Re: Support logical replication of DDLs

On Thu, Mar 30, 2023 at 3:16 PM vignesh C <vignesh21(at)gmail(dot)com> wrote:
>
> On Thu, 30 Mar 2023 at 13:29, houzj(dot)fnst(at)fujitsu(dot)com
> <houzj(dot)fnst(at)fujitsu(dot)com> wrote:
> >
> >
> >
> > > -----Original Message-----
> > > From: houzj(dot)fnst(at)fujitsu(dot)com <houzj(dot)fnst(at)fujitsu(dot)com>
> > > Sent: Thursday, March 30, 2023 2:37 PM
> > >
> > > On Tuesday, March 28, 2023 12:13 PM houzj(dot)fnst(at)fujitsu(dot)com
> > > <houzj(dot)fnst(at)fujitsu(dot)com> wrote:
> > > >
> > > > On Monday, March 27, 2023 8:08 PM Amit Kapila
> > > <amit(dot)kapila16(at)gmail(dot)com>
> > > > wrote:
> > > > > On Mon, Mar 27, 2023 at 12:07 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
> > > > > wrote:
> > > > > >
> > > > > > On Mon, Mar 27, 2023 at 2:52 AM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> > > > > > >
> > > > > >
> > > > > > > I suggest taking a couple of steps back from the minutiae of the
> > > > > > > patch, and spending some hard effort thinking about how the thing
> > > > > > > would be controlled in a useful fashion (that is, a real design
> > > > > > > for the filtering that was mentioned at the very outset), and
> > > > > > > about the security issues, and about how we could get to a
> > > committable
> > > > patch.
> > > > > > >
> > > > > >
> > > > > > Agreed. I'll try to summarize the discussion we have till now on
> > > > > > this and share my thoughts on the same in a separate email.
> > > > > >
> > > > >
> > > > > The idea to control what could be replicated is to introduce a new
> > > > > publication option 'ddl' along with current options 'publish' and
> > > > > 'publish_via_partition_root'. The values of this new option could be
> > > > > 'table', 'function', 'all', etc. Here 'all' enables the replication of
> > > > > all supported DDL commands. Example usage for this would be:
> > > > > Example:
> > > > > Create a new publication with all ddl replication enabled:
> > > > > CREATE PUBLICATION pub1 FOR ALL TABLES with (ddl = 'all');
> > > > >
> > > > > Enable table ddl replication for an existing Publication:
> > > > > ALTER PUBLICATION pub2 SET (ddl = 'table');
> > > > >
> > > > > This is what seems to have been discussed but I think we can even
> > > > > extend it to support based on operations/commands, say one would like
> > > > > to publish only 'create' and 'drop' of tables. Then we can extend the
> > > > > existing publish option to have values like 'create', 'alter', and 'drop'.
> > > > >
> > > > > Another thing we are considering related to this is at what level
> > > > > these additional options should be specified. We have three variants
> > > > > FOR TABLE, FOR ALL TABLES, and FOR TABLES IN SCHEMA that enables
> > > > > replication. Now, for the sake of simplicity, this new option is
> > > > > discussed to be provided only with FOR ALL TABLES variant but I think
> > > > > we can provide it with other variants with some additional
> > > > > restrictions like with FOR TABLE, we can only specify 'alter' and
> > > > > 'drop' for publish option. Now, though possible, it brings additional
> > > > > complexity to support it with variants other than FOR ALL TABLES
> > > > > because then we need to ensure additional filtering and possible
> > > > > modification of the content we have to send to downstream. So, we can
> > > even
> > > > decide to first support it only FOR ALL TABLES variant.
> > > > >
> > > > > The other point to consider for publish option 'ddl = table' is
> > > > > whether we need to allow replicating dependent objects like say some
> > > > > user-defined type is used in the table. I guess the difficulty here
> > > > > would be to identify which dependents we want to allow.
> > > > >
> > > > > I think in the first version we should allow to replicate only some of
> > > > > the objects instead of everything. For example, can we consider only
> > > > > allowing tables and indexes in the first version? Then extend it in a phased
> > > > manner?
> > > >
> > > > I think supporting table related stuff in the first version makes sense and the
> > > > patch size could be reduced to a suitable size.
> > >
> > > Based on the discussion, I split the patch into four parts: Table DDL
> > > replication(0001,0002), Index DDL replication(0003), ownership stuff for table
> > > and index(0004), other DDL's replication(0005).
> > >
> > > In this version, I mainly tried to split the patch set, and there are few
> > > OPEN items we need to address later:
> > >
> > > 1) The current publication "ddl" option only have two values: table, all. We
> > > also need to add index and maybe other objects in the list.
> > >
> > > 2) Need to improve the syntax stuff. Currently, we store the option value of
> > > the "with (ddl = xx)" via different columns in the catalog, the
> > > catalog(pg_publication) will have more and more columns as we add
> > > support
> > > for logical replication of other objects in the future. We could store it as
> > > an text array instead.
> > >
> > > OTOH, since we have proposed some other more flexible syntax to -hackers,
> > > the current
> > > syntax might be changed which might also solve this problem.
> > >
> > > 3) The test_ddl_deparse_regress test module is not included in the set,
> > > because
> > > I think we also need to split it into table stuff, index stuff and others,
> > > we can share it after finishing that.
> > >
> > > 4) The patch set could be spitted further to make it easier for reviewer like:
> > > infrastructure for deparser, deparser, logical-decoding, built-in logical
> > > replication, We can do it after some analysis.
> > >
> > > [1]
> > > https://www.postgresql.org/message-id/OS0PR01MB571646874A3E165D939
> > > 99A9D94889%40OS0PR01MB5716.jpnprd01.prod.outlook.com
> >
> > The patch needs a rebase due to a recent commit da324d6, here is the rebased version.
>
> Thanks for the patches, Few comments:
> 1: create unlogged is replicated but the insert on the same is not replicated:
> create unlogged table t3(c1 int); -- The insert on this is not replicated
>

I don't see a reason to replicate unlogged tables when we don't
replicate the corresponding inserts.

>
> 4: Should we document tablespace creation should be taken care of by user:
> create table t1(c1 int) tablespace tbs1; -- As tablespace tbs1 does
> not exist in the subscriber, should we add some documentation for
> this.
>
> 5: temporary table is not replicated, should we add some documentation for this:
> create global temporary table t2(c1 int); -- Should we add some
> documentation for this
>

Yeah, I think it would be better to document the operations which
won't be replicated.

--
With Regards,
Amit Kapila.