Re: GUC for cleanup indexes threshold.

On 20 February 2017 at 10:27, Amit Kapila <amit.kapila16@gmail.com> wrote:
> On Mon, Feb 20, 2017 at 3:01 PM, Simon Riggs <simon@2ndquadrant.com> wrote:
>> On 20 February 2017 at 09:15, Amit Kapila <amit.kapila16@gmail.com> wrote:
>>> On Mon, Feb 20, 2017 at 7:26 AM, Masahiko Sawada <sawada.mshk@gmail.com> wrote:
>>>> On Fri, Feb 17, 2017 at 3:41 AM, Robert Haas <robertmhaas@gmail.com> wrote:
>>>>> On Thu, Feb 16, 2017 at 6:17 AM, Simon Riggs <simon@2ndquadrant.com> wrote:
>>>>>> On 15 February 2017 at 08:07, Masahiko Sawada <sawada.mshk@gmail.com> wrote:
>>>>>>> It's a bug. Attached latest version patch, which passed make check.
>>
>>>>>> 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.
>>
>> Agreed.... which is why
>> On 16 February 2017 at 11:17, Simon Riggs <simon@2ndquadrant.com> wrote:
>>> I suggest that we store the number of half-dead pages in the metapage
>>> after each VACUUM, so we can decide whether to skip the scan or not.
>>
>>
>>> Also, I think we do reclaim the
>>> complete page while allocating a new page in btree.
>>
>> That's not how it works according to the README at least.
>>
>
> I am referring to code (_bt_getbuf()->if (_bt_page_recyclable(page))),
> won't that help us in reclaiming the space?

Not unless the README is incorrect, no.

That section of code is just a retest of pages retrieved from FSM;
they aren't even added there until two scans have occurred and even
then it may not be possible to recycle.

-- 
Simon Riggs                http://www.2ndQuadrant.com/
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