Re: Freeze avoidance of very large table.

On Mon, Jul 13, 2015 at 7:46 PM, Amit Kapila <amit.kapila16@gmail.com> wrote:
> On Mon, Jul 13, 2015 at 3:39 PM, Sawada Masahiko <sawada.mshk@gmail.com>
> wrote:
>>
>> On Tue, Jul 7, 2015 at 9:07 PM, Simon Riggs <simon@2ndquadrant.com> wrote:
>> > On 6 July 2015 at 17:28, Simon Riggs <simon@2ndquadrant.com> wrote:
>> >
>> >> I think we need something for pg_upgrade to rewrite existing VMs.
>> >> Otherwise a large read only database would suddenly require a massive
>> >> revacuum after upgrade, which seems bad. That can wait for now until we
>> >> all
>> >> agree this patch is sound.
>> >
>> >
>> > Since we need to rewrite the "vm" map, I think we should call the new
>> > map
>> > "vfm"
>> >
>> > That way we will be able to easily check whether the rewrite has been
>> > conducted on all relations.
>> >
>> > Since the maps are just bits there is no other way to tell that a map
>> > has
>> > been rewritten
>>
>> To avoid revacuum after upgrade, you meant that we need to rewrite
>> each bit of vm to corresponding bits of vfm, if it's from
>> not-supporting vfm version(i.g., 9.5 or earlier ). right?
>> If so, we will need to do whole scanning table, which is expensive as
>> well.
>> Clearing vm and do revacuum would be nice, rather than doing in
>> upgrading, I think.
>>
>
> How will you ensure to have revacuum for all the tables after
> upgrading?

We use script file which are generated by pg_upgrade.

>  Till the time Vacuum is done on the tables that
> have vm before upgrade, any queries on those tables can
> become slower.

Even If we implement rewriting tool for vm into pg_upgrade, it will
take time as much as revacuum because it need whole scanning table.
I meant that we rewrite vm using by existing facility (i.g., vacuum
(freeze)), instead of implementing new rewriting tool for vm.

Regards,

--
Masahiko Sawada