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Subject: Re: Replication slot WAL reservation
From: Christophe Pettus <xof@thebuild.com>
In-Reply-To: <CAGAwPgR=B9VTDwKkHZRHjYTmQzvJ+30mK-uyc7MMw6Tf9s9Fsw@mail.gmail.com>
Date: Tue, 25 Mar 2025 10:07:12 -0700
Cc: pgsql-general@postgresql.org
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Message-Id: <8760F666-DF81-4BEA-9CA9-80BBC569210E@thebuild.com>
References: <CAGAwPgR=B9VTDwKkHZRHjYTmQzvJ+30mK-uyc7MMw6Tf9s9Fsw@mail.gmail.com>
To: Phillip Diffley <phillip6402@gmail.com>
Archived-At: <https://www.postgresql.org/message-id/8760F666-DF81-4BEA-9CA9-80BBC569210E%40thebuild.com>
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> On Mar 25, 2025, at 09:56, Phillip Diffley <phillip6402@gmail.com> =
wrote:
> 1. Every DML operation (insert, update, delete, truncate) will have a =
row in the WAL and that row will have an LSN assigned to it.=20
> 2. The LSNs are assigned when the operation happens.
> 3. Operations within a transaction are written to the WAL in a =
contiguous block when the transaction commits.
> 4. Each transaction block in the WAL has a commit timestamp associated =
with it, and these timestamps are monotonically increasing (I'm only =
inferring this, so please correct me if this is not always the case).

This isn't *quite* right.  LSNs are byte offsets, not operation sequence =
numbers (despite the name), so they don't have to be "assigned"; they're =
just an offset to a particular point in the (virtual) WAL bytestream.

The WAL is written as it is generated (allowing for buffering etc.).  A =
transaction end flushes out any remaining WAL that is involved in that =
transaction (and any other WAL that might be between operations for that =
transaction), unless you are not in synchronous_commit mode.

It's not quite the "transaction block" that has the commit timestamp; =
it's the commit record for a particular transaction.  I can't come up =
with a scenario off the top of my head in which the commit timestamp =
would go down, but I wouldn't absolutely bet on it never doing so.=