Per-table resync for logical replication subscriptions

Hi hackers,

I would like to propose a subscriber-side way to re-copy a single table
that is already part of a logical replication subscription.

At the moment, I do not think PostgreSQL has a supported way to do this for
one table on one subscriber. ALTER SUBSCRIPTION ... REFRESH PUBLICATION
only handles tables that were added to the subscribed publications since
the subscription was created, or since the last refresh. It does not
re-copy tables that are already known to the subscription.

That is fine for the normal “new table was added to the publication” case,
but it leaves a gap when only one subscriber needs one table to be
re-seeded. Some examples are:

-

the subscriber table became inconsistent;
-

a row filter changed and the subscriber needs a fresh copy;
-

a forwarder/subscriber needs to repair one table without disturbing
other subscribers.

The usual workaround is to temporarily change publication membership,
refresh the subscription, truncate the local table, add the table back, and
refresh again. That works in simple topologies, but it is not really local
to the subscriber. In a fan-out topology it can affect sibling subscribers,
and in a forwarding topology the truncate can be replicated further
downstream if truncates are published.

There is also the unsupported option of updating pg_subscription_rel by
hand to put the table back into the initial sync state. That is not safe
either, because it races with the apply worker and does not handle the
surrounding cleanup and invalidation work.

There is already a similar mechanism for sequences. ALTER SUBSCRIPTION ...
REFRESH SEQUENCES can re-synchronize sequence data for sequences already
known to the subscription, while REFRESH PUBLICATION only discovers newly
added ones. This proposal is essentially the table-side equivalent of that
idea.

What I am proposing is something like:

ALTER SUBSCRIPTION name REFRESH TABLE table_name
[ WITH (copy_data = true, truncate = true) ];

or, if people prefer the wording:

ALTER SUBSCRIPTION name RESYNC TABLE table_name
[ WITH (copy_data = true, truncate = true) ];

The command would be subscriber-local and would not change publication
membership. Internally, it would reuse the existing table synchronization
machinery by resetting the selected relation back to the initial sync
state, so that a tablesync worker copies it again and moves it through the
normal i -> d -> s -> r states.

I tested the current behavior on current devel with a small three-node
setup:

-

upstream publisher U;
-

forwarding subscriber/publisher F;
-

downstream subscriber D;
-

sibling subscriber S.

The test script keeps a writer running and prints per-table checksums at
each step.

The important observations were:

1.

ALTER SUBSCRIPTION ... REFRESH PUBLICATION on F is a no-op for an
already subscribed table. The table remained in ready state, the tuple
metadata did not change, no WAL was generated, and no sync worker was
started.
2.

The publication-membership workaround is not local. In the forwarding
case, truncating the table on F also wiped the table on D. In the
fan-out case, the sibling subscriber S was left behind because the
publication changes were global but the repair was only intended for F.
3.

A controlled subscriber-local reset of only the selected table on F
produced matching checksums again without touching the publication or S.
Doing the same thing as a bare catalog update while the subscription
remained enabled was not reliable; the apply worker could continue with
cached state and the table diverged.

So I do not think the hard part is inventing a new synchronization
mechanism. The existing tablesync path already does most of the work. The
missing piece is a supported command that performs the reset safely:
stopping or coordinating with workers, cleaning up per-table sync state and
slots/origins where needed, optionally truncating the target table,
invalidating caches, taking the right locks, and validating that the
requested table is actually part of the subscription.

For a first version, I think it would be reasonable to require the
subscription to be disabled. That avoids the apply-worker race and keeps
the patch smaller. A later version could support the enabled case, probably
by making the running worker notice and re-read the changed relation state.

I have a WIP patch for the disabled-subscription, executor changes, and a
TAP test. Before polishing it further, I would like to get feedback on
whether this direction makes sense ?

Regards,

Cagri Biroglu