使用IN vs NOT IN时Postgresql的巨大性能差异

Question

我有2张桌子,"transaksi"和"buku"."transaksi"有大约250k行,buku大约有170k行.两个表都有名为"k999a"的列,两个表都不使用索引.现在我检查这两个陈述.

声明1:

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

声明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);

声明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NOT IN查询中,postgresql会循环连接,使得查询需要很长时间？

PS:Windows 10上的postgresql版本9.6.1

Answer 1

这是可以预料的.WHERE NOT EXISTS相反,您可能会获得更好的性能:

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

以下是每个方法的原因:https://explainextended.com/2009/09/16/not-in-vs-not-exists-vs-left-join-is-null-postgresql/