not in改寫為anti join或not exists
更新時間 2025-02-05 09:36:58
最近更新時間: 2025-02-05 09:36:58
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本頁介紹天翼云TeleDB數據庫通過將not in改寫為anti join或not exists來提高查詢性能的優化案例。
通過調大work_mem減少io訪問達到提高性能的目的，但內存不可能無限擴大，下面通過改寫語句也可以達到提高查詢的性能。
例如，下面的SQL，就是將not in改寫為anti join的執行計劃和耗時：
teledb=# explain select * from t1 left outer join t2 on t1.f2 = t2.f2 where t2.f2 is null;
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------------------------
Remote Subquery Scan on all (dn01,dn02,dn03,dn04,dn05,dn06,dn07,dn08,dn09,dn10)
(cost=6405.00..9260.75 rows=1 width=734)
-> Hash Anti Join (cost=6405.00..9260.75 rows=1 width=734)
Hash Cond: (t1.f2 = t2.f2)
-> Remote Subquery Scan on all
(dn01,dn02,dn03,dn04,dn05,dn06,dn07,dn08,dn09,dn10) (cost=100.00..682.00 rows=1000 width=367)
Distribute results by S: f2
-> Seq Scan on t1 (cost=0.00..210.00 rows=1000 width=367)
-> Hash (cost=21940.00..21940.00 rows=50000 width=367)
-> Remote Subquery Scan on all
(dn01,dn02,dn03,dn04,dn05,dn06,dn07,dn08,dn09,dn10) (cost=100.00..21940.00 rows=50000 width=367)
Distribute results by S: f2
-> Seq Scan on t2 (cost=0.00..3240.00 rows=50000 width=367)
(10 rows)
Time: 1.047 ms
teledb=# select * from t1 left outer join t2 on t1.f2 = t2.f2 where t2.f2 is null;
f1 | f2 | f3 | f4 | f5 | f6 | f7 | f8 | f9 | f10 | f11 | f12 | f1 | f2 | f3 | f4 | f5 | f6 | f7 | f8 | f9 | f10 | f11 | f12
----+----+----+----+----+----+----+----+----+-----+-----+-----+----+----+----+----+----+----+----+----+----+-----+-----+-----
(0 rows)
Time: 107.233 ms
也可以改寫為not exists寫法，改寫后的SQL執行計劃和耗時如下：
teledb=# explain select * from t1 where not exists( select 1 from t2 where t1.f2=t2.f2);
QUERY PLAN
----------------------------------------------------------------------------------------------------------------------
-------------------------
Remote Subquery Scan on all (dn01,dn02,dn03,dn04,dn05,dn06,dn07,dn08,dn09,dn10) (cost=3865.00..4078.75 rows=1 width=367)
-> Hash Anti Join (cost=3865.00..4078.75 rows=1 width=367)
Hash Cond: (t1.f2 = t2.f2)
-> Remote Subquery Scan on all (dn01,dn02,dn03,dn04,dn05,dn06,dn07,dn08,dn09,dn10) (cost=100.00..682.00 rows=1000 width=367)
Distribute results by S: f2
-> Seq Scan on t1 (cost=0.00..210.00 rows=1000 width=367)
-> Hash (cost=5240.00..5240.00 rows=50000 width=33)
-> Remote Subquery Scan on all (dn01,dn02,dn03,dn04,dn05,dn06,dn07,dn08,dn09,dn10) (cost=100.00..5240.00 rows=50000 width=33)
Distribute results by S: f2
-> Seq Scan on t2 (cost=0.00..3240.00 rows=50000 width=33)
(10 rows)
Time: 0.974 ms
teledb=# select * from t1 where not exists( select 1 from t2 where t1.f2=t2.f2);
f1 | f2 | f3 | f4 | f5 | f6 | f7 | f8 | f9 | f10 | f11 | f12
----+----+----+----+----+----+----+----+----+-----+-----+-----
(0 rows)
Time: 42.944 ms
可以看到改寫為not exists后效果也很好。
