Re: Freezing without cleanup lock

On 10/22/15 6:39 PM, Alvaro Herrera wrote:
> Jim Nasby wrote:
>
>> That would be the minimal-impact version, yes. But I suspect if we went
>> through the trouble to do that, it would be just as easy to attempt the
>> freeze regardless of what scan_all is set to.
>
> You mean if !scan_all we conditional-get the cleanup lock, if we get it
> then prune, if not then freeze?  That seems nice on paper but I think
> it's useless because unless scan_all is true, then relfrozenxid doesn't
> advance anyway.

Actually, advancing relfrozenxid only depends on having hit all pages in 
the table, which can happen even if !scan_all. Aside from that, once the 
freeze map hits this would be useful in setting bits there.

>> What I wish I knew is whether this problem was worth worrying about or not.
>> Hopefully the extra logging in 9.5 will shed some light at some point...
>
> As I recall, Andres says it isn't, but I have recollections of scans
> that take a very long time to finish because they keep running into a
> vacuum that has a page locked.

I guess lets see if the new logging we have on this sheds some light then.
-- 
Jim Nasby, Data Architect, Blue Treble Consulting, Austin TX
Experts in Analytics, Data Architecture and PostgreSQL
Data in Trouble? Get it in Treble! http://BlueTreble.com