Re: Clear empty space in a page.

Hi,

Andres Freund wrote 2021-05-31 00:07:
> Hi,
> 
> On 2021-05-30 03:10:26 +0300, Yura Sokolov wrote:
>> While this result is not directly applied to stock PostgreSQL, I 
>> believe
>> page compression is important for full_page_writes with 
>> wal_compression
>> enabled. And probably when PostgreSQL is used on filesystem with
>> compression enabled (ZFS?).
> 
> I don't think the former is relevant, because the hole is skipped in 
> wal page
> compression (at some cost).

Ah, forgot about. Yep, you are right.

>> Therefore I propose clearing page's empty space with zero in
>> PageRepairFragmentation, PageIndexMultiDelete, PageIndexTupleDelete 
>> and
>> PageIndexTupleDeleteNoCompact.
>> 
>> Sorry, didn't measure impact on raw performance yet.
> 
> I'm worried that this might cause O(n^2) behaviour in some cases, by
> repeatedly memset'ing the same mostly already zeroed space to 0. Why do 
> we
> ever need to do memset_hole() instead of accurately just zeroing out 
> the space
> that was just vacated?

It is done exactly this way: memset_hole accepts "old_pd_upper" and 
cleans between
old and new one.

regards,
Yura