PostgreSQL并行计算算法及参数强制并行度设置方法

postgres=# show  ;      
 max_worker_processes     
----------------------    
 128    
(1 row)   

max_parallel_workers    

postgres=# show max_parallel_workers_per_gather ;    
 max_parallel_workers_per_gather     
---------------------------------    
 0    
(1 row)    

Min(parallel_workers（表级设置，没有设置则，根据表大小计算得到）, max_parallel_workers_per_gather)    

postgres=# alter table pa set (parallel_workers =32);    
ALTER TABLE    

min (max_worker_processes - 已运行workers ,     
     max_parallel_workers - 其他会话当前真实启用的并行度 ,      
     Min(parallel_workers（表级设置，没有设置则，根据表大小计算得到）, max_parallel_workers_per_gather)     
)   

postgres=# show parallel_tuple_cost ;    
 parallel_tuple_cost     
---------------------    
 0    
(1 row)    
postgres=# show parallel_setup_cost ;    
 parallel_setup_cost     
---------------------    
 0    
(1 row)    

postgres=# show min_parallel_table_scan_size ;    
 min_parallel_table_scan_size     
------------------------------    
 0    
(1 row)    
postgres=# show min_parallel_index_scan_size ;    
 min_parallel_index_scan_size     
------------------------------    
 0    
(1 row)    

#force_parallel_mode = on

postgres=# show max_parallel_maintenance_workers ;    
 max_parallel_maintenance_workers     
----------------------------------    
 24    
(1 row)   

#enable_partitionwise_join = on    

#enable_partitionwise_aggregate = on    

#enable_parallel_hash = on    

parallel_leader_participation = on 

#enable_parallel_append = on    

1、总的可开启的WORKER足够大  
postgres=# show max_worker_processes ;  
 max_worker_processes   
----------------------  
 128  
(1 row)  
  
2、所有会话同时执行并行计算的并行度足够大  
postgres=# set max_parallel_workers=64;  
SET  
3、单个QUERY中并行计算NODE开启的WORKER=24  
postgres=# set max_parallel_workers_per_gather =24;  
4、所有表和索引扫描允许并行  
postgres=# set min_parallel_table_scan_size =0;  
postgres=# set min_parallel_index_scan_size =0;  
5、并行计算优化器成本设置为0  
postgres=# set parallel_tuple_cost =0;  
postgres=# set parallel_setup_cost =0;  
6、设置表级并行度为24  
postgres=# alter table pa set (parallel_workers =24);  
ALTER TABLE  
7、效果，强制24并行。  
postgres=# explain (analyze) select count(*) from pa;  
                                                             QUERY PLAN                                                                
-------------------------------------------------------------------------------------------------------------------------------------  
 Finalize Aggregate  (cost=1615.89..1615.89 rows=1 width=8) (actual time=81.711..81.711 rows=1 loops=1)  
   ->  Gather  (cost=1615.83..1615.83 rows=24 width=8) (actual time=81.572..90.278 rows=25 loops=1)  
         Workers Planned: 24  
         Workers Launched: 24  
         ->  Partial Aggregate  (cost=1615.83..1615.83 rows=1 width=8) (actual time=58.411..58.411 rows=1 loops=25)  
               ->  Parallel Seq Scan on pa  (cost=0.00..712.71 rows=416667 width=0) (actual time=0.012..35.428 rows=400000 loops=25)  
 Planning Time: 0.449 ms  
 Execution Time: 90.335 ms  
(8 rows)  

create or replace function ftest(int) returns boolean as $$    
  select $1<1000;    
$$ language sql strict    
parallel safe;    
    
-- parallel safe 语法  

combinefunc