Re: Support logical replication of DDLs

On Thu, Apr 7, 2022 at 3:46 PM Amit Kapila <amit.kapila16@gmail.com> wrote:

>

> On Wed, Mar 23, 2022 at 10:39 AM Japin Li <japinli@hotmail.com> wrote:

>

> 2. For DDL replication, do we need to wait for a consistent point of

> snapshot? For DMLs, that point is a convenient point to initialize

> replication from, which is why we export a snapshot at that point,

> which is used to read normal data. Do we have any similar needs for

> DDL replication?

>

I have thought a bit more about this and I think we need to build the

snapshot for DML replication as we need to read catalog tables to

decode the corresponding WAL but it is not clear to me if we have a

similar requirement for DDL replication. If the catalog access is

required then it makes sense to follow the current snapshot model,

otherwise, we may need to think differently for DDL replication.

One more related point is that for DML replication, we do ensure that

we copy the entire data of the table (via initial sync) which exists

even before the publication for that table exists, so do we want to do

something similar for DDLs? How do we sync the schema of the table

before the user has defined the publication? Say the table has been

created before the publication is defined and after that, there are

only Alter statements, so do we expect, users to create the table on

the subscriber and then we can replicate the Alter statements? And

even if we do that it won't be clear which Alter statements will be

replicated after publication is defined especially if those Alters

happened concurrently with defining publications?