Re: [PATCH] Add `truncate` option to subscription commands

On Mon, May 24, 2021 at 11:01 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > 1) Whether a table the sync worker is trying to truncate is having any
> > referencing (foreign key) tables on the subscriber? If yes, whether
> > all the referencing tables are present in the list of subscription
> > tables (output of fetch_table_list)? In this case, the sync worker is
> > truncating the primary key/referenced table.
> >
> > One way to solve the above problem is by storing the table oids of the
> > subscription tables (output of fetch_table_list) in a new column in
> > the pg_subscription catalog (like subpublications text[] column). In
> > the sync worker, before truncation of a table, use
> > heap_truncate_find_FKs to get all the referencing tables of the given
> > table and get all the subscription tables from the new pg_subscription
> > column. If all the referencing tables exist in the subscription
> > tables, then truncate the table, otherwise don't, just skip it.
> >
>
> Here, silently skipping doesn't seem like a good idea when the user
> has asked to truncate the table. Shouldn't we allow it if the user has
> provided say cascade with a truncate option?

We could do that. In that case, the truncate option just can't be a
boolean, but it has to be an enum accepting "restrict", "cascade",
maybe "restart identity" or "continue identity" too. I have a concern
here - what if the ExecuteTruncateGuts fails with the cascade option
for whatever reason? Should the table sync worker be trapped in that
error? Will that table ever finish initial table sync/data copy?
Basically, how will this error info be known to the user other than
from the subscriber logs?

Or should it just continue by skipping the error?

If required, we could introduce another rel state, say,
SUBREL_STATE_READY_WITH_TRUNCATION_DONE if the table is truncated as
per the user expectation. Otherwise just SUBREL_STATE_READY if there
has been any error occurred while truncating.

Another thing is that, if we allow the cascade option we must document
it saying that the truncate might cascade down to any subscriber local
tables that are not part of the subscription.

Thoughts?

> > There
> > can be a problem here if there are many subscription tables, the size
> > of the new column in pg_susbcription can be huge. However, we can
> > choose to store the table ids in this new column only when the
> > truncate option is specified.
> >
> > Another way is to let each table sync worker scan the
> > pg_subscription_rel to get all the relations that belong to a
> > subscription. But I felt this was costly.
> >
>
> I feel it is better to use pg_subscription_rel especially because we
> will do so when the user has given the truncate option and note that
> we are already accessing it in sync worker for both reading and
> writing. See LogicalRepSyncTableStart.

Note that in pg_subscription_rel, there can exist multiple rows for
each table for a given subscription. Say, t1 is a table that the sync
worker is trying to truncate and copy. Say, t1_dep1, t1_dep2, t1_dep3
.... are the dependent tables (we can find these using
heap_truncate_find_FKs). Now, we need to see if all the t1_dep1,
t1_dep2, t1_dep3 .... tables are in the pg_suscription_rel with the
same subscription id, then only we can delete all of them with
EexecuteTruncateGuts() using cascade option. If any of the t1_depX is
either not in the pg_subscription_rel or it is subscribed in another
subscription, then is it okay if we scan pg_suscription_rel in a loop
with t1_depX relid's? Isn't it costiler? Or since it is a cache
lookup, maybe that's okay?

/* Try finding the mapping. */
tup = SearchSysCache2(SUBSCRIPTIONRELMAP,
ObjectIdGetDatum(relid),
ObjectIdGetDatum(subid));

> One other problem discussed in this thread was what to do when the
> same table is part of multiple subscriptions and the user has provided
> a truncate option while operating on such a subscription.

I think we can just skip truncating a table when it is part of
multiple subscriptions. We can tell this clearly in the documentation.

With Regards,
Bharath Rupireddy.
EnterpriseDB: http://www.enterprisedb.com