以下是关于该场景的测试情况：

1. work_mem默认4MB时，SQL执行计划和耗时情况如下：

   可以看到，优化器选择走了Hash Semi Join，耗时3.7秒。

   teledb=# explain select count(1) from t1 where exists(select 1 from t2 where t2.t1_f1=t1.f1);
                                                         QUERY PLAN                             
   -----------------------------------------------------------------------------------------------------------------------
    Finalize Aggregate  (cost=242218.32..242218.33 rows=1 width=8)
      ->  Remote Subquery Scan on all (dn001,dn002)  (cost=242218.30..242218.32 rows=1 width=0)
            ->  Partial Aggregate  (cost=242118.30..242118.31 rows=1 width=8)
                  ->  Hash Semi Join  (cost=110248.00..242118.30 rows=505421 width=0)
                        Hash Cond: (t1.f1 = t2.t1_f1)
                        ->  Seq Scan on t1  (cost=0.00..17420.00 rows=1000000 width=4)
                        ->  Hash  (cost=79340.00..79340.00 rows=3000000 width=4)
                              ->  Remote Subquery Scan on all (dn001,dn002)  (cost=100.00..79340.00 rows=3000000 width=4)
                                    Distribute results by S: t1_f1
                                    ->  Seq Scan on t2  (cost=0.00..52240.00 rows=3000000 width=4)
   (10 rows)
   Time: 1.091 ms
   teledb=# select count(1) from t1 where exists(select 1 from t2 where t2.t1_f1=t1.f1);   
    count  
   --------
    500000
   (1 row)
   Time: 3779.401 ms (00:03.779)
   

2. work_mem调整为128MB后，SQL执行计划和耗时情况如下：

   可以看到，优化器选择走了Hash Join，耗时1.9秒。

   teledb=# set work_mem to '128MB';
   SET
   Time: 0.368 ms
   teledb=# explain select count(1) from t1 where exists(select 1 from t2 where t2.t1_f1=t1.f1);
                                                       QUERY PLAN                                          
   ------------------------------------------------------------------------------------------------------------------
    Finalize Aggregate  (cost=101763.76..101763.77 rows=1 width=8)
      ->  Remote Subquery Scan on all (dn001,dn002)  (cost=101763.75..101763.76 rows=1 width=0)
            ->  Partial Aggregate  (cost=101663.75..101663.76 rows=1 width=8)
                  ->  Hash Join  (cost=89660.00..101663.75 rows=505421 width=0)
                        Hash Cond: (t2.t1_f1 = t1.f1)
                        ->  Remote Subquery Scan on all (dn001,dn002)  (cost=59840.00..69443.00 rows=505421 width=4)
                              Distribute results by S: t1_f1
                              ->  HashAggregate  (cost=59740.00..64794.21 rows=505421 width=4)
                                    Group Key: t2.t1_f1
                                    ->  Seq Scan on t2  (cost=0.00..52240.00 rows=3000000 width=4)
                        ->  Hash  (cost=17420.00..17420.00 rows=1000000 width=4)
                              ->  Seq Scan on t1  (cost=0.00..17420.00 rows=1000000 width=4)
   (12 rows)
   Time: 4.739 ms
   teledb=# select count(1) from t1 where exists(select 1 from t2 where t2.t1_f1=t1.f1);   
    count  
   --------
    500000
   (1 row)
   Time: 1942.037 ms (00:01.942)