[HACKERS] moving some partitioning code to executor

Hi.

It seems to me that some of the code in partition.c is better placed
somewhere under the executor directory.  There was even a suggestion
recently [1] to introduce a execPartition.c to house some code around
tuple-routing.

IMO, catalog/partition.c should present an interface for handling
operations on a *single* partitioned table and avoid pretending to support
any operations on the whole partition tree.  For example, the
PartitionDispatch structure embeds some knowledge about the partition tree
it's part of, which is useful when used for tuple-routing, because of the
way it works now (lock and form ResultRelInfos of *all* leaf partitions
before the first input row is processed).

So, let's move that structure, along with the code that creates and
manipulates the same, out of partition.c/h and to execPartition.c/h.
Attached patch attempts to do that.

While doing the same, I didn't move *all* of get_partition_for_tuple() out
to execPartition.c, instead modified its signature as shown below:

-extern int get_partition_for_tuple(PartitionDispatch *pd,
-                        TupleTableSlot *slot,
-                        EState *estate,
-                        PartitionDispatchData **failed_at,
-                        TupleTableSlot **failed_slot);
+extern int get_partition_for_tuple(Relation relation, Datum *values,
+                            bool *isnull);

That way, we keep the core partition bound comparison logic inside
partition.c and move rest of the stuff to its caller ExecFindPartition(),
which includes navigating the enveloping PartitionDispatch's.

Thoughts?

PS: 0001 of the attached is the patch from [2] which is here to be applied
on HEAD before applying the main patch (0002) itself

Thanks,
Amit

[1]
https://www.postgresql.org/message-id/CA%2BTgmoafr%3DhUrM%3Dcbx-k%3DBDHOF2OfXa
w95HQSNAK4mHBwmSjtw%40mail.gmail.com

[2]
https://www.postgresql.org/message-id/7fe0007b-7ad1-a593-df11-ad05364ebce4%40l
ab.ntt.co.jp

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