Feedback-ID: ia2694551:Fastmail
Date: Thu, 2 Feb 2023 11:58:59 +0100
From: Alvaro Herrera <alvherre@alvh.no-ip.org>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, pgsql-hackers@lists.postgresql.org,
	Andrew <pgsqlhackers@andrewrepp.com>
Subject: Re: pg_dump versus hash partitioning
Message-ID: <20230202105859.f6wp77s2j3qsoyvz@alvherre.pgsql>
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On 2023-Feb-01, Robert Haas wrote:

> I think you can construct plausible cases where it's not just
> academic. For instance, suppose I intend to use some kind of logical
> replication system, not necessarily the one built into PostgreSQL, to
> replicate data between two systems. Before engaging that system, I
> need to make the initial database contents match. The system is
> oblivious to partitioning, and just replicates each table to a table
> with a matching name.

This only works if that other system's hashing behavior is identical to
Postgres' for hashing that particular enum; there's no other way that
you could make the tables match exactly in the way you propose.  What
this tells me is that it's not really reasonable for users to expect
that this situation would actually work.  It is totally reasonable for
range and list, but not for hash.


If the idea of --load-via-partition-root=auto is going to be the fix for
this problem, then it has to consider that hash partitioning might be in
a level below the topmost one.  For example,

create type colors as enum ('blue', 'red', 'green');
create table topmost (prim int, col colors, a int) partition by range (prim);
create table parent partition of topmost for values from (0) to (1000) partition by hash (col);
create table child1 partition of parent for values with (modulus 3, remainder 0);
create table child2 partition of parent for values with (modulus 3, remainder 1);
create table child3 partition of parent for values with (modulus 3, remainder 2);

If you dump this with --load-via-partition-root, for child1 it'll give you this:

--
-- Data for Name: child1; Type: TABLE DATA; Schema: public; Owner: alvherre
--

COPY public.topmost (prim, col, a) FROM stdin;
\.

which is what we want; so for --load-via-partition-root=auto (or
whatever), we need to ensure that we detect hash partitioning all the
way down from the topmost to the leaves.

-- 
Álvaro Herrera               48°01'N 7°57'E  —  https://www.EnterpriseDB.com/
"La libertad es como el dinero; el que no la sabe emplear la pierde" (Alvarez)