Re: Freeze avoidance of very large table.

On Fri, Jul 10, 2015 at 3:42 AM, Jeff Janes <jeff.janes@gmail.com> wrote:
> On Wed, Jul 8, 2015 at 10:10 PM, Sawada Masahiko <sawada.mshk@gmail.com>
> wrote:
>>
>> On Thu, Jul 9, 2015 at 4:31 AM, Jeff Janes <jeff.janes@gmail.com> wrote:
>> > On Fri, Jul 3, 2015 at 1:25 AM, Sawada Masahiko <sawada.mshk@gmail.com>
>> > wrote:
>> >>
>> >> It's impossible to have VM bits set to frozen but not visible.
>> >> These bit are controlled independently. But eventually, when
>> >> all-frozen bit is set, all-visible is also set.
>> >
>> >
>> > If that combination is currently impossible, could it be used indicate
>> > that
>> > the page is all empty?
>>
>> Yeah, the status of that VM bits set to frozen but not visible is
>> impossible, so we could use this status for another something status
>> of the page.
>>
>> > Having a crash-proof bitmap of all-empty pages would make vacuum
>> > truncation
>> > scans much more efficient.
>>
>> The empty page is always marked all-visible by vacuum today, it's not
>> enough?
>
>
> The "current" vacuum can just remember that they were empty as well as
> all-visible.
>
> But the next vacuum that occurs on the table won't know that they are empty,
> just that they are all-visible, so it can't truncate them away without
> having to read each one first.

Yeah, it would be effective for vacuum empty page.

>
> It is a minor thing, but if there is no other use for this fourth
> "bit-space", it seems a shame to waste it when there is some use for it.  I
> haven't looked at the code around this area to know how hard it would be to
> implement the setting and clearing of the bit.

I think so too, we would be able to use unused fourth status of bits
efficiently.
Should I include these improvement into this patch?
This topic should be discussed on another thread after this feature is
committed, I think.

Regards,

--
Sawada Masahiko