DDR爱好者之家

postgresql insert into select无法使用并行查询的解决
站长资源 2024/11/26 佚名
66 1538
DDR爱好者之家 Design By 杰米

本文信息基于PG13.1。

从PG9.6开始支持并行查询。PG11开始支持CREATE TABLE … AS、SELECT INTO以及CREATE MATERIALIZED VIEW的并行查询。

先说结论：

换用create table as 或者select into或者导入导出。

首先跟踪如下查询语句的执行计划：

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)

可以看到走了两个Workers。

下边看一下insert into select：

postgres=# explain analyze insert into va select count(*) from test t1,test1 t2 where t1.id = t2.id ;     
                                  QUERY PLAN                                  
--------------------------------------------------------------------------------------------------------------------------------------------------
 Insert on va (cost=73228.00..73228.02 rows=1 width=4) (actual time=3744.179..3744.187 rows=0 loops=1)
  -> Subquery Scan on "*SELECT*" (cost=73228.00..73228.02 rows=1 width=4) (actual time=3743.343..3743.352 rows=1 loops=1)
     -> Aggregate (cost=73228.00..73228.01 rows=1 width=8) (actual time=3743.247..3743.254 rows=1 loops=1)
        -> Hash Join (cost=30832.00..70728.00 rows=1000000 width=0) (actual time=1092.295..3511.301 rows=1000000 loops=1)
           Hash Cond: (t1.id = t2.id)
           -> Seq Scan on test t1 (cost=0.00..14425.00 rows=1000000 width=4) (actual time=0.030..421.537 rows=1000000 loops=1)
           -> Hash (cost=14425.00..14425.00 rows=1000000 width=4) (actual time=1090.078..1090.081 rows=1000000 loops=1)
              Buckets: 131072 Batches: 16 Memory Usage: 3227kB
              -> Seq Scan on test1 t2 (cost=0.00..14425.00 rows=1000000 width=4) (actual time=0.021..422.768 rows=1000000 loops=1)
 Planning Time: 0.511 ms
 Execution Time: 3745.633 ms
(11 rows)

可以看到并没有Workers的指示，没有启用并行查询。

即使开启强制并行，也无法走并行查询。

postgres=# set force_parallel_mode =on;
SET
postgres=# explain analyze insert into va select count(*) from test t1,test1 t2 where t1.id = t2.id ;
                                  QUERY PLAN                                  
--------------------------------------------------------------------------------------------------------------------------------------------------
 Insert on va (cost=73228.00..73228.02 rows=1 width=4) (actual time=3825.042..3825.049 rows=0 loops=1)
  -> Subquery Scan on "*SELECT*" (cost=73228.00..73228.02 rows=1 width=4) (actual time=3824.976..3824.984 rows=1 loops=1)
     -> Aggregate (cost=73228.00..73228.01 rows=1 width=8) (actual time=3824.972..3824.978 rows=1 loops=1)
        -> Hash Join (cost=30832.00..70728.00 rows=1000000 width=0) (actual time=1073.587..3599.402 rows=1000000 loops=1)
           Hash Cond: (t1.id = t2.id)
           -> Seq Scan on test t1 (cost=0.00..14425.00 rows=1000000 width=4) (actual time=0.034..414.965 rows=1000000 loops=1)
           -> Hash (cost=14425.00..14425.00 rows=1000000 width=4) (actual time=1072.441..1072.443 rows=1000000 loops=1)
              Buckets: 131072 Batches: 16 Memory Usage: 3227kB
              -> Seq Scan on test1 t2 (cost=0.00..14425.00 rows=1000000 width=4) (actual time=0.022..400.624 rows=1000000 loops=1)
 Planning Time: 0.577 ms
 Execution Time: 3825.923 ms
(11 rows)

原因在官方文档有写：

The query writes any data or locks any database rows. If a query contains a data-modifying operation either at the top level or within a CTE, no parallel plans for that query will be generated. As an exception, the commands CREATE TABLE … AS, SELECT INTO, and CREATE MATERIALIZED VIEW which create a new table and populate it can use a parallel plan.

解决方案有如下三种：

1.select into

postgres=# explain analyze select count(*) into vaa 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=742.736..774.923 rows=1 loops=1)
  -> Gather (cost=34243.95..34244.16 rows=2 width=8) (actual time=740.223..774.907 rows=3 loops=1)
     Workers Planned: 2
     Workers Launched: 2
     -> Partial Aggregate (cost=33243.95..33243.96 rows=1 width=8) (actual time=731.408..731.413 rows=1 loops=3)
        -> Parallel Hash Join (cost=15428.00..32202.28 rows=416667 width=0) (actual time=489.880..700.830 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=0.033..87.479 rows=333333 loops=3)
           -> Parallel Hash (cost=8591.67..8591.67 rows=416667 width=4) (actual time=266.839..266.840 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=0.058..106.874 rows=333333 loops=3)
 Planning Time: 0.319 ms
 Execution Time: 783.300 ms
(13 rows)

2.create table as

postgres=# explain analyze create table vb as 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=540.120..563.733 rows=1 loops=1)
  -> Gather (cost=34243.95..34244.16 rows=2 width=8) (actual time=537.982..563.720 rows=3 loops=1)
     Workers Planned: 2
     Workers Launched: 2
     -> Partial Aggregate (cost=33243.95..33243.96 rows=1 width=8) (actual time=526.602..527.136 rows=1 loops=3)
        -> Parallel Hash Join (cost=15428.00..32202.28 rows=416667 width=0) (actual time=334.532..502.793 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=0.018..57.819 rows=333333 loops=3)
           -> Parallel Hash (cost=8591.67..8591.67 rows=416667 width=4) (actual time=189.502..189.503 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=0.023..77.786 rows=333333 loops=3)
 Planning Time: 0.189 ms
 Execution Time: 565.448 ms
(13 rows)

3.或者通过导入导出的方式，例如：

psql -h localhost -d postgres -U postgres -c "select count(*) from test t1,test1 t2 where t1.id = t2.id " -o result.csv -A -t -F ","
psql -h localhost -d postgres -U postgres -c "COPY va FROM 'result.csv' WITH (FORMAT CSV, DELIMITER ',', HEADER FALSE, ENCODING 'windows-1252')"

一些场景下也会比非并行快。

DDR爱好者之家 Design By 杰米
广告合作：本站广告合作请联系QQ：858582 申请时备注：广告合作（否则不回）
免责声明：本站资源来自互联网收集,仅供用于学习和交流,请遵循相关法律法规,本站一切资源不代表本站立场,如有侵权、后门、不妥请联系本站删除！
DDR爱好者之家 Design By 杰米
ddrfans.com DDR爱好者之家
8,675无损音乐
1,324高清电影
213破解软件
70,141收录资讯

稳了！魔兽国服回归的3条重磅消息！官宣时间再确认！

昨天有一位朋友在大神群里分享，自己亚服账号被封号之后居然弹出了国服的封号信息对话框。

这里面让他访问的是一个国服的战网网址，com.cn和后面的zh都非常明白地表明这就是国服战网。

而他在复制这个网址并且进行登录之后，确实是网易的网址，也就是我们熟悉的停服之后国服发布的暴雪游戏产品运营到期开放退款的说明。这是一件比较奇怪的事情，因为以前都没有出现这样的情况，现在突然提示跳转到国服战网的网址，是不是说明了简体中文客户端已经开始进行更新了呢？

更新日志

友情链接

杰晶网络 DDR爱好者之家 南强小屋 黑松山资源网 白云城资源网
DDR爱好者之家 Design By 杰米