[HACKERS] Not getting error if ALTER SUBSCRIPTION syntax is wrong.

Hi,

While testing 'logical replication' against v10 , i encountered one 
issue where data stop migrating after ALTER PUBLICATION.

X Server
\\ Make sure wal_level is set to logical in postgresql.conf file
\\create table/Insert 1 row -> create table test(n int); insert into t 
values (1);
\\create publication for all -> create publication pub for ALL TABLES ;


Y server

\\ Make sure wal_level is set to logical in postgresql.conf file
\\create table -> create table test(n int);

\\create Subscription

CREATE SUBSCRIPTION sub CONNECTION 'host=localhost dbname=postgres 
port=5432 ' PUBLICATION pub;

postgres=# select * from test; n
--- 1
(1 row)

\\Alter subscription
postgres=# alter subscription sub connection 'host=localhost 
dbname=postgres PUBLICATION pub';
ALTER SUBSCRIPTION

X server
postgres=# insert into test values (1);
INSERT 0 1
postgres=# select * from test; n
--- 1 1
(2 rows)

Y server
postgres=# select * from test; n
--- 1
(1 row)

I think probably syntax of alter subscription is not correct but 
surprisingly it is not throwing an error.

-- 
regards,tushar
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