Re: pg_dump is broken for partition tablespaces

On Thu, 7 Mar 2019 at 11:37, Andres Freund <andres(at)anarazel(dot)de> wrote:
>
> On 2019-03-07 11:31:15 +1300, David Rowley wrote:
> > Do you think it's fine to reword the docs to make this point more
> > clear, or do you see this as a fundamental problem with the patch?
>
> Hm, both? I mean I wouldn't necessarily characterize it as "fundamental"
> problem, but ...

Okay, so if I understand you correctly, you're complaining about the
fact that if the user does:

CREATE TABLE p (a int) PARTITION BY LIST(a) TABLESPACE pg_default;

that the user intended that all future partitions go to pg_default and
not whatever default_tablespace is set to at the time?

If so, that seems like a genuine concern.

I see in heap_create() we do;

/*
* Never allow a pg_class entry to explicitly specify the database's
* default tablespace in reltablespace; force it to zero instead. This
* ensures that if the database is cloned with a different default
* tablespace, the pg_class entry will still match where CREATE DATABASE
* will put the physically copied relation.
*
* Yes, this is a bit of a hack.
*/
if (reltablespace == MyDatabaseTableSpace)
reltablespace = InvalidOid;

which will zero pg_class.reltablespace if the specified tablespace
happens to match pg_database.dattablespace. This causes future
partitions to think that no tablespace was specified and therefore
DefineRelation() consults the default_tablespace.

I see that this same problem exists for partitioned indexes too:

create table listp (a int) partition by list(a);
create index on listp (a) tablespace pg_default;
set default_Tablespace = n;
create table listp1 partition of listp for values in(1);
\d listp1
Table "public.listp1"
Column | Type | Collation | Nullable | Default
--------+---------+-----------+----------+---------
a | integer | | |
Partition of: listp FOR VALUES IN (1)
Indexes:
"listp1_a_idx" btree (a), tablespace "n"
Tablespace: "n"

If I understand what you're saying correctly, then the listp1_a_idx
should have been created in pg_default since that's what the default
partitioned index tablespace was set to.

> I don't think the argument that the user intended to explicitly set a
> tablespace holds much water if it was just set via default_tablespace,
> rather than an explicit TABLESPACE. I think iff you really want
> something like this feature, you'd have to mark a partition's
> reltablespace as 0 unless an *explicit* assignment of the tablespace
> happened. In which case you also would need to explicitly emit a
> TABLESPACE for the partitioned table in pg_dump, to restore that.

I think emitting an explicit tablespace in pg_dump for partitioned
tables (when non-zero) might have issues for pg_restore's
--no-tablespaces option.

--
David Rowley http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services