Subject: Logical replication with temporary tables

From: Stuart Campbell <[email-masked]>
Date: Wed, 3 Jul 2024 10:47:32 +1000

Hello,

I'm trying to use a blue/green deployment in Amazon RDS, which uses
Postgres logical replication to keep blue and green environments in sync.

Some parts of our app use temporary tables, and in some cases we CREATE
INDEX on those tables and sometimes DROP and recreate them.

The CREATE INDEX and DROP TABLE statements are causing problems with the
blue/green replication. (They are detected by RDS, and cause the deployment
to be marked as "degraded" and subsequently unusable.)

My (limited) understanding is that this happens because CREATE TEMPORARY
TABLE doesn't get written to the WAL, but the other statements do.

My question is whether there is some workaround that will let me create
indexes on a temporary table, and also let me drop the temporary table, in
a way that doesn't end up "degrading" replication? (Presumably that means
avoiding writing to the WAL?)

I've discovered that I can at least create a primary key inline as part of
the temporary table definition, which gives me a way to create a single
(unique) index, i.e.:

    CREATE TEMPORARY TABLE foo (id INT PRIMARY KEY, …)

But I haven't discovered a way to drop the table, or to create other
indexes.

Cheers,
Stuart

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From: Christophe Pettus <[email-masked]>
Date: Tue, 2 Jul 2024 17:49:57 -0700

> On Jul 2, 2024, at 17:47, Stuart Campbell <[email-masked]> wrote:
> My question is whether there is some workaround that will let me create indexes on a temporary table, and also let me drop the temporary table, in a way that doesn't end up "degrading" replication? (Presumably that means avoiding writing to the WAL?)

This is a question for AWS.  Community PostgreSQL doesn't have any of these concepts, and this is all proprietary modifications to PostgreSQL by Amazon.

---

From: Stuart Campbell <[email-masked]>
Date: Wed, 3 Jul 2024 11:16:42 +1000

>
> This is a question for AWS.  Community PostgreSQL doesn't have any of
> these concepts, and this is all proprietary modifications to PostgreSQL by
> Amazon.

My understanding was that under the hood, AWS uses the logical replication
features that are present in community Postgres. If that's incorrect then
I'm sorry for the off-topic post.

Maybe my question can be re-summarised as: do DDL operations on temporary
tables necessarily have to be written to the WAL? Is there a way to avoid
that?

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From: "David G. Johnston" <[email-masked]>
Date: Tue, 2 Jul 2024 18:19:51 -0700

> tables necessarily have to be written to the WAL? Is there a way to avoid
Yes, and no.  The tables those commands update are WAL-logged and that
status is all-or-nothing.

David J.