On 3/23/17 1:54 AM, Masahiko Sawada wrote:
> On Wed, Mar 15, 2017 at 7:51 AM, Peter Geoghegan <pg@bowt.ie> wrote:
>> On Tue, Mar 14, 2017 at 3:10 PM, Peter Geoghegan <pg@bowt.ie> wrote:
>>> We already have BTPageOpaqueData.btpo, a union whose contained type
>>> varies based on the page being dead. We could just do the same with
>>> some other field in that struct, and then store epoch there. Clearly
>>> nobody really cares about most data that remains on the page. Index
>>> scans just need to be able to land on it to determine that it's dead,
>>> and VACUUM needs to be able to determine whether or not there could
>>> possibly be such an index scan at the time it considers recycling..
>>
>> ISTM that we need all of the fields within BTPageOpaqueData even for
>> dead pages, actually. The left links and right links still need to be
>> sane, and the flag bits are needed. Plus, the field that stores an XID
>> already is clearly necessary. Even if they weren't needed, it would
>> probably still be a good idea to keep them around for forensic
>> purposes. However, the page header field pd_prune_xid is currently
>> unused for indexes, and is the same width as CheckPoint.nextXidEpoch
>> (the extra thing we might want to store -- the epoch).
>>
>> Maybe you could store the epoch within that field when B-Tree VACUUM
>> deletes a page, and then compare that within _bt_page_recyclable(). It
>> would come before the existing XID comparison in that function. One
>> nice thing about this idea is that pd_prune_xid will be all-zero for
>> index pages from the current format, so there is no need to take
>> special care to make sure that databases that have undergone
>> pg_upgrade don't break.
>>
>
> Thank you for the suggestion!
> If we store the poch within union field, I think we will not be able
> to use BTPageOpaqueData.btpo.xact at the same time. Since comparing
> btpo.xact is still necessary to determine if that page is recyclable
> we cannot store the epoch into the same union field. And if we store
> it into BTPageOpaqueData, it would break disk compatibility.
I have marked this patch "Waiting for Author".
This thread has been idle for five days. Please respond with a new
patch by 2017-03-30 00:00 AoE (UTC-12) or this submission will be marked
"Returned with Feedback".
Thanks,
--
-David
david@pgmasters.net
