Re: GUC for cleanup indexes threshold.

On Mon, Feb 20, 2017 at 6:15 PM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> On Mon, Feb 20, 2017 at 7:26 AM, Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
>> On Fri, Feb 17, 2017 at 3:41 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>>> On Thu, Feb 16, 2017 at 6:17 AM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
>>>> On 15 February 2017 at 08:07, Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
>>>>> It's a bug. Attached latest version patch, which passed make check.
>>>>
>>>> In its current form, I'm not sure this is a good idea. Problems...
>>>>
>>>> 1. I'm pretty sure the world doesn't need another VACUUM parameter
>>>>
>>>> I suggest that we use the existing vacuum scale factor/4 to reflect
>>>> that indexes are more sensitive to bloat.
>>>
>>> I do not think it's a good idea to control multiple behaviors with a
>>> single GUC. We don't really know that dividing by 4 will be right for
>>> everyone, or even for most people. It's better to have another
>>> parameter with a sensible default than to hardcode a ratio that might
>>> work out poorly for some people.
>>>
>>>> 2. The current btree vacuum code requires 2 vacuums to fully reuse
>>>> half-dead pages. So skipping an index vacuum might mean that second
>>>> index scan never happens at all, which would be bad.
>>>
>>> Maybe. If there are a tiny number of those half-dead pages in a huge
>>> index, it probably doesn't matter. Also, I don't think it would never
>>> happen, unless the table just never gets any more updates or deletes -
>>> but that case could also happen today. It's just a matter of
>>> happening less frequently.
>>
>
> Yeah thats right and I am not sure if it is worth to perform a
> complete pass to reclaim dead/deleted pages unless we know someway
> that there are many such pages. Also, I think we do reclaim the
> complete page while allocating a new page in btree.
>
>> The half-dead pages are never cleaned up if the ratio of pages
>> containing garbage is always lower than threshold.
>>
>
> Which threshold are you referring here?
>

I meant the new parameter in current patch.

Regards,

--
Masahiko Sawada
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center