Subject: Re: Introduction of a new field in pg_class indicating presence of a large object in a table

From: "David G. Johnston" <[email-masked]>
Date: Tue, 30 Apr 2024 12:13:52 -0700

On Tue, Apr 30, 2024 at 11:57 AM Gaurav Pant <[email-masked]>
wrote:

> I wanted to know if there is any such system table that we can use to
> identify and map the fields containing large objects and the respective
> tables and if it is not already there, do we have any plans to incorporate
> the same in pg_class like we have for pg_toast?
>
>
https://www.postgresql.org/docs/current/catalog-pg-largeobject.html

Large Objects are nothing like TOAST.  There is no system level association
between large objects and tables.  Sure, the DBA can choose to store a
large object OID in a table, but how you'd go about figuring out which
columns contain those is going to be installation specific.  Though
hopefully they used a bigint data type and maybe added "oid" to the column
name...I suppose it would be interesting if one could define a FK on a
table and point it at pg_largeobject_metadata but that I suspect would be
the extent to which we'd do something along the lines of your request.

David J.

---

From: Tom Lane <[email-masked]>
Date: Tue, 30 Apr 2024 15:28:05 -0400

> columns contain those is going to be installation specific.

Yeah.  You might want to look at contrib/vacuumlo, but realize that
that's fairly heuristic.

> Though

That would solve the opposite problem, of preventing a column from
containing any OIDs that *weren't* large object OIDs.  Given that
recording a large object OID elsewhere in the database is purely
an application decision, I don't think there's a reasonable way
for the system to track it.

			regards, tom lane