Re: Allow CLUSTER, VACUUM FULL and REINDEX to change tablespace on the fly

On Mon, Jan 25, 2021 at 11:11:38PM +0300, Alexey Kondratov wrote:
> I updated comment with CCI info, did pgindent run and renamed new function
> to SetRelationTableSpace(). New patch is attached.
>
> [...]
>
> Yeah, all these checks we complicated from the beginning. I will try to find
> a better place tomorrow or put more info into the comments at least.

I was reviewing that, and I think that we can do a better
consolidation on several points that will also help the features
discussed on this thread for VACUUM, CLUSTER and REINDEX.

If you look closely, ATExecSetTableSpace() uses the same logic as the
code modified here to check if a relation can be moved to a new
tablespace, with extra checks for mapped relations,
GLOBALTABLESPACE_OID or if attempting to manipulate a temp relation
from another session.  There are two differences though:
- Custom actions are taken between the phase where we check if a
relation can be moved to a new tablespace, and the update of
pg_class.
- ATExecSetTableSpace() needs to be able to set a given relation
relfilenode on top of reltablespace, the newly-created one.

So I think that the heart of the problem is made of two things here:
- We should have one common routine for the existing code paths and
the new code paths able to check if a tablespace move can be done or
not.  The case of a cluster, reindex or vacuum on a list of relations
extracted from pg_class would still require a different handling
as incorrect relations have to be skipped, but the case of individual
relations can reuse the refactoring pieces done here
(see CheckRelationTableSpaceMove() in the attached).
- We need to have a second routine able to update reltablespace and
optionally relfilenode for a given relation's pg_class entry, once the
caller has made sure that CheckRelationTableSpaceMove() validates a
tablespace move.

Please note that was a bug in your previous patch 0002: shared
dependencies need to be registered if reltablespace is updated of
course, but also iff the relation has no physical storage.  So
changeDependencyOnTablespace() requires a check based on
RELKIND_HAS_STORAGE(), or REINDEX would have registered shared
dependencies even for relations with storage, something we don't
want per the recent work done by Alvaro in ebfe2db.
--
Michael