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Re: Re: Hot Standby query cancellation and Streaming Replication integration

From: Greg Stark <gsstark(at)mit(dot)edu>
To: Greg Smith <greg(at)2ndquadrant(dot)com>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Re: Hot Standby query cancellation and Streaming Replication integration
Date: 2010-02-28 13:54:48
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
On Sun, Feb 28, 2010 at 6:07 AM, Greg Smith <greg(at)2ndquadrant(dot)com> wrote:
> Not forced to--have the option of.  There are obviously workloads where you
> wouldn't want this.  At the same time, I think there are some pretty common
> ones people are going to expect HS+SR to work on transparently where this
> would obviously be the preferred trade-off to make, were it available as one
> of the options.  The test case I put together shows an intentionally
> pathological but not completely unrealistic example of such a workload.

Well if we're forced to eventually have both then it kind of takes the
wind out of Tom's arguments. We had better get both features working
so it becomes only a question of which is worth doing first and which
can be held off. Since there aren't any actual bugs in evidence for
the current setup and we already have it that's a pretty easy

> What I am sure of is that a SR-based xmin passing approach is simpler,
> easier to explain, more robust for some common workloads, and less likely to
> give surprised "wow, I didn't think *that* would cancel my standby query"
> reports from the field

Really? I think we get lots of suprised wows from the field from the
idea that a long-running read-only query can cause your database to
bloat. I think the only reason that's obvious to us is that we've been
grappling with that problem for so long.

> And since I never like to bet against Tom's gut feel, having it
> around as a "plan B" in case he's right about an overwhelming round of bug
> reports piling up against the max_standby_delay etc. logic doesn't hurt
> either.

Agreed. Though I think it'll be bad in that case even if we have a
plan B. It'll mean no file-based log shipping replicas and no
guarantee that what you run on the standby can't affect the master --
which is a pretty nice guarantee. It'll also mean it'll be much more
fragile against network interruptions.


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