用IN的postgres查询非常慢 [英] postgres query with IN is very slow

问题描述

我有一个表，该表的索引为（第A列，B列）。而且我正在运行一个查询，如下所示：

I have a table which has an index on (column A, column B). And I'm running a query that looks like this:

SELECT * FROM table WHERE (A, B) IN ((a_1, b_1), (a_2, b_2), ..., (a_5000, b_5000))

此查询很慢！
该计划如下所示：

This query is very slow ! The plan looks like:

Bitmap Heap Scan on table
  Recheck Cond: (((A = a_1) AND (B = b_1)) OR ((A = a_2) AND (B = b_2)) OR ...
  ->  BitmapOr
        ->  Bitmap Index Scan on idx
              Index Cond: ((A = a_1) AND (B = b_1))
        ->  Bitmap Index Scan on idx
              Index Cond: ((A = a_2) AND (B = b_2))
        ...(5000 other Bitmax Index Scan)

而不是用5000进行一次索引扫描值，postgres似乎一次执行5000个索引扫描，一次解释一个值，这解释了为什么查询如此缓慢。

Instead of doing one index scan with 5000 values, postgres seems to be doing 5000 index scan with one value at a time, which explains why the query is so slow.

实际上，这样做的速度更快：

Actually it is way faster to do someting like:

SELECT * FROM table WHERE A IN (a_1, ..., a_5000)

获取结果，然后在应用程序内部的B列（Python）上进行过滤。

fetch the results and then filter on column B inside the app (python).

我真的更希望结果已经由Postgres在合理的运行时间下进行过滤。是一种解决方法？

I'd really prefer to have the results already filtered by postgres with a reasonable running time. Is there a workaround ?

推荐答案

尝试加入CTE：

with value_list (a,b) as (
  values 
      (a_1, b_1), 
      (a_2, b_2), ..., 
      (a_5000, b_5000) 
)
select *
from table t
  join value_list v on (t.a, t.b) = (v.a, v.b);

（假定值列表中没有重复项）

(This assumes you have no duplicates in the list of values)

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