Re: HOT chain validation in verify_heapam()

On Wed, Nov 16, 2022 at 12:41 PM Himanshu Upadhyaya <
upadhyaya(dot)himanshu(at)gmail(dot)com> wrote:

>
>
>> > + }
>> > +
>> > + /* Loop over offsets and validate the data in the
>> predecessor array. */
>> > + for (OffsetNumber currentoffnum = FirstOffsetNumber;
>> currentoffnum <= maxoff;
>> > + currentoffnum = OffsetNumberNext(currentoffnum))
>> > + {
>> > + HeapTupleHeader pred_htup;
>> > + HeapTupleHeader curr_htup;
>> > + TransactionId pred_xmin;
>> > + TransactionId curr_xmin;
>> > + ItemId pred_lp;
>> > + ItemId curr_lp;
>> > +
>> > + ctx.offnum = predecessor[currentoffnum];
>> > + ctx.attnum = -1;
>> > +
>> > + if (ctx.offnum == 0)
>> > + {
>> > + /*
>> > + * Either the root of the chain or an
>> xmin-aborted tuple from
>> > + * an abandoned portion of the HOT chain.
>> > + */
>>
>> Hm - couldn't we check that the tuple could conceivably be at the root of
>> a
>> chain? I.e. isn't HEAP_HOT_UPDATED? Or alternatively has an aborted xmin?
>>
>>
> I don't see a way to check if tuple is at the root of HOT chain because
> predecessor array will always be having either xmin from non-abandoned
> transaction or it will be zero. We can't differentiate root or tuple
> inserted via abandoned transaction.
>
> I was wrong here. I think this can be done and will be doing these changes
in my next patch.

--
Regards,
Himanshu Upadhyaya
EnterpriseDB: http://www.enterprisedb.com