一、概述

Plan Hint是PG社区官方版”永远”不考虑引入的功能之一，社区开发者的理念是，引入Hint功能，会掩盖优化器本身的问题，导致缺陷不被暴露出来。但对于使用者来讲，遇到某些SQL的查询计划不好，性能出了问题，其他方法又不奏效的情况下，首先的目标还是想尽快解决问题，而Hint就可以在这种时候帮助到我们。

二、配置

在postgresql.conf中修改shared_preload_libraries=‘pg_hint_plan'

三、示例

1、初始化测试数据

create table t1 (id int, t int, name varchar(255));

create table t2 (id int , salary int);
create table t3 (id int , age int);
insert into t1 values (1,200,'jack');
insert into t1 values (2,300,'tom');
insert into t1 values (3,400,'john');
insert into t2 values (1,40000);
insert into t2 values (2,38000);
insert into t2 values (3,18000);
insert into t3 values (3,38);
insert into t3 values (2,55);
insert into t3 values (1,12);
explain analyze select * from t1 left join t2 on t1.id=t2.id left join t3 on t1.id=t3.id;
              QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------
 Hash Right Join (cost=89.82..337.92 rows=17877 width=540) (actual time=0.053..0.059 rows=3 loops=1)
 Hash Cond: (t3.id = t1.id)
 -> Seq Scan on t3 (cost=0.00..32.60 rows=2260 width=8) (actual time=0.002..0.002 rows=3 loops=1)
 -> Hash (cost=70.05..70.05 rows=1582 width=532) (actual time=0.042..0.043 rows=3 loops=1)
   Buckets: 2048 Batches: 1 Memory Usage: 17kB
   -> Hash Right Join (cost=13.15..70.05 rows=1582 width=532) (actual time=0.034..0.039 rows=3 loops=1)
    Hash Cond: (t2.id = t1.id)
    -> Seq Scan on t2 (cost=0.00..32.60 rows=2260 width=8) (actual time=0.002..0.002 rows=3 loops=1)
    -> Hash (cost=11.40..11.40 rows=140 width=524) (actual time=0.017..0.017 rows=3 loops=1)
      Buckets: 1024 Batches: 1 Memory Usage: 9kB
      -> Seq Scan on t1 (cost=0.00..11.40 rows=140 width=524) (actual time=0.010..0.011 rows=3 loops=1)
 Planning time: 0.154 ms
 Execution time: 0.133 ms
 
create index idx_t1_id on t1(id);

create index idx_t2_id on t2(id);
create index idx_t3_id on t3(id);
explain analyze select * from t1 left join t2 on t1.id=t2.id left join t3 on t1.id=t3.id;
             QUERY PLAN
--------------------------------------------------------------------------------------------------------------
 Hash Left Join (cost=2.14..3.25 rows=3 width=540) (actual time=0.045..0.047 rows=3 loops=1)
 Hash Cond: (t1.id = t3.id)
 -> Hash Left Join (cost=1.07..2.14 rows=3 width=532) (actual time=0.030..0.032 rows=3 loops=1)
   Hash Cond: (t1.id = t2.id)
   -> Seq Scan on t1 (cost=0.00..1.03 rows=3 width=524) (actual time=0.005..0.006 rows=3 loops=1)
   -> Hash (cost=1.03..1.03 rows=3 width=8) (actual time=0.007..0.007 rows=3 loops=1)
    Buckets: 1024 Batches: 1 Memory Usage: 9kB
    -> Seq Scan on t2 (cost=0.00..1.03 rows=3 width=8) (actual time=0.002..0.003 rows=3 loops=1)
 -> Hash (cost=1.03..1.03 rows=3 width=8) (actual time=0.005..0.005 rows=3 loops=1)
   Buckets: 1024 Batches: 1 Memory Usage: 9kB
   -> Seq Scan on t3 (cost=0.00..1.03 rows=3 width=8) (actual time=0.002..0.002 rows=3 loops=1)
 Planning time: 0.305 ms
 Execution time: 0.128 ms

2、强制走Index Scan

explain (analyze,buffers) /*+ indexscan(t1) */select * from t1 where id=2;

             QUERY PLAN
----------------------------------------------------------------------------------------------------------------
 Index Scan using idx_t1_id on t1 (cost=0.13..8.15 rows=1 width=524) (actual time=0.044..0.046 rows=1 loops=1)
 Index Cond: (id = 2)
 Buffers: shared hit=1 read=1
 Planning time: 0.145 ms
 Execution time: 0.072 ms
 
explain (analyze,buffers) /*+ indexscan(t1 idx_t1_id) */select * from t1 where id=2;
             QUERY PLAN
----------------------------------------------------------------------------------------------------------------
 Index Scan using idx_t1_id on t1 (cost=0.13..8.15 rows=1 width=524) (actual time=0.016..0.017 rows=1 loops=1)
 Index Cond: (id = 2)
 Buffers: shared hit=2
 Planning time: 0.079 ms
 Execution time: 0.035 ms

3、强制多条件组合

/*+ indexscan(t2) indexscan(t1 idx_t1_id) */

/*+ seqscan(t2) indexscan(t1 idx_t1_id) */
explain analyze SELECT * FROM t1 JOIN t2 ON (t1.id = t2.id);
            QUERY PLAN
--------------------------------------------------------------------------------------------------------
 Hash Join (cost=1.07..2.14 rows=3 width=532) (actual time=0.018..0.020 rows=3 loops=1)
 Hash Cond: (t1.id = t2.id)
 -> Seq Scan on t1 (cost=0.00..1.03 rows=3 width=524) (actual time=0.006..0.007 rows=3 loops=1)
 -> Hash (cost=1.03..1.03 rows=3 width=8) (actual time=0.005..0.005 rows=3 loops=1)
   Buckets: 1024 Batches: 1 Memory Usage: 9kB
   -> Seq Scan on t2 (cost=0.00..1.03 rows=3 width=8) (actual time=0.001..0.003 rows=3 loops=1)
 Planning time: 0.114 ms
 Execution time: 0.055 ms
(8 rows)
 
/*+ indexscan(t2) indexscan(t1 idx_t1_id) */explain analyze SELECT * FROM t1 JOIN t2 ON (t1.id = t2.id);

              QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------
 Merge Join (cost=0.26..24.40 rows=3 width=532) (actual time=0.047..0.053 rows=3 loops=1)
 Merge Cond: (t1.id = t2.id)
 -> Index Scan using idx_t1_id on t1 (cost=0.13..12.18 rows=3 width=524) (actual time=0.014..0.015 rows=3 loops=1)
 -> Index Scan using idx_t2_id on t2 (cost=0.13..12.18 rows=3 width=8) (actual time=0.026..0.028 rows=3 loops=1)
 
/*+ seqscan(t2) indexscan(t1 idx_t1_id) */explain analyze SELECT * FROM t1 JOIN t2 ON (t1.id = t2.id);

              QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------
 Nested Loop (cost=0.13..13.35 rows=3 width=532) (actual time=0.025..0.032 rows=3 loops=1)
 Join Filter: (t1.id = t2.id)
 Rows Removed by Join Filter: 6
 -> Index Scan using idx_t1_id on t1 (cost=0.13..12.18 rows=3 width=524) (actual time=0.016..0.018 rows=3 loops=1)
 -> Materialize (cost=0.00..1.04 rows=3 width=8) (actual time=0.002..0.003 rows=3 loops=3)
   -> Seq Scan on t2 (cost=0.00..1.03 rows=3 width=8) (actual time=0.004..0.005 rows=3 loops=1)
 

4、强制指定join method

/*+ NestLoop(t1 t2) MergeJoin(t1 t2 t3) Leading(t1 t2 t3) */

explain analyze select * from t1 left join t2 on t1.id=t2.id left join t3 on t1.id=t3.id;
              QUERY PLAN
--------------------------------------------------------------------------------------------------------------------
 Merge Left Join (cost=3.28..3.34 rows=3 width=540) (actual time=0.093..0.096 rows=3 loops=1)
 Merge Cond: (t1.id = t3.id)
 -> Sort (cost=2.23..2.23 rows=3 width=532) (actual time=0.077..0.078 rows=3 loops=1)
   Sort Key: t1.id
   Sort Method: quicksort Memory: 25kB
   -> Nested Loop Left Join (cost=0.00..2.20 rows=3 width=532) (actual time=0.015..0.020 rows=3 loops=1)
    Join Filter: (t1.id = t2.id)
    Rows Removed by Join Filter: 6
    -> Seq Scan on t1 (cost=0.00..1.03 rows=3 width=524) (actual time=0.005..0.005 rows=3 loops=1)
    -> Materialize (cost=0.00..1.04 rows=3 width=8) (actual time=0.002..0.003 rows=3 loops=3)
      -> Seq Scan on t2 (cost=0.00..1.03 rows=3 width=8) (actual time=0.002..0.003 rows=3 loops=1)
 -> Sort (cost=1.05..1.06 rows=3 width=8) (actual time=0.012..0.013 rows=3 loops=1)
   Sort Key: t3.id
   Sort Method: quicksort Memory: 25kB
   -> Seq Scan on t3 (cost=0.00..1.03 rows=3 width=8) (actual time=0.002..0.003 rows=3 loops=1)
 
/*+ NestLoop(t1 t2 t3) MergeJoin(t2 t3) Leading(t1 (t2 t3)) */

explain analyze select * from t1 left join t2 on t1.id=t2.id left join t3 on t1.id=t3.id;
QUERY PLAN
--------------------------------------------------------------------------------------------------------------
 Nested Loop Left Join (cost=1.07..3.31 rows=3 width=540) (actual time=0.036..0.041 rows=3 loops=1)
 Join Filter: (t1.id = t3.id)
 Rows Removed by Join Filter: 6
 -> Hash Left Join (cost=1.07..2.14 rows=3 width=532) (actual time=0.030..0.032 rows=3 loops=1)
   Hash Cond: (t1.id = t2.id)
   -> Seq Scan on t1 (cost=0.00..1.03 rows=3 width=524) (actual time=0.008..0.009 rows=3 loops=1)
   -> Hash (cost=1.03..1.03 rows=3 width=8) (actual time=0.007..0.007 rows=3 loops=1)
    Buckets: 1024 Batches: 1 Memory Usage: 9kB
    -> Seq Scan on t2 (cost=0.00..1.03 rows=3 width=8) (actual time=0.002..0.004 rows=3 loops=1)
 -> Materialize (cost=0.00..1.04 rows=3 width=8) (actual time=0.001..0.002 rows=3 loops=3)
   -> Seq Scan on t3 (cost=0.00..1.03 rows=3 width=8) (actual time=0.002..0.003 rows=3 loops=1)

5、控制单条SQL的cost

/*+ set(seq_page_cost 20.0) seqscan(t1) */

/*+ set(seq_page_cost 20.0) seqscan(t1) */explain analyze select * from t1 where id > 1;
           QUERY PLAN
-----------------------------------------------------------------------------------------------
 Seq Scan on t1 (cost=0.00..20.04 rows=1 width=524) (actual time=0.011..0.013 rows=2 loops=1)
 Filter: (id > 1)
 Rows Removed by Filter: 1

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