Re: [HACKERS] GUC for cleanup indexes threshold.

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.

You might be referring to cleaning out killed tuples just before a
page split? That's something different.

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