From: | Merlin Moncure <mmoncure(at)gmail(dot)com> |
---|---|
To: | Greg Stark <stark(at)mit(dot)edu> |
Cc: | Kevin Grittner <kgrittn(at)ymail(dot)com>, Simon Riggs <simon(at)2ndquadrant(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Limiting setting of hint bits by read-only queries; vacuum_delay |
Date: | 2013-03-25 23:15:43 |
Message-ID: | CAHyXU0xTtfMrj09E1rZdT3K7QDW7da6SUxXjfe8ZDxrjQDS+=A@mail.gmail.com |
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Lists: | pgsql-hackers |
On Mon, Mar 25, 2013 at 5:57 PM, Greg Stark <stark(at)mit(dot)edu> wrote:
> On Mon, Mar 25, 2013 at 9:53 PM, Kevin Grittner <kgrittn(at)ymail(dot)com> wrote:
>> That would make it harder to construct a degenerate case
>
> I don't think it's hard at all. It's the same as the case Simon wants
> to solve except that the cost is incurred in a different way. Imagine
> a system where there's a huge data load to a table which is then
> read-only for an OLTP system. Until vacuum comes along -- and it may
> never since the table never sees deletes or updates -- every
> transaction needs to do a clog lookup for every tuple it sees. That
> means a significant cpu slowdown for every row lookup forever more. To
> save a one-time i/o cost.
That is simply not the case, unless every tuple was created by a
unique (or at least non-sequential) transaction xid. There is a 'last
transaction id' guard over clog lookup which is pretty effective. The
real cost is actually i/o from writing hint bits.
merlin
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