Re: BUG #17485: Records missing from Primary Key index when doing REINDEX INDEX CONCURRENTLY

> On 25 May 2022, at 00:01, Andres Freund <andres@anarazel.de> wrote:
> 
>>> On 24 May 2022, at 23:15, Andres Freund <andres@anarazel.de> wrote:
>>> 
>>> I suspect the problem might be related to pruning done during the validation
>>> scan. Once PROC_IN_SAFE_IC is set, the backend itself will not preserve tids
>>> its own snapshot might need. Which will wreak havoc during the validation
>>> scan.
>> 
>> I observe that removing PROC_IN_SAFE_IC for index_validate() fixes tests.
>> But why it's not a problem for index_build() scan?
> 
> I now suspect it's a problem for both, just more visible for index_validate().
No, I understood now. index_build() in CIC can be safely replaced by index_build_empty().
index_build() is only called because many AMs can make index more efficient than index_build_empty()+inserts.
BTW for other indexes that are built just by inserts we can safely optimize CIC by calling index_build_empty() instead of index_build() in Phase 1.

>> And I do not understand why it's a problem that tuple is pruned during the scan... How does this "wreak havoc" happen?
> 
> Basically snapshots don't work anymore

I got it! (almost)
Thank you!
Now I see why tests in 002_cic.pl did not catch that: there were no HOT updates.

So, can we leave the feature enabled for all the process besides index validation? We don't seem to need a valid snapshot in CIC anywhere else. Or it's safer to revert whole commit?

Thanks!

Best regards, Andrey Borodin.