本文信息基于PG13.1。

從PG9.6開始支持并行查詢。PG11開始支持CREATE TABLE … AS、SELECT INTO以及CREATE MATERIALIZED VIEW的并行查詢。

先說結論：

換用create table as 或者select into或者導入導出。

首先跟蹤如下查詢語句的執行計劃：

select count(*) from test t1,test1 t2 where t1.id = t2.id ;
postgres=# explain analyze select count(*) from test t1,test1 t2 where t1.id = t2.id ;
QUERY PLAN
——————————————————————————————-
Finalize Aggregate (cost=34244.16..34244.17 rows=1 width=8) (actual time=683.246..715.324 rows=1 loops=1)
-> Gather (cost=34243.95..34244.16 rows=2 width=8) (actual time=681.474..715.311 rows=3 loops=1)
Workers Planned: 2
Workers Launched: 2
-> Partial Aggregate (cost=33243.95..33243.96 rows=1 width=8) (actual time=674.689..675.285 rows=1 loops=3)
-> Parallel Hash Join (cost=15428.00..32202.28 rows=416667 width=0) (actual time=447.799..645.689 rows=333333 loops=3)
Hash Cond: (t1.id = t2.id)
-> Parallel Seq Scan on test t1 (cost=0.00..8591.67 rows=416667 width=4) (actual time=0.025..74.010 rows=333333 loops=3)
-> Parallel Hash (cost=8591.67..8591.67 rows=416667 width=4) (actual time=260.052..260.053 rows=333333 loops=3)
Buckets: 131072 Batches: 16 Memory Usage: 3520kB
-> Parallel Seq Scan on test1 t2 (cost=0.00..8591.67 rows=416667 width=4) (actual time=0.032..104.804 rows=333333 loops=3)
Planning Time: 0.420 ms
Execution Time: 715.447 ms
(13 rows)