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

Re: Not HOT enough

From: Simon Riggs <simon(at)2ndQuadrant(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Not HOT enough
Date: 2011-11-23 22:43:15
Message-ID: CA+U5nM+aTpv2Q3Qmf=cMftL1Ve2VZMhj0bE9SRyZ5T-Ozqx27g@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Wed, Nov 23, 2011 at 9:55 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Simon Riggs <simon(at)2ndQuadrant(dot)com> writes:
>> My patch actually improves the speed of snapshots, rather than slowing
>> them as Tom's would.
>
> It can be arbitrarily fast if it doesn't have to get the right answer.

(LOL) - laughing with you

> Unfortunately, you're not producing the right answer.  You can not
> exclude XIDs in other databases from the snapshot, at least not unless
> you know that the snapshot will not be used for examining any shared
> catalogs ... and GetSnapshotData certainly cannot know that.
>
> I think that the idea of computing a different cutoff on the
> probably-rare occasions where we need to prune a shared catalog page
> has some merit, but the change you are currently proposing to
> GetSnapshotData flat out does not work.

All true, but I already said that myself in a direct reply to you 2 hours ago.

And I proposed a solution, which was to use SnapshotNow as an override
for user queries against shared catalog tables.

Computing two cutoffs is overkill for the rare event of pruning a
shared catalog page. I did think of that already and its not a good
solution. I'm tempted by the idea of getting rid of multiple databases
altogether. The hassle of supporting that feature far outweighs the
fairly low benefit.

-- 
 Simon Riggs                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services

In response to

Responses

pgsql-hackers by date

Next:From: Robert HaasDate: 2011-11-23 22:47:39
Subject: Re: Not HOT enough
Previous:From: Magnus HaganderDate: 2011-11-23 22:36:57
Subject: Snapshot build updates

Privacy Policy | About PostgreSQL
Copyright © 1996-2014 The PostgreSQL Global Development Group