Re: Freeze avoidance of very large table.

On 4/24/15 6:52 AM, Robert Haas wrote:
> On Thu, Apr 23, 2015 at 9:03 PM, Michael Paquier
> <michael.paquier@gmail.com> wrote:
>> On Thu, Apr 23, 2015 at 10:42 PM, Robert Haas wrote:
>>> On Thu, Apr 23, 2015 at 4:19 AM, Simon Riggs  wrote:
>>>> We only need a freeze/backup map for larger relations. So if we map 1000
>>>> blocks per map page, we skip having a map at all when size < 1000.
>>>
>>> Agreed.  We might also want to map multiple blocks per map slot - e.g.
>>> one slot per 32 blocks.  That would keep the map quite small even for
>>> very large relations, and would not compromise efficiency that much
>>> since reading 256kB sequentially probably takes only a little longer
>>> than reading 8kB.
>>>
>>> I think the idea of integrating the freeze map into the VM fork is
>>> also worth considering.  Then, the incremental backup map could be
>>> optional; if you don't want incremental backup, you can shut it off
>>> and have less overhead.
>>
>> When I read that I think about something configurable at
>> relation-level.There are cases where you may want to have more
>> granularity of this information at block level by having the VM slots
>> to track less blocks than 32, and vice-versa.
>
> What are those cases?  To me that sounds like making things
> complicated to no obvious benefit.

Tables that get few/no dead tuples, like bulk insert tables. You'll have 
large sections of blocks with the same visibility.

I suspect the added code to allow setting 1 bit for multiple pages 
without having to lock all those pages simultaneously will probably 
outweigh making this a reloption anyway.
-- 
Jim Nasby, Data Architect, Blue Treble Consulting
Data in Trouble? Get it in Treble! http://BlueTreble.com