RE: [POC] Fast COPY FROM command for the table with foreign partitions

From: Amit Langote <amitlangote09@gmail.com>
> Andrey's original patch had the flag to, as I understand it, make the
> partitioning case work correctly.  When inserting into a
> non-partitioned table, there's only one relation to care about.  In
> that case, CopyFrom() can use either the new COPY interface or the
> INSERT interface for the entire operation when talking to a foreign
> target relation's FDW driver.  With partitions, that has to be
> considered separately for each partition.  What complicates the matter
> further is that while the original target relation (the root
> partitioned table in the partitioning case) is fully initialized in
> CopyFrom(), partitions are lazily initialized by ExecFindPartition().

Yeah, I felt it a bit confusing to see the calls to Begin/EndForeignInsert() in both CopyFrom() and
ExecInitRoutingInfo().


> Note that the initialization of a given target relation can also
> optionally involve calling the FDW to perform any pre-COPY
> initializations.  So if a given partition is a foreign table, whether
> the copy operation was initialized using the COPY interface or the
> INSERT interface is determined away from CopyFrom().  Andrey created
> ri_usesMultiInsert to remember which was used so that CopyFrom() can
> use the correct interface during the subsequent interactions with the
> partition's driver.
> 
> Now, it does not seem outright impossible to do this without the flag,
> but maybe Andrey thinks it is good for readability?  If it is
> confusing from a modularity standpoint, maybe we should rethink that.
> That said, I still think that there should be a way for CopyFrom() to
> tell ExecFindPartition() which FDW interface to initialize a given
> foreign table partition's copy operation with -- COPY if the copy
> allows multi-insert, INSERT if not.  Maybe the multi_insert parameter
> I mentioned earlier would serve that purpose.

I agree with your idea of adding multi_insert argument to ExecFindPartition() to request a multi-insert-capable
partition. At first, I thought ExecFindPartition() is used for all operations, insert/delete/update/select, so I found
itodd to add multi_insert argument.  But ExecFindPartion() is used only for insert, so multi_insert argument seems
okay.


Regards
Takayuki Tsunakawa