Re: Support logical replication of DDLs

On Thu, Mar 24, 2022 at 3:32 PM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
>
> On Mon, Mar 21, 2022 at 1:43 PM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> >
> > On Thu, Mar 17, 2022 at 2:47 AM Zheng Li <zhengli10(at)gmail(dot)com> wrote:
> > >
> > > Hi,
> > >
> > > >If you don't mind, would you like to share the POC or the branch for this work?
> > >
> > > The POC patch is attached. It currently supports the following functionalities:
> >
> > Thanks for sharing, I will look into it.
> >
> > > >In such cases why don't we just log the table creation WAL for DDL
> > > >instead of a complete statement which creates the table and inserts
> > > >the tuple? Because we are already WAL logging individual inserts and
> > > >once you make sure of replicating the table creation I think the exact
> > > >data insertion on the subscriber side will be taken care of by the
> > > >insert WALs no?
> > >
> > > The table creation WAL and table insert WAL are available. The tricky
> > > part is how do we break down this command into two parts (a normal
> > > CREATE TABLE followed by insertions) either from the parsetree or the
> > > WALs. I’ll have to dig more on this.
> >
> > I agree that this is a bit tricky, anyway I will also put more thoughts on this.
>
> I had put some more thought about this, basically, during CTAS we are
> generating the CreateStmt inside "create_ctas_internal" and executing
> it first before inserting the tuple, so can't we generate the
> independent sql just for creating the tuple maybe using deparsing or
> something?
>
> Apart from that I have one more question, basically if you are
> directly logging the sql query then how you are identifying under
> which schema you need to create that table, are you changing the sql
> and generating schema-qualified name?

I was going through the patch and it seems you are logging the search
path as well along with the query so I think this will probably work.
I have got one more query while looking into the code. In the below
code snippet you are logging DDL command only if it is a top level
query but there are no comments explaining what sort of queries we
don't want to log. Suppose I am executing a DDL statement inside a PL
then that will not be a top level statement so is your intention to
block that as well or that is an unintentional side effect?

+ /*
+ * Consider logging the DDL command if logical logging is
enabled and this is
+ * a top level query.
+ */
+ if (XLogLogicalInfoActive() && isTopLevel)
+ LogLogicalDDLCommand(parsetree, queryString);
+

--
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com