Postgresql使用left join 优化 not in

Postgresql使用left join 优化 not in

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场景：
查看在t1表中的数据，而这些数据不能出现在t2表。初学者容易想到 not in。这个问题可以使用left join 解决，如果两个表的结构一致，也可以采用差集来解决

使用例子

1. 建立测试数据
   新建两个表t1, t2, 每个表有100万的数据。有999990是相同的。现在要找到在t1的数据，而不再t2的数据。

   create table t1(id int, info text);
   create table t2(id int, info text);
   
   insert into t1(id, info) select i, md5(i::text) from generate_series(1, 1000000) t(i);
   insert into t2(id, info) select i, md5(i::text) from generate_series(11, 1000000 + 10) t(i);
   
   create index on t1(id);
   create index on t2(id);
   
   postgres=# select count(*) from t1;
     count
   ---------
    1000000
   (1 row)
   
   postgres=# select count(*) from t2;
     count
   ---------
    1000000
   (1 row) 

2. 使用not in 效果

   postgres=# explain  select * from t1 where id not in (select id from t2);
                                  QUERY PLAN
   --------------------------------------------------------------------------------
    Gather  (cost=1000.00..6196106209.00 rows=500000 width=37)
      Workers Planned: 2
      ->  Parallel Seq Scan on t1  (cost=0.00..6196055209.00 rows=208333 width=37)
            Filter: (NOT (SubPlan 1))
            SubPlan 1
              ->  Materialize  (cost=0.00..27241.00 rows=1000000 width=4)
                    ->  Seq Scan on t2  (cost=0.00..18334.00 rows=1000000 width=4)
   (7 rows) 

3. 使用left join 的效果

   postgres=# explain  select t1.* from t1 left join t2 on t1.id = t2.id where t2.* is  null;
                                     QUERY PLAN
   ---------------------------------------------------------------------------------------
    Gather  (cost=23592.00..51286.28 rows=5000 width=37)
      Workers Planned: 2
      ->  Parallel Hash Left Join  (cost=22592.00..49786.28 rows=2083 width=37)
            Hash Cond: (t1.id = t2.id)
            Filter: (t2.* IS NULL)
            ->  Parallel Seq Scan on t1  (cost=0.00..12500.67 rows=416667 width=37)
            ->  Parallel Hash  (cost=12500.67..12500.67 rows=416667 width=65)
                  ->  Parallel Seq Scan on t2  (cost=0.00..12500.67 rows=416667 width=65)
   (8 rows) 

对比结果
6196106209 / 51286 = 120814
基本上相差12万倍