Re: speedup COPY TO for partitioned table.

Hi!
I reviewed v7. Maybe we should add a multi-level partitioning case
into copy2.sql regression test?

I also did quick benchmarking for this patch:

==== DDL

create table ppp(i int) partition by range (i);

genddl.sh:

for i in `seq 0 200`; do echo "create table p$i partition of ppp for
values from ( $((10 * i)) ) to ( $((10 * (i + 1))) ); "; done

=== insert data data:
insert into ppp select i / 1000 from generate_series(0, 2000000)i;

=== results:

for 2000001 rows speedup is 1.40 times : 902.604 ms (patches) vs
1270.648 ms (unpatched)

for 4000002 rows speedup is 1.20 times : 1921.724 ms (patches) vs
2343.393 ms (unpatched)

for 8000004 rows speedup is 1.10 times : 3932.361 ms (patches) vs
4358.489ms (unpatched)

So, this patch indeed speeds up some cases, but with larger tables
speedup becomes negligible.

--
Best regards,
Kirill Reshke