Re: Ambiguous description on new columns

On Wed, May 29, 2024 at 8:04 PM vignesh C <vignesh21(at)gmail(dot)com> wrote:
>
> On Wed, 22 May 2024 at 14:26, Peter Smith <smithpb2250(at)gmail(dot)com> wrote:
> >
> > On Tue, May 21, 2024 at 8:40 PM PG Doc comments form
> > <noreply(at)postgresql(dot)org> wrote:
> > >
> > > The following documentation comment has been logged on the website:
> > >
> > > Page: https://www.postgresql.org/docs/16/logical-replication-col-lists.html
> > > Description:
> > >
> > > The documentation on this page mentions:
> > >
> > > "If no column list is specified, any columns added later are automatically
> > > replicated."
> > >
> > > It feels ambiguous what this could mean. Does it mean:
> > >
> > > 1/ That if you alter the table on the publisher and add a new column, it
> > > will be replicated
> > >
> > > 2/ If you add a column list later and add a column to it, it will be
> > > replicated
> > >
> > > In both cases, does the subscriber automatically create this column if it
> > > wasn't there before?
> >
> > No, the subscriber will not automatically create the column. That is
> > already clearly said at the top of the same page you linked "The table
> > on the subscriber side must have at least all the columns that are
> > published."
> >
> > All that "If no column list..." paragraph was trying to say is:
> >
> > CREATE PUBLICATION pub FOR TABLE T;
> >
> > is not quite the same as:
> >
> > CREATE PUBLICATION pub FOR TABLE T(a,b,c);
> >
> > The difference is, in the 1st case if you then ALTER the TABLE T to
> > have a new column 'd' then that will automatically start replicating
> > the 'd' data without having to do anything to either the PUBLICATION
> > or the SUBSCRIPTION. Of course, if TABLE T at the subscriber side does
> > not have a column 'd' then you'll get an error because your subscriber
> > table needs to have *at least* all the replicated columns. (I
> > demonstrate this error below)
> >
> > Whereas in the 2nd case, even though you ALTER'ed the TABLE T to have
> > a new column 'd' then that won't be replicated because 'd' was not
> > named in the PUBLICATION's column list.
> >
> > ~~~~
> >
> > Here's an example where you can see this in action
> >
> > Here is an example of the 1st case -- it shows 'd' is automatically
> > replicated and also shows the subscriber-side error caused by the
> > missing column:
> >
> > test_pub=# CREATE TABLE T(a int,b int, c int);
> > test_pub=# CREATE PUBLICATION pub FOR TABLE T;
> >
> > test_sub=# CREATE TABLE T(a int,b int, c int);
> > test_sub=# CREATE SUBSCRIPTION sub CONNECTION 'dbname=test_pub' PUBLICATION pub;
> >
> > See the replication happening
> > test_pub=# INSERT INTO T VALUES (1,2,3);
> > test_sub=# SELECT * FROM t;
> > a | b | c
> > ---+---+---
> > 1 | 2 | 3
> > (1 row)
> >
> > Now alter the publisher table T and insert some new data
> > test_pub=# ALTER TABLE T ADD COLUMN d int;
> > test_pub=# INSERT INTO T VALUES (5,6,7,8);
> >
> > This will cause subscription errors like:
> > 2024-05-22 11:53:19.098 AEST [16226] ERROR: logical replication
> > target relation "public.t" is missing replicated column: "d"
> >
> > ~~~~
> >
> > I think the following small change will remove any ambiguity:
> >
> > BEFORE
> > If no column list is specified, any columns added later are
> > automatically replicated.
> >
> > SUGGESTION
> > If no column list is specified, any columns added to the table later
> > are automatically replicated.
> >
> > ~~
> >
> > I attached a small patch to make the above change.
>
> A small recommendation:
> It would enhance clarity to include a line break following "If no
> column list is specified, any columns added to the table later are":
> - If no column list is specified, any columns added later are automatically
> + If no column list is specified, any columns added to the table
> later are automatically
> replicated. This means that having a column list which names all columns

Hi Vignesh,

IIUC you're saying my v1 patch *content* and rendering is OK, but you
only wanted the SGML text to have better wrapping for < 80 chars
lines. So I have attached a patch v2 with improved wrapping. If you
meant something different then please explain.

======
Kind Regards,
Peter Smith.
Fujitsu Australia