Re: Microvacuum support for Hash Index

>>> Sure, but we are not clearing in conditionally. I am not sure, how
>>> after recovery it will be cleared it gets set during normal operation.
>>> Moreover, btree already clears similar flag during replay (refer
>>> btree_xlog_delete).
>>
>> You were right. In case datachecksum is enabled or wal_log_hint is set
>> to true, 'LH_PAGE_HAS_DEAD_TUPLES' will get wal logged and therefore
>> needs to be cleared on the standby as well.
>>
>
> I was thinking what bad can happen if we don't clear this flag during
> replay, the main thing that comes to mind is that after crash
> recovery, if the flag is set the inserts on that page might need to
> traverse all the tuples on that page once the page is full even if
> there are no dead tuples in that page. It can be later cleared when
> there are dead tuples in that page and we actually delete them, but I
> don't think it is worth the price to pay for not clearing the flag
> during replay.

Yes, you are absolutely correct. If we do not clear this flag during
replay then there is a possibility of _hash_doinsert() unnecessarily
scanning the page with no space assuming that the page has got some
dead tuples in it which is not true.

>
>> Attached is the patch that
>> clears this flag on standby during replay.
>>
>
> Don't you think, we should also clear it during the replay of
> XLOG_HASH_DELETE? We might want to log the clear of flag along with
> WAL record for XLOG_HASH_DELETE.
>

Yes, it should be cleared. I completely missed this part in a hurry.
Thanks for informing. I have taken care of it in the attached v2
patch.

--
With Regards,
Ashutosh Sharma
EnterpriseDB:http://www.enterprisedb.com