在大表查询时，为充分利用服务器资源，可以尝试开启并行，多进程并发查询，提升查询效率。在执行计划中，算子前增加关键字Partial，同时有关键字 Workers Planned: xx，表示SQL使用了并行。

例如下面的SQL，未开启并行。

teledb=# explain select count(1) from teledb_1;
 QUERY PLAN 
---------------------------------------------------------------------------------------
Finalize Aggregate (cost=118.81..118.83 rows=1 width=8)
 -> Remote Subquery Scan on all (dn001,dn002) (cost=118.80..118.81 rows=1 width=0)
 -> Partial Aggregate (cost=18.80..18.81 rows=1 width=8)
 -> Seq Scan on teledb_1 (cost=0.00..18.80 rows=880 width=0)
(4 rows)

下面是开启并行查询后的执行计划。

teledb=# explain select count(1) from teledb_1;
 QUERY PLAN 
----------------------------------------------------------------------------------------------------
Parallel Finalize Aggregate (cost=14728.45..14728.46 rows=1 width=8)
 -> Parallel Remote Subquery Scan on all (dn001,dn002) (cost=14728.33..14728.45 rows=1 width=0)
 -> Gather (cost=14628.33..14628.44 rows=1 width=8)
 Workers Planned: 2
 -> Partial Aggregate (cost=13628.33..13628.34 rows=1 width=8)
 -> Parallel Seq Scan on teledb_1 (cost=0.00..12586.67 rows=416667 width=0)
(6 rows)

建议不要全局开启并行，仅在需要开启并行的具体SQL上开启，可通过会话级设置并行，或hint方式指定并行。

会话级设置如：

set max_parallel_workers_per_gather=2;
执行SQL;
set max_parallel_workers_per_gather=0;