Inheriting table AMs for partitioned tables

Hi,

In the pluggable storage patch [1], one thing that I'm wondering about
is how exactly to inherit the storage AM across partitions. I think
that's potentially worthy of a discussion with a wider audience than I'd
get in that thread. It seems also related to the recent discussion in [2]

Consider (excerpted from the tests):

CREATE TABLE tableam_parted_heap2 (a text, b int) PARTITION BY list (a) USING heap2;

SET default_table_access_method = 'heap';
CREATE TABLE tableam_parted_a_heap2 PARTITION OF tableam_parted_heap2 FOR VALUES IN ('a');

SET default_table_access_method = 'heap2';
CREATE TABLE tableam_parted_b_heap2 PARTITION OF tableam_parted_heap2 FOR VALUES IN ('b');

CREATE TABLE tableam_parted_c_heap2 PARTITION OF tableam_parted_heap2 FOR VALUES IN ('c') USING heap;
CREATE TABLE tableam_parted_d_heap2 PARTITION OF tableam_parted_heap2 FOR VALUES IN ('d') USING heap2;

It seems pretty clear that tableam_parted_heap2, tableam_parted_d_heap2
would be stored via heap2, and tableam_parted_c_heap2 via heap.

But for tableam_parted_a_heap2 tableam_parted_b_heap2 the answer isn't
quite as clear. I think it'd both be sensible for new partitions to
inherit the AM from the root, but it'd also be sensible to use the
current default.

Out of laziness (it's how it works rn) I'm inclined to to go with using
the current default, but I'd be curious if others disagree.

Greetings,

Andres Freund

[1] https://postgr.es/m/20180703070645.wchpu5muyto5n647%40alap3.anarazel.de
[2] https://www.postgresql.org/message-id/201902041630.gpadougzab7v%40alvherre.pgsql