Re: Toast issues with OldestXmin going backwards

>>>>> "Amit" == Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> writes:

>>> (Or do we need to track it across restarts? maybe we do, to deal with
>>> replication slaves without slots, or changes in parameters)

>> Yeah, I'm worried that it might need to be persistent across restarts.
>>
>> One idea that occurred to me is to somehow record -- I guess in
>> pg_class using non-transactional updates -- the last cutoff XID used
>> to vacuum any given table. Then we could just make a rule that you
>> can't vacuum the TOAST table with an XID that's newer than the last
>> one used for the main table. That would preserve the property that
>> you can vacuum the tables separately while avoiding dangling pointers.

Amit> Won't this lead to a bloat in toast tables when there is a big
Amit> difference between the cutoff XID of the main heap table and the
Amit> latest values of OldestXmin?

Yes. What we need is actually the reverse of what Robert describes -
when we vacuum the _main_ table, we must use the _later_ of the
currently calculated OldestXmin or the OldestXmin last used to vacuum
the toast table.

--
Andrew (irc:RhodiumToad)