Postgres 没有使用部分索引

问题描述

我尝试使用部分索引计算表中的 id，但性能非常低

SELECT COUNT("refuel_request"."refuel_request_id") as "count" 
FROM "refuel_request" 
WHERE "refuel_request"."refuel_request_status_id" 
IN ('1','2','3')

我创建索引

CREATE INDEX idx_refuel_request_status_id10 ON refuel_request (refuel_request_status_id)
WHERE "refuel_request"."refuel_request_status_id" 
IN ('1','3')

你能解释一下我做错了什么吗？

解释

[
 {
"Plan": {
  "Node Type": "Seq Scan","Parallel Aware": false,"Relation Name": "refuel_request","Alias": "refuel_request","Startup Cost": 0,"Total Cost": 160442.88,"Plan Rows": 4700100,"Plan Width": 16,"Filter": "(refuel_request_status_id = ANY ('{1,2,3}'))"
   }
  }
 ]

解释（分析，缓冲区）

解释（分析，格式化文本）

Finalize Aggregate  (cost=114931.68..114931.69 rows=1 width=8) (actual time=570.019..570.020 rows=1 loops=1)
  ->  Gather  (cost=114931.47..114931.68 rows=2 width=8) (actual time=569.735..575.504 rows=3 loops=1)
        Workers Planned: 2
        Workers Launched: 2
        ->  Partial Aggregate  (cost=113931.47..113931.48 rows=1 idth=8) (actual time=528.094..528.094 rows=1 loops=3)
              ->  Parallel Seq Scan on refuel_request cost=0.00..109035.53 rows=1958375 width=16) (actual time=0.070..452.908 rows=1566700 loops=3)
                    Filter: (refuel_request_status_id = ANY ('{1,3}'))
Planning Time: 0.665 ms
Execution Time: 575.538 ms

解决方法

过滤器在顺序扫描中没有删除任何行，因此表中的所有行似乎都有 refuel_request_status_id 等于 '1'、'2' 或 '3'。所以索引中的 WHERE 条件没有区别。

您可以尝试使用 enable_seqscan = off 来查看 PostgreSQL 是否做出了正确的选择。如果不是，则可能是 random_page_cost 没有为您的硬件正确设置。

如果事实证明顺序扫描确实是最快的方式，那么您无能为力来加速查询：您可以获得更快的存储空间或更多的 RAM 来缓存表。

Counting is slow business.