Re: Always bump PG_CONTROL_VERSION?

On 5/12/21 4:18 PM, Tom Lane wrote:
> David Steele <david(at)pgmasters(dot)net> writes:
>> On 5/12/21 3:21 PM, Alvaro Herrera wrote:
>>> Why don't you use the PG_VERSION file in the datadir?
>
>> Mostly because there is other data we need in pg_control and it is
>> simpler to read one file than two.
>
> I'm disinclined to change the longstanding rule in this area for
> a reason as weak as that.

It's a bit more than that -- for instance we have pg_control in every
backup but in order to get PG_VERSION we may need to reference a prior
backup since it never changes. pg_control is also checked on every
archive_command/restore_command so reading an extra file adds up when
performance is paramount. There are also many unit tests that need to
write this data, etc.

In short, it would be very nice to have one place to get info about a
cluster.

> Even if we did change the rule going forward, you'd still need to
> do it properly for existing releases, so I don't see that you're
> going to save anything.

It's not really a burden for existing releases. The issue is during the
alpha/beta phase when the CATALOG_VERSION_NO can change several times in
a few months.

Perhaps it was unwise to frame this in the requirements for an external
tool, but I still think eeca4cd3 and 99dd8b05a argue for it being a good
idea.

Or perhaps we could just add the version number to pg_control? At least
then pg_controldata could display the version.

Regards,
--
-David
david(at)pgmasters(dot)net