Re: wal_level logical for streaming replication

On Wed, 2019-08-28 at 21:44 +0530, Vijaykumar Jain wrote:
> If I change wal_level back to replica, will it corrupt wal? coz it
> will then be having diff information ( r format of data ?)

That's why you have to restart the server when you change that
parameter. This way, there will be a checkpoint marking the change.

> What is the base reason as to why ddls are not sent via logical
> replication but the hot standby has ddl changes reflected absolutely
> fine ?

Physical streaming replication just replicates the database files,
so it also replicates DDL statements, which are just changes to the
catalog tables.

Basically, anything that can be recovered can be replicated.

Logical replication has to perform "logical decoding", that is,
it has to translate the (physical) WAL information into logical
information (waht row was modified in which table).

So this is much more complicated. It should be possible in theory,
but nobody has got around to solving the difficulties involved yet.

> and there is one large limitation on large object support for logical
> replication?
> Where can I see the limitation on size or is it just certain data
> types ?

This has nothing to do with the size; I guess the answer is the same as
above. One proble that I can see immediately is that primary and
standby don't share the same OIDs, yet every large object is identified
by its OID. So I think this is a fundamental problem that cannot be
solved.

Yours,
Laurenz Albe
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