PostgreSQL 10.0 preview 功能增强 - 增加ProcArrayGroupUpdate等待事件

背景

ProcArrayGroupUpdate等待事件，指事务结束后，将当前xid从ProcArray移除的事件。

 506     /*  
 507      * If the list was not empty, the leader will clear our XID.  It is  
 508      * impossible to have followers without a leader because the first process  
 509      * that has added itself to the list will always have nextidx as  
 510      * INVALID_PGPROCNO.  
 511      */  
 512     if (nextidx != INVALID_PGPROCNO)  
 513     {  
 514         int         extraWaits = 0;  
 515   
 516         /* Sleep until the leader clears our XID. */  
 517         pgstat_report_wait_start(WAIT_EVENT_PROCARRAY_GROUP_UPDATE);  
 518         for (;;)  
 519         {  
 520             /* acts as a read barrier */  
 521             PGSemaphoreLock(proc->sem);  
 522             if (!proc->procArrayGroupMember)  
 523                 break;  
 524             extraWaits++;  
 525         }  
 526         pgstat_report_wait_end();  
 527   
 528         Assert(pg_atomic_read_u32(&proc->procArrayGroupNext) == INVALID_PGPROCNO);  
 529   
 530         /* Fix semaphore count for any absorbed wakeups */  
 531         while (extraWaits-- > 0)  
 532             PGSemaphoreUnlock(proc->sem);  
 533         return;  
 534     }  

patch 如下

Add ProcArrayGroupUpdate wait event.  
  
author	Robert Haas <rhaas@postgresql.org>	  
Sat, 8 Apr 2017 01:41:47 +0800 (13:41 -0400)  
committer	Robert Haas <rhaas@postgresql.org>	  
Sat, 8 Apr 2017 01:41:47 +0800 (13:41 -0400)  
commit	d4116a771925379c33cf4c6634ca620ed08b551d  
tree	27f53d0c2833c66a78809bdb2db57c23d20cb0b9	tree | snapshot  
parent	dbb2a931478a397a2b655eb77e8be8c1ca136f63	commit | diff  
Add ProcArrayGroupUpdate wait event.  
  
Discussion: http://postgr.es/m/CA+TgmobgWHcXDcChX2+BqJDk2dkPVF85ZrJFhUyHHQmw8diTpA@mail.gmail.com  

这个patch的讨论，详见邮件组，本文末尾URL。

PostgreSQL社区的作风非常严谨，一个patch可能在邮件组中讨论几个月甚至几年，根据大家的意见反复的修正，patch合并到master已经非常成熟，所以PostgreSQL的稳定性也是远近闻名的。

参考

https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=d4116a771925379c33cf4c6634ca620ed08b551d

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