Skip site navigation (1) Skip section navigation (2)

Re: lazy_truncate_heap()

From: Simon Riggs <simon(at)2ndQuadrant(dot)com>
To: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>
Cc: Greg Stark <greg(dot)stark(at)enterprisedb(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: lazy_truncate_heap()
Date: 2009-01-01 17:57:32
Message-ID: 1230832652.4032.106.camel@ebony.2ndQuadrant (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
On Thu, 2009-01-01 at 12:00 +0200, Heikki Linnakangas wrote:
> Greg Stark wrote:
> > 
> > On 31 Dec 2008, at 13:21, Simon Riggs <simon(at)2ndQuadrant(dot)com> wrote:
> >>
> >> Both of these bugs are minor, but the effect of either/both of them is
> >> to cause more AccessExclusiveLocks than we might expect.
> >>
> >> For Hot Standby this means that many VACUUMs take AccessExclusiveLocks
> >> on relations, which would potentially lead to having queries cancelled
> >> for no reason at all.
> > 
> > Well by default it would just cause wal to pause briefly until the 
> > queries with those locks finish, no?
> Wait a minute. Why does an AccessExclusiveLock lead to cancelled queries 
> or pausing WAL application? I thought it'd just block other queries 
> trying to acquire a conflicting lock in the standby, just like holding 
> an AccessExclusiveLock on the primary does. It's unrelated to the xmin 
> horizon issue.

Yes, it is unrelated to the xmin horizon issue. There are two reasons
for delaying WAL apply:
* locks
* xmin horizon

When a lock is acquired on the primary it almost always precedes an
action which cannot occur concurrently. For example, if VACUUM did
truncate a table then queries could get errors because parts of their
table disappear from under them. Others are drop table etc..

 Simon Riggs 
 PostgreSQL Training, Services and Support

In response to


pgsql-hackers by date

Next:From: Magnus HaganderDate: 2009-01-01 17:58:21
Subject: Kerberos options requiring restart
Previous:From: Simon RiggsDate: 2009-01-01 17:54:33
Subject: Re: lazy_truncate_heap()

Privacy Policy | About PostgreSQL
Copyright © 1996-2017 The PostgreSQL Global Development Group