Horton

Re: POC: enable logical decoding when wal_level = 'replica' without a server restart

Amit Kapila <amit.kapila16@gmail.com>

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From: Amit Kapila <amit.kapila16@gmail.com>
To: Masahiko Sawada <sawada.mshk@gmail.com>
Cc: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>, Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>, PostgreSQL-development <pgsql-hackers@postgresql.org>
Date: 2025-04-24T12:30:07Z
Lists: pgsql-hackers

Commits

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  1. Fix regression test failure when wal_level is set to minimal.

  2. Toggle logical decoding dynamically based on logical slot presence.

  3. Disallow server start with sync_replication_slots = on and wal_level < logical.

On Wed, Apr 23, 2025 at 9:35 PM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
>
> On Wed, Apr 23, 2025 at 5:46 AM Amit Kapila <amit.kapila16@gmail.com> wrote:
> >
> > BTW, did we consider the idea to automatically transition to 'logical'
> > when the first logical slot is created and transition back to
> > 'replica' when last logical slot gets dropped? I see some ideas around
> > this last time we discussed this topic.
>
> Yes. Bertrand pointed out that a drawback is that the primary server
> needs to create a logical slot in order to execute logical decoding on
> the standbys[1].
>

True, but if we want to avoid that, we can still keep 'logical' as
wal_level for the ease of users. We can also have another API like the
one you originally proposed (pg_activate_logical_decoding) for the
ease of users. But the minimum requirement would be that one creates a
logical slot to enable logical decoding/replication.

Additionally, shall we do some benchmarking, if not done already, to
show the cases where the performance and WAL volume can hurt users if
we make wal_level as 'logical'?

-- 
With Regards,
Amit Kapila.