Inconsistent/confusing handling of tablespaces for partitioned tables

Hi hackers,

The following sequence of statements:

CREATE SCHEMA testschema;
CREATE TABLE testschema.part (a int) PARTITION BY LIST (a);
SET default_tablespace TO pg_global;
ALTER TABLE testschema.part SET TABLESPACE pg_default;
CREATE TABLE testschema.part_78 PARTITION OF testschema.part FOR VALUES 
IN (7, 8) PARTITION BY LIST (a);

Produce error
ERROR:  only shared relations can be placed in pg_global tablespace
when been executed in database with default tablespace, but produce no 
error in database with assigned tablespace.

create tablespace my_tblspc location '/tmp/tblspc';
create databse test;
alter database test set tablespace my_tblspc;


There is the following code in tablecmds.c:

     else if (stmt->partbound)
     {
         /*
          * For partitions, when no other tablespace is specified, we 
default
          * the tablespace to the parent partitioned table's.
          */
         Assert(list_length(inheritOids) == 1);
         tablespaceId = get_rel_tablespace(linitial_oid(inheritOids));
     }

In first case get_rel_tablespace returns 0 (because parent table has no 
explicit tablespace)
and in the second: pg_default


Also I am confused that the following statement is rejected:

SET default_tablespace TO pg_default;
CREATE TABLE testschema.part (a int) PARTITION BY LIST (a);
ERROR:  cannot specify default tablespace for partitioned relations

but still it is possible to set tablespace of parent table to pg_default 
using alter tablespace command:

RESET default_tablespace;
CREATE TABLE testschema.part (a int) PARTITION BY LIST (a);
ALTER TABLE testschema.part SET TABLESPACE pg_default;

But ... it has no effect: testschema.part is till assumed to belong to 
default tablespace.
Because of the following code in tablecmds.c:


     /*
      * No work if no change in tablespace.
      */
     oldTableSpace = rel->rd_rel->reltablespace;
     if (newTableSpace == oldTableSpace ||
         (newTableSpace == MyDatabaseTableSpace && oldTableSpace == 0))
     {
         InvokeObjectPostAlterHook(RelationRelationId,
                                   RelationGetRelid(rel), 0);

         relation_close(rel, NoLock);
         return;
     }


I found the thread discussing the similar problem:

https://www.postgresql.org/message-id/flat/BY5PR18MB3170E372542F34694E630B12F10C0%40BY5PR18MB3170.namprd18.prod.outlook.com

and looks like the decision was to change nothing and leave everything 
as it is.

 From my point of view the source of the problem is that pg_default 
(oid=1663) is treated as database default tablespace.
pg_default stands for concrete tablespace and it is not clear why it is 
treated in different way comparing with any other tablepsace.

-- 
Konstantin Knizhnik
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company