I found the way that MergeAttributes() handles column compression and
storage settings very weird.
For example, I don't understand the purpose of this error:
create table t1 (a int, b text compression pglz);
create table t1a (b text compression lz4) inherits (t1);
...
ERROR: 42804: column "b" has a compression method conflict
DETAIL: pglz versus lz4
or this:
create table t2 (a int, b text compression lz4);
create table t12 () inherits (t1, t2);
...
ERROR: column "b" has a compression method conflict
DETAIL: pglz versus lz4
And we can't override it in the child, per the first example.
But this works:
create table t1a (a int, b text compression lz4);
alter table t1a inherit t1;
Also, you can change the settings in the child using ALTER TABLE ... SET
COMPRESSION (which is also how pg_dump will represent the above
constructions), so the restrictions at CREATE TABLE time don't seem to
make much sense.
Looking at the code, I suspect these rules were just sort of
copy-and-pasted from the nearby rules for types and collations. The
latter are needed so that a table with inheritance children can present
a logically consistent view of the data. But compression and storage
are physical properties that are not logically visible, so every table
in an inheritance hierarchy can have their own setting.
I think the rules should be approximately like this (both for
compression and storage):
- A newly created child inherits the settings from the parent.
- A newly created child can override the settings.
- Attaching a child changes nothing.
- When inheriting from multiple parents with different settings, an
explicit setting in the child is required.
Thoughts?
