Re: Make COPY format extendable: Extract COPY TO format implementations

Hi,

In <CAD21AoBkDE4JwjPgcLxSEwqu3nN4VXjkYS9vpRQDwA2GwNQCsg(at)mail(dot)gmail(dot)com>
"Re: Make COPY format extendable: Extract COPY TO format implementations" on Tue, 4 Feb 2025 22:20:51 -0800,
Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:

>> I was just looking at bit at this series of patch labelled with v31,
>> to see what is happening here.
>>
>> In 0001, we have that:
>>
>> + /* format-specific routines */
>> + const CopyToRoutine *routine;
>> [...]
>> - CopySendEndOfRow(cstate);
>> + cstate->routine->CopyToOneRow(cstate, slot);
>>
>> Having a callback where the copy state is processed once per row is
>> neat in terms of design for the callbacks and what extensions can do,
>> and this is much better than what 2889fd23be5 has attempted (later
>> reverted in 1aa8324b81fa) because we don't do indirect function calls
>> for each attribute. Still, I have a question here: what happens for a
>> COPY TO that involves one attribute, a short field size like an int2
>> and many rows (the more rows the more pronounced the effect, of
>> course)? Could this level of indirection still be the cause of some
>> regressions in a case like that? This is the worst case I can think
>> about, on top of my mind, and I am not seeing tests with few
>> attributes like this one, where we would try to make this callback as
>> hot as possible. This is a performance-sensitive area.
>
> FYI when Sutou-san last measured the performance[1], it showed a
> slight speed up even with fewer columns (5 columns) in both COPY TO
> and COPY FROM cases. The callback design has not changed since then.
> But it would be a good idea to run the benchmark with a table having a
> single small size column.
>
> [1] https://www.postgresql.org/message-id/20241114.161948.1677325020727842666.kou%40clear-code.com

I measured v31 patch set with 1,6,11,16,21,26,31 int2
columns. See the attached PDF for 0001 and 0002 result.

0001 - to:

It's faster than master when the number of rows are
1,000,000-5,000,000.

It's almost same as master when the number of rows are
6,000,000-10,000,000.

There is no significant slow down when the number of columns
is 1.

0001 - from:

0001 doesn't change COPY FROM code. So the differences are
not real difference.

0002 - to:

0002 doesn't change COPY TO code. So "0001 - to" and "0002 -
to" must be the same result. But 0002 is faster than master
for all cases. It shows that the CopyToOneRow() approach
doesn't have significant slow down.

0002 - from:

0002 changes COPY FROM code. So this may have performance
impact.

It's almost same as master when data is smaller
((1,000,000-2,000,000 rows) or (3,000,000 rows and 1,6,11,16
columns)).

It's faster than master when data is larger.

There is no significant slow down by 0002.

Thanks,
--
kou