Re: speedup COPY TO for partitioned table.

hi.

In the V10 patch, there will be some regression if the partition column
ordering is different from the root partitioned table.

because in V10 CopyThisRelTo
+ while (table_scan_getnextslot(scandesc, ForwardScanDirection, slot))
+ {
+ if (map != NULL)
+ {
+ original_slot = slot;
+ root_slot = MakeSingleTupleTableSlot(rootdesc, &TTSOpsBufferHeapTuple);
+ slot = execute_attr_map_slot(map, slot, root_slot);
+ }
+ else
+ slot_getallattrs(slot);
+
+ if (original_slot != NULL)
+ ExecDropSingleTupleTableSlot(original_slot);
+}
as you can see, for each slot in the partition, i called
MakeSingleTupleTableSlot to get the dumpy root_slot
and ExecDropSingleTupleTableSlot too.
that will cause overhead.

we can call produce root_slot before the main while loop.
like the following:
+ if (root_rel != NULL)
+ {
+ rootdesc = RelationGetDescr(root_rel);
+ root_slot = table_slot_create(root_rel, NULL);
+ map = build_attrmap_by_name_if_req(rootdesc, tupdesc, false);
+ }
....
+ while (table_scan_getnextslot(scandesc, ForwardScanDirection, slot))
+ {
+ TupleTableSlot *copyslot;
+ if (map != NULL)
+ copyslot = execute_attr_map_slot(map, slot, root_slot);
+ else
+ {
+ slot_getallattrs(slot);
+ copyslot = slot;
+ }
+
please check CopyThisRelTo in v11. so, with v11, there is no
regression for case when
partition column ordering differs from partitioned.

I have tested 30 partitions, 10 columns, all the column ordering
is different with the root partitioned table.

copy pp to '/tmp/2.txt'
is still faster than
copy (select * from pp) to '/tmp/1.txt';
(359.463 ms versus  376.371 ms)

I am using -Dbuildtype=release
PostgreSQL 18beta1_release_build on x86_64-linux, compiled by gcc-14.1.0, 64-bit

you may see the attached test file.