Postgresql使用IN vs NOT IN时的巨大性能差异 [英] Postgresql huge performance difference when using IN vs NOT IN

问题描述

我有2张桌子， transaksi和 buku。 transaksi大约有25万行，而buku大约有17万行。两个表的列均称为 k999a，并且两个表均不使用索引。现在我检查这两个语句。

I have 2 tables, "transaksi" and "buku". "transaksi" has around ~250k rows, and buku has around ~170k rows. Both tables have column called "k999a", and both tables use no indexes. Now I check these 2 statements.

声明1：

explain select k999a from transaksi where k999a not in (select k999a from buku);

陈述1输出：

 Seq Scan on transaksi  (cost=0.00..721109017.46 rows=125426 width=9)
   Filter: (NOT (SubPlan 1))
   SubPlan 1
     ->  Materialize  (cost=0.00..5321.60 rows=171040 width=8)
           ->  Seq Scan on buku  (cost=0.00..3797.40 rows=171040 width=8)

声明2：

explain select k999a from transaksi where k999a in (select k999a from buku);

陈述2输出：

Hash Semi Join  (cost=6604.40..22664.82 rows=250853 width=9)
   Hash Cond: (transaksi.k999a = buku.k999a)
   ->  Seq Scan on transaksi  (cost=0.00..6356.53 rows=250853 width=9)
   ->  Hash  (cost=3797.40..3797.40 rows=171040 width=8)
         ->  Seq Scan on buku  (cost=0.00..3797.40 rows=171040 width=8)

为什么在不在查询中，postgresql会循环联接，使查询花费很长时间吗？

Why in the NOT IN query, postgresql does loop join, making the query takes a long time?

PS：Windows 10上的postgresql版本9.6.1

PS: postgresql version 9.6.1 on windows 10

推荐答案

这是预料之中的。使用 WHERE NOT EXISTS 可能会获得更好的性能：

This is to be expected. You may get better performance using WHERE NOT EXISTS instead:

SELECT k999a
FROM transaksi
WHERE NOT EXISTS (
    SELECT 1 FROM buku WHERE buku.k999a = transaksi.k999a LIMIT 1
);

这里很好地解释了每种方法的原因： https://explainextended.com/2009 / 09/16 / not-in-vs-not-exists-vs-left-join-is-null-postgresql /

Here is a good explanation as to why for each of the methods: https://explainextended.com/2009/09/16/not-in-vs-not-exists-vs-left-join-is-null-postgresql/

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