PostgreSQL已支持并行创建索引、并行查询、并行VACUUM等并行操作，PostgreSQL 13 版本的系统视图pg_stat_activity新增leader_pid字段，可以显示父进程号,本文简单演示下。

发行说明

Add leader_pid to pg_stat_activity to report parallel worker ownership (Julien Rouhaud)

Leader_pid 说明

pg_stat_activity.leader_pid字段手册说明如下:

leader_pid integer

Process ID of the parallel group leader if this process is or has been involved in parallel query, or null. 
This field is set when a process wants to cooperate with parallel workers, and remains set as long as the process exists. 
For a parallel group leader, this field is set to its own process ID.
For a parallel worker, this field is set to the process ID of the parallel group leader.

对于并行操作的父进程，leader_pid显示为该进程的进程号，对于并行操作的子进程，则显示并行操作父进程的进程号。

计划对以下两种并行场景进行演示:

场景一: 并行创建索引
场景二: 并行查询

环境准备

创建测试表，并插入数据，如下:

1
2

CREATE TABLE big(user_id int4,user_name text,ctime timestamp(6) without time zone default clock_timestamp() );
INSERT INTO big(user_id,user_name) SELECT  n ,n || '_data' FROM generate_series(1,20000000) n;

场景一: 并行创建索引

会话1: 设置max_parallel_maintenance_workers=4，并创建索引，如下:

1 2	SET max_parallel_maintenance_workers =4; CREATE INDEX idx_big_ctime ON big USING BTREE(ctime);

会话2: 监控pg_stat_activity的活动会话，如下:

postgres=# SELECT datname,pid,leader_pid,state,query FROM pg_stat_activity WHERE state='active' and usename='pguser' and pid <> pg_backend_pid();
 datname |  pid   | leader_pid | state  |                         query
---------+--------+------------+--------+-------------------------------------------------------
 mydb    | 128984 |     128984 | active | CREATE INDEX idx_big_ctime ON big USING BTREE(ctime);
 mydb    | 129717 |     128984 | active | CREATE INDEX idx_big_ctime ON big USING BTREE(ctime);
(2 rows)

显示了两个索引创建进程，128984为创建索引的父进程。

场景二: 并行查询