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From: Achilleas Mantzios <[email protected]>
To: Moreno Andreo <[email protected]>
To: PostgreSQL mailing lists <[email protected]>
Subject: Re: Logical replication, need to reclaim big disk space
Date: Fri, 16 May 2025 22:33:29 +0300
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>

On 16/5/25 18:45, Moreno Andreo wrote:

> Hi,
>     we are moving our old binary data approach, moving them from bytea 
> fields in a table to external storage (making database smaller and 
> related operations faster and smarter).
> In short, we have a job that runs in background and copies data from 
> the table to an external file and then sets the bytea field to NULL.
> (UPDATE tbl SET blob = NULL, ref = 'path/to/file' WHERE id = <uuid>)
>
> This results, at the end of the operations, to a table that's less 
> than one tenth in size.
> We have a multi-tenant architecture (100s of schemas with identical 
> architecture, all inheriting from public) and we are performing the 
> task on one table per schema.
>
So? toasted data are kept on separate TOAST tables, unless those bytea 
cols are selected, you won't even touch them. I cannot understand what 
you are trying to achieve here.

Years ago, when I made the mistake to go for a coffee and let my 
developers "improvise" , the result was a design similar to what you are 
trying to achieve. Years after, I am seriously considering moving those 
data back to PostgreSQL.


> The problem is: this is generating BIG table bloat, as you may imagine.
> Running a VACUUM FULL on an ex-22GB table on a standalone test server 
> is almost immediate.
> If I had only one server, I'll process a table a time, with a nightly 
> script, and issue a VACUUM FULL to tables that have already been 
> processed.
>
> But I'm in a logical replication architecture (we are using a 
> multimaster system called pgEdge, but I don't think it will make big 
> difference, since it's based on logical replication), and I'm building 
> a test cluster.
>
So you use PgEdge , but you wanna lose all the benefits of multi-master 
, since your binary data won't be replicated ...
> I've been instructed to issue VACUUM FULL on both nodes, nightly, but 
> before proceeding I read on docs that VACUUM FULL can disrupt logical 
> replication, so I'm a bit concerned on how to proceed. Rows are 
> cleared one a time (one transaction, one row, to keep errors to the 
> record that issued them)
>
PgEdge is based on the old pg_logical, the old 2ndQuadrant extension, 
not the native logical replication we have since pgsql 10. But I might 
be mistaken.
> I read about extensions like pg_squeeze, but I wonder if they are 
> still not dangerous for replication.
>
What's pgEdge take on that, I mean the bytea thing you are trying to 
achieve here.
> Thanks for your help.
> Moreno.-
>
>
>

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