Re: logical decoding and replication of sequences, take 2

On Wed, Mar 15, 2023 at 7:51 PM Tomas Vondra <tomas(dot)vondra(at)enterprisedb(dot)com>
wrote:
>
>
>
> On 3/14/23 08:30, John Naylor wrote:
> > I tried a couple toy examples with various combinations of use styles.
> >
> > Three with "automatic" reading from sequences:
> >
> > create table test(i serial);
> > create table test(i int GENERATED BY DEFAULT AS IDENTITY);
> > create table test(i int default nextval('s1'));
> >
> > ...where s1 has some non-default parameters:
> >
> > CREATE SEQUENCE s1 START 100 MAXVALUE 100 INCREMENT BY -1;
> >
> > ...and then two with explicit use of s1, one inserting the 'nextval'
> > into a table with no default, and one with no table at all, just
> > selecting from the sequence.
> >
> > The last two seem to work similarly to the first three, so it seems like
> > FOR ALL TABLES adds all sequences as well. Is that expected?
>
> Yeah, that's a bug - we shouldn't replicate the sequence changes, unless
> the sequence is actually added to the publication. I tracked this down
> to a thinko in get_rel_sync_entry() which failed to check the object
> type when puballtables or puballsequences was set.
>
> Attached is a patch fixing this.

Okay, I can verify that with 0001-0006, sequences don't replicate unless
specified. I do see an additional change that doesn't make sense: On the
subscriber I no longer see a jump to the logged 32 increment, I see the
very next value:

# alter system set wal_level='logical';
# port 7777 is subscriber

echo
echo "PUB:"
psql -c "drop table if exists test;"
psql -c "drop publication if exists pub1;"

echo
echo "SUB:"
psql -p 7777 -c "drop table if exists test;"
psql -p 7777 -c "drop subscription if exists sub1 ;"

echo
echo "PUB:"
psql -c "create table test(i int GENERATED BY DEFAULT AS IDENTITY);"
psql -c "CREATE PUBLICATION pub1 FOR ALL TABLES;"
psql -c "CREATE PUBLICATION pub2 FOR ALL SEQUENCES;"

echo
echo "SUB:"
psql -p 7777 -c "create table test(i int GENERATED BY DEFAULT AS IDENTITY);"
psql -p 7777 -c "CREATE SUBSCRIPTION sub1 CONNECTION 'host=localhost
dbname=postgres application_name=sub1 port=5432' PUBLICATION pub1;"
psql -p 7777 -c "CREATE SUBSCRIPTION sub2 CONNECTION 'host=localhost
dbname=postgres application_name=sub2 port=5432' PUBLICATION pub2;"

echo
echo "PUB:"
psql -c "insert into test default values;"
psql -c "insert into test default values;"
psql -c "select * from test;"
psql -c "select * from test_i_seq;"

sleep 1

echo
echo "SUB:"
psql -p 7777 -c "select * from test;"
psql -p 7777 -c "select * from test_i_seq;"

psql -p 7777 -c "drop subscription sub1 ;"
psql -p 7777 -c "drop subscription sub2 ;"

psql -p 7777 -c "insert into test default values;"
psql -p 7777 -c "select * from test;"
psql -p 7777 -c "select * from test_i_seq;"

The last two queries on the subscriber show:

i
---
1
2
3
(3 rows)

last_value | log_cnt | is_called
------------+---------+-----------
3 | 30 | t
(1 row)

...whereas before with 0001-0003 I saw:

i
----
1
2
34
(3 rows)

last_value | log_cnt | is_called
------------+---------+-----------
34 | 32 | t

> > The documentation for CREATE PUBLICATION mentions sequence options,
> > but doesn't really say how these options should be used.
> Good point. The idea is that we handle tables and sequences the same
> way, i.e. if you specify 'sequence' then we'll replicate increments for
> sequences explicitly added to the publication.
>
> If this is not clear, the docs may need some improvements.

Aside from docs, I'm not clear what some of the tests are doing:

+CREATE PUBLICATION testpub_forallsequences FOR ALL SEQUENCES WITH (publish
= 'sequence');
+RESET client_min_messages;
+ALTER PUBLICATION testpub_forallsequences SET (publish = 'insert,
sequence');

What does it mean to add 'insert' to a sequence publication?

Likewise, from a brief change in my test above, 'sequence' seems to be a
noise word for table publications. I'm not fully read up on the background
of this topic, but wanted to make sure I understood the design of the
syntax.

--
John Naylor
EDB: http://www.enterprisedb.com