pg_upgrade: transfer pg_largeobject_metadata's files when possible

(new thread)

On Fri, Jul 18, 2025 at 11:05:04AM -0500, Nathan Bossart wrote:
> I'm cautiously optimistic that we can find some better gains for upgrades
> from v16 and newer.  That would involve dumping lo_create() commands for
> all LOs with comments/seclabels, dumping the relevant pg_shdepend rows, and
> then copying/linking the pg_largeobject_metadata files like we did prior to
> v12.

Here is a patch.  For background, the reason this is limited to upgrades
from v16 and newer is because the aclitem data type (needed by
pg_largeobject_metadata.lomacl) changed its storage format in v16 (see
commit 7b378237aa).  Note that the patch is essentially a revert of commit
12a53c732c, but there are enough differences that it should be considered a
fresh effort.

Something I hadn't anticipated is that we need to take special care to
transfer the relfilenode of pg_largeobject_metadata and its index, as was
done for pg_largeobject in commits d498e052b4 and bbe08b8869.  In fact, the
majority of the patch is dedicated to that.

My testing showed some decent, but not earth-shattering performance
improvements from this patch.  For upgrades with many large objects with
NULL lomacl/lomowner columns, pg_upgrade was 50% faster.  With non-NULL
lomacl/lomowner, that dropped to 25%.  When each large object had a
comment, there was no change.  I'm assuming that its rare to have lots of
large objects with comments or security labels, so I don't see any need to
expend energy trying to optimize that use-case.

I am a bit concerned that we'll forget to add checks for new types of
dependencies similar to comments and security labels.  If we do, pg_upgrade
should just fail to restore the schema, and fixing the code should be easy
enough.  Also, we'll need to remember to revisit this code if there's
another storage format change for one of pg_largeobject_metadata's columns,
but that seems unlikely to happen anytime soon.  On the whole, I'm not too
worried about either of these points.

-- 
nathan