PostgreSQL UPDATE 中包含子查询的性能优化

一位网友使用update t1 set info=(select info from t2 where t1.id=t2.id) where t1.id<9999;这种查询时，发现性能很低。
而单独执行时很快的。
原因分析：
postgres=# create table t1(id int,info text);
CREATE TABLE
postgres=# create table t2(id int,info text);
CREATE TABLE

postgres=# insert into t1 select generate_series(1,10000) ;
INSERT 0 10000
postgres=# insert into t2 select generate_series(1,1000) ;
INSERT 0 1000
这个SubPlan循环了9998次，慢就慢在这里，虽然单次subquery只需要0.132毫秒，乘以9998后就是1300多毫秒了。
postgres=# explain (analyze,verbose,timing,buffers) update t1 set info=(select info from t2 where t1.id=t2.id) where t1.id<9999;
                                                       QUERY PLAN                                                       
------------------------------------------------------------------------------------------------------------------------
 Update on public.t1  (cost=0.00..145134.50 rows=9999 width=10) (actual time=1368.041..1368.041 rows=0 loops=1)
   Buffers: shared hit=39870
   ->  Seq Scan on public.t1  (cost=0.00..145134.50 rows=9999 width=10) (actual time=0.218..1337.192 rows=9998 loops=1)
         Output: t1.id, (SubPlan 1), t1.ctid
         Filter: (t1.id < 9999)
         Rows Removed by Filter: 2
         Buffers: shared hit=20020
         SubPlan 1
           ->  Seq Scan on public.t2  (cost=0.00..14.50 rows=1 width=32) (actual time=0.127..0.132 rows=0 loops=9998)
                 Output: t2.info
                 Filter: (t1.id = t2.id)
                 Rows Removed by Filter: 1000
                 Buffers: shared hit=19996
 Planning time: 0.095 ms
 Execution time: 1368.077 ms
(15 rows)
将SQL修改为update ... set ... from ... where ...;
LOOP消失，性能立马提升
postgres=# explain (analyze,verbose,timing,buffers) update t1 set info=t2.info from t2 where t1.id=t2.id and t1.id<9999;
                                                         QUERY PLAN                                                          
-----------------------------------------------------------------------------------------------------------------------------
 Update on public.t1  (cost=24.50..221.00 rows=1000 width=48) (actual time=8.103..8.103 rows=0 loops=1)
   Buffers: shared hit=1940
   ->  Hash Join  (cost=24.50..221.00 rows=1000 width=48) (actual time=0.569..5.295 rows=1000 loops=1)
         Output: t1.id, t2.info, t1.ctid, t2.ctid
         Hash Cond: (t1.id = t2.id)
         Buffers: shared hit=26
         ->  Seq Scan on public.t1  (cost=0.00..149.00 rows=9999 width=10) (actual time=0.038..2.600 rows=9998 loops=1)
               Output: t1.id, t1.ctid
               Filter: (t1.id < 9999)
               Rows Removed by Filter: 2
               Buffers: shared hit=24
         ->  Hash  (cost=12.00..12.00 rows=1000 width=42) (actual time=0.518..0.518 rows=1000 loops=1)
               Output: t2.info, t2.ctid, t2.id
               Buckets: 1024  Batches: 1  Memory Usage: 51kB
               Buffers: shared hit=2
               ->  Seq Scan on public.t2  (cost=0.00..12.00 rows=1000 width=42) (actual time=0.025..0.240 rows=1000 loops=1)
                     Output: t2.info, t2.ctid, t2.id
                     Buffers: shared hit=2
 Planning time: 0.237 ms
 Execution time: 8.156 ms
(20 rows)

对于多个子查询的修改例子：
update u_md_rs.s_tmp_zb010s_top4_show t1
     set sal_store_num_this = (select sal_store_num_this from u_md_rs.s_tmp_zb010s_top4_show t2
                                where t1.region_no = t2.region_no
                                  and t1.region_name = t2.region_name
                                  and t2.product_code='total_sal_num'),
         inv_store_num = (select inv_store_num from u_md_rs.s_tmp_zb010s_top4_show t2
                           where t1.region_no = t2.region_no
                             and t1.region_name = t2.region_name
                             and t2.product_code='total_inv_num')
   where t1.merge_flag='top'; 
改为
update u_md_rs.s_tmp_zb010s_top4_show t1 set 
sal_store_num_this = case when t2.product_code='total_sal_num' then t2.sal_store_num_this else t1.sal_store_num_this end ,
inv_store_num = case when t2.product_code='total_inv_num' then t2.inv_store_num else t1.inv_store_num end 
from s_tmp_zb010s_top4_show t2 
where t1.region_no = t2.region_no 
and t1.region_name = t2.region_name 
and t1.merge_flag='top' 
and (t2.product_code='total_sal_num' or t2.product_code='total_inv_num');