqscan = off也没有用，因为读取索引的速度与seq扫描一样慢，可能更糟。

查询必须定期运行，始终返回最后一个。

使用Postgres 9.3。

解释/分析（有170万条记录）：

set enable_seqscan=true;
explain analyze select max(created_at),equipment_id from geoposition_records group by equipment_id;
"HashAggregate  (cost=47803.77..47804.34 rows=57 width=12) (actual time=1935.536..1935.556 rows=58 loops=1)"
"  ->  Seq Scan on geoposition_records  (cost=0.00..39544.51 rows=1651851 width=12) (actual time=0.029..494.296 rows=1651851 loops=1)"
"Total runtime: 1935.632 ms"

set enable_seqscan=false;
explain analyze select max(created_at),equipment_id from geoposition_records group by equipment_id;
"GroupAggregate  (cost=0.00..2995933.57 rows=57 width=12) (actual time=222.034..11305.073 rows=58 loops=1)"
"  ->  Index Scan using geoposition_records_equipment_id_created_at_idx on geoposition_records  (cost=0.00..2987673.75 rows=1651851 width=12) (actual time=0.062..10248.703 rows=1651851 loops=1)"
"Total runtime: 11305.161 ms"

普通的多列b树索引毕竟应该起作用：

CREATE INDEX foo_idx
ON geoposition_records (equipment_id, created_at DESC NULLS LAST);

为什么DESC NULLS LAST呢

• 日期范围中未使用的索引查询

功能

如果您对查询计划程序说不清话，那么遍历设备表的函数应该可以解决问题。一次查找一个equipment_id将使用索引。对于一小部分（从您的EXPLAIN ANALYZE输出判断为57 ），那是很快的。
可以安全地假设您有equipment桌子吗？

CREATE OR REPLACE FUNCTION f_latest_equip()
  RETURNS TABLE (equipment_id int, latest timestamp) AS
$func$
BEGIN
FOR equipment_id IN
   SELECT e.equipment_id FROM equipment e ORDER BY 1
LOOP
   SELECT g.created_at
   FROM   geoposition_records g
   WHERE  g.equipment_id = f_latest_equip.equipment_id
                           -- prepend function name to disambiguate
   ORDER  BY g.created_at DESC NULLS LAST
   LIMIT  1
   INTO   latest;

   RETURN NEXT;
END LOOP;
END  
$func$  LANGUAGE plpgsql STABLE;

也打个不错的电话：

SELECT * FROM f_latest_equip();

相关子查询

想到这一点，使用此equipment表，您可以通过低关联的子查询对肮脏的工作产生巨大的效果：

SELECT equipment_id
     ,(SELECT created_at
       FROM   geoposition_records
       WHERE  equipment_id = eq.equipment_id
       ORDER  BY created_at DESC NULLS LAST
       LIMIT  1) AS latest
FROM   equipment eq;

表现非常好。

LATERAL 加入Postgres 9.3+

SELECT eq.equipment_id, r.latest
FROM   equipment eq
LEFT   JOIN LATERAL (
   SELECT created_at
   FROM   geoposition_records
   WHERE  equipment_id = eq.equipment_id
   ORDER  BY created_at DESC NULLS LAST
   LIMIT  1
   ) r(latest) ON true;

详细说明：

• 优化GROUP BY查询以检索每个用户的最新记录

与相关子查询的性能类似。比较，，函数，相关子查询的性能max()，DISTINCT ON并LATERAL在此进行比较：

SQL小提琴。

尝试1

如果

1. 我有一张单独的equipment桌子，
2. 我有一个索引 geoposition_records(equipment_id, created_at desc)

那么以下对我有用：

select id as equipment_id, (select max(created_at)
                            from geoposition_records
                            where equipment_id = equipment.id
                           ) as max_created_at
from equipment;

我是不是能够迫使PG做一个快速的查询，以确定双方的名单equipment_idS和相关的max(created_at)。但是明天我要再试一次！

尝试2

我找到了此链接：http: //zogovic.com/post/44856908222/optimizing-postgresql-query-for-distinct-values 将此技术与尝试1中的查询相结合，得到：

WITH RECURSIVE equipment(id) AS (
    SELECT MIN(equipment_id) FROM geoposition_records
  UNION
    SELECT (
      SELECT equipment_id
      FROM geoposition_records
      WHERE equipment_id > equipment.id
      ORDER BY equipment_id
      LIMIT 1
    )
    FROM equipment WHERE id IS NOT NULL
)
SELECT id AS equipment_id, (SELECT MAX(created_at)
                            FROM geoposition_records
                            WHERE equipment_id = equipment.id
                           ) AS max_created_at
FROM equipment;

这很有效！但是你需要

1. 此超扭曲查询表单，以及
2. 的索引geoposition_records(equipment_id, created_at desc)。