Re: speedup COPY TO for partitioned table.

Hi Jian,

Thanks for the patch. After reviewing it, I got a few small comments:

> On Oct 9, 2025, at 15:10, jian he <jian.universality@gmail.com> wrote:
> 
> 
> Please check the attached v16.
> <v16-0001-support-COPY-partitioned_table-TO.patch>



1
```
+	List	   *partitions;		/* oid list of partition oid for copy to */
```

The comment doesn’t look very good. First, it repeats “oid”; second, as “List *partitions” implies multiple partitions, the comment should use plural OIDs. Maybe change the comment to “/* list of partition OIDs for COPY TO */" 

2
```
+			/*
+			 * Collect a list of partitions containing data, so that later
+			 * DoCopyTo can copy the data from them.
+			*/
+			children = find_all_inheritors(RelationGetRelid(rel),
+										   AccessShareLock,
+										   NULL);
+
+			foreach_oid(childreloid, children)
+			{
+				char		 relkind = get_rel_relkind(childreloid);
+
+				if (relkind == RELKIND_FOREIGN_TABLE)
+				{
+					char	   *relation_name;
+
+					relation_name = get_rel_name(childreloid);
+					ereport(ERROR,
+							errcode(ERRCODE_WRONG_OBJECT_TYPE),
+							errmsg("cannot copy from foreign table \"%s\"", relation_name),
+							errdetail("Partition \"%s\" is a foreign table in the partitioned table \"%s\"",
+									  relation_name, RelationGetRelationName(rel)),
+							errhint("Try the COPY (SELECT ...) TO variant."));
+				}
+
+				if (RELKIND_HAS_PARTITIONS(relkind))
+					children = foreach_delete_current(children, childreloid);
+			}
```

Is it better to move the RELKIND_HAS_PARTIONS() check to before FOREIGH_TABLE check and continue after foreach_delete_current()? Now every childreloid goes through the both checks, if we do the movement, then HAS_PARTIONS child will go through 1 check. This is a tiny optimization.

3
```
+				if (RELKIND_HAS_PARTITIONS(relkind))
+					children = foreach_delete_current(children, childreloid);
+			}
```

I wonder if there is any specially consideration of using RELKIND_HAS_PARTITIONS() here? Because according to the function comment of find_all_inheritors(), it will only return OIDs of relations; while RELKIND_HAS_PARTITIONS checks for both relations and views. Logically using this macro works, but it may lead to some confusion to code readers.

4
```
@@ -722,6 +754,7 @@ BeginCopyTo(ParseState *pstate,
 		DestReceiver *dest;
 
 		cstate->rel = NULL;
+		cstate->partitions = NIL;
```

Both NULL assignment are not needed as cstate is allocated by palloc0().  

5
```
+static void
+CopyRelTo(CopyToState cstate, Relation rel, Relation root_rel,
+		  uint64 *processed)
```

Instead of using a pointer to pass out processed count, I think it’s better to return the process count. I understand the current implementation allows continuous increment while calling this function in a loop. However, it’s a bit error-prone, a caller must make sure “processed” is well initialized. With returning a unit64, the caller’s code is still simple:

```
processed += CopyRelTo(cstate, …);
``` 

6. In BeginCopyTo(), “children” list is created before “cstate” is created, it is not allocated under “cstate->copycontext”, so in EndCopyTo(), we should also free memory of “cstate->partitions”.

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/