Re: Using CTID system column as a "temporary" primary key

How large can the string representation of a CTID (TID type) be?

This page mentions 6 bytes for t_ctid / ItemPointerData...
=> how can I deduce the max size of its string representation?

https://www.postgresql.org/docs/14/storage-page-layout.html

Seb
________________________________
From: Sebastien Flaesch <sebastien(dot)flaesch(at)4js(dot)com>
Sent: Tuesday, March 28, 2023 11:57 AM
To: pgsql-general <pgsql-general(at)lists(dot)postgresql(dot)org>
Subject: Re: Using CTID system column as a "temporary" primary key

EXTERNAL: Do not click links or open attachments if you do not recognize the sender.

I mean Oracle's ROWID of course, not ROWNUM.
ROWNUM is temporary in the context of the SELECT, so it cannot be used in subsequent SQL statements.
Seb
________________________________
From: Sebastien Flaesch <sebastien(dot)flaesch(at)4js(dot)com>
Sent: Tuesday, March 28, 2023 11:28 AM
To: pgsql-general <pgsql-general(at)lists(dot)postgresql(dot)org>
Subject: Using CTID system column as a "temporary" primary key

EXTERNAL: Do not click links or open attachments if you do not recognize the sender.

Hello!

We are looking for an equivalent of Informix ROWID or Oracle's ROWNUM.

Is the CTID a good choice?

I assume it must be used in a specific context, and of course not considered as permanent primary key.

I understand that if the row is updated, the CTID may change.

Where can we find details about the validity and lifetime of the value such column?

Will CTID be supported long term or is there any plan to remove it or hide it some day?

Of course, one should use a real primary key definition. However, we have legacy code to adapt to PostgreSQL, and in some cases, tables have a composite primary key. A first SELECT uses that primary key, but it also fetches the ROWID, and will use that one in a subsequent SELECT, UPDATE or DELETE, instead of carrying the composite pkey values.

Seb