SQL子查询的感悟[亲测有效]

postgres=# create table t1(id int primary key, info text, reg_time timestamp);
CREATE TABLE
postgres=# create table t2(id int, name text);
CREATE TABLE
postgres=# insert into t1 select generate_series(1, 10000),"lottu", now();
INSERT 0 10000
postgres=# insert into t2 select (random()*1000)::int, "lottu"||id  from generate_series(1,1000) id;
INSERT 0 1000
postgres=# create index ind_t1_id on t1(id);
CREATE INDEX

代码100分

代码100分select * from t1 where id in (select id from t2);

postgres=# explain (analyze,verbose,costs,timing) select * from t1 where id in (select id from t2);
                                                             QUERY PLAN                                                             
----------------------------------------------------------------------
 Merge Join  (cost=54.25..99.73 rows=628 width=18) (actual time=1.319..2.365 rows=628 loops=1)
   Output: t1.id, t1.info, t1.reg_time
   Inner Unique: true
   Merge Cond: (t1.id = t2.id)
   ->  Index Scan using ind_t1_id on public.t1  (cost=0.29..337.29 rows=10000 width=18) (actual time=0.014..0.421 rows=997 loops=1)
         Output: t1.id, t1.info, t1.reg_time
   ->  Sort  (cost=53.97..55.54 rows=628 width=4) (actual time=1.298..1.387 rows=628 loops=1)
         Output: t2.id
         Sort Key: t2.id
         Sort Method: quicksort  Memory: 54kB
         ->  HashAggregate  (cost=18.50..24.78 rows=628 width=4) (actual time=0.730..0.877 rows=628 loops=1)
               Output: t2.id
               Group Key: t2.id
               ->  Seq Scan on public.t2  (cost=0.00..16.00 rows=1000 width=4) (actual time=0.013..0.267 rows=1000 loops=1)
                     Output: t2.id
 Planning Time: 0.454 ms
 Execution Time: 2.507 ms
(17 rows)

代码100分postgres=# explain (analyze,verbose,costs,timing) select t1,* from t1 , t2 where t1.id = t2.id ;
                                                             QUERY PLAN                                                             
------------------------------------------------------------------------------------------------------------------------------------
 Merge Join  (cost=66.11..117.17 rows=1000 width=72) (actual time=0.601..2.184 rows=1000 loops=1)
   Output: t1.*, t1.id, t1.info, t1.reg_time, t2.id, t2.name
   Merge Cond: (t1.id = t2.id)
   ->  Index Scan using ind_t1_id on public.t1  (cost=0.29..337.29 rows=10000 width=60) (actual time=0.021..0.726 rows=997 loops=1)
         Output: t1.*, t1.id, t1.info, t1.reg_time
   ->  Sort  (cost=65.83..68.33 rows=1000 width=12) (actual time=0.573..0.721 rows=1000 loops=1)
         Output: t2.id, t2.name
         Sort Key: t2.id
         Sort Method: quicksort  Memory: 71kB
         ->  Seq Scan on public.t2  (cost=0.00..16.00 rows=1000 width=12) (actual time=0.013..0.226 rows=1000 loops=1)
               Output: t2.id, t2.name
 Planning Time: 0.288 ms
 Execution Time: 2.421 ms
(13 rows)

postgres=# explain (analyze,verbose,costs,timing) select * from t1 where id = any(array(select id from t2));
                                                        QUERY PLAN                                                         
---------------------------------------------------------------------------------------------------------------------------
 Index Scan using ind_t1_id on public.t1  (cost=16.29..59.03 rows=10 width=18) (actual time=0.418..1.108 rows=628 loops=1)
   Output: t1.id, t1.info, t1.reg_time
   Index Cond: (t1.id = ANY ($0))
   InitPlan 1 (returns $0)
     ->  Seq Scan on public.t2  (cost=0.00..16.00 rows=1000 width=4) (actual time=0.014..0.127 rows=1000 loops=1)
           Output: t2.id
 Planning Time: 0.106 ms
 Execution Time: 1.178 ms
(8 rows)

select * from t1 where id in (select id from t2 where id <= 1000);
或者
with t as
(select id from t2 where id <= 1000)
select t1.* from t1 where id in (select id from t);

select id from t2 where id <= 1000);