Re: COPY FREEZE and setting PD_ALL_VISIBLE/visibility map bits

On Tue, Mar 12, 2019 at 4:54 PM Pavan Deolasee <pavan.deolasee@gmail.com> wrote:
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> On Mon, Mar 11, 2019 at 1:37 PM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
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>> I might be missing something but why do we need to recheck whether
>> each pages is all-frozen after insertion? I wonder if we can set
>> all-frozen without checking all tuples again in this case.
>
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> It's possible that the user may have inserted unfrozen rows (via regular INSERT or COPY without FREEZE option) before
insertingfrozen rows. 

I think that since COPY FREEZE can be executed only when the table is
created or truncated within the transaction other users cannot insert
any rows during COPY FREEZE.

> So we can't set the all-visible/all-frozen flag unconditionally. I also find it safer to do an explicit check to
ensurewe never accidentally mark a page as all-frozen. Since the check is performed immediately after the page becomes
fulland only once per page, there shouldn't be any additional IO cost and the check should be quite fast. 

I'd suggest to measure performance overhead. I can imagine one use
case of COPY FREEZE is the loading a very large table. Since in
addition to set visibility map bits this patch could scan a very large
table I'm concerned that how much performance is degraded by this
patch.
Regards,

--
Masahiko Sawada
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center