Re: 64-bit XIDs again

On 31 July 2015 at 22:46, Heikki Linnakangas <hlinnaka(at)iki(dot)fi> wrote:

> On 07/31/2015 12:29 AM, Josh Berkus wrote:
>
>> On 07/30/2015 07:24 AM, Heikki Linnakangas wrote:
>>
>>>
>>> You'd never be forced to do anti-wraparound
>>> vacuums, you could just let the clog grow arbitrarily large
>>>
>>
>> When I introduced the same idea a few years back, having the clog get
>> arbitrarily large was cited as a major issue. I was under the
>> impression that clog size had some major performance impacts.
>>
>
> Well, sure, if you don't want the clog to grow arbitrarily large, then you
> need to freeze.

This statement isn't quite right, things are better than that.

We don't need to freeze in order to shrink the clog, we just need to hint
and thereby ensure we move forwards the lowest unhinted xid. That does
involve scanning, but doesn't need to scan indexes. That scan won't produce
anywhere near as much additional WAL traffic or I/O.

In practice, larger clog would only happen with higher transaction rate,
which means more system resources, so I don't think its too much of a
problem overall.

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