Re: Downgrade pgsql 17 to pgsql 12 question

On Fri, 2025-09-26 at 17:48 +0530, Ashish Mukherjee wrote:
> I have a strange requirement to downgrade from pgsql 17 to pgsql 12. This is
> because we found in production certain incompatibilities between both versions
> for our database. It should have been caught in testing but was not.
>
> The clean way seems to be text file dump and restore but this would be too
> huge and too slow for our database of 3T. If I use pg_dump v17 and then
> restore with pg_restore v 17 on a pgsql v12 database, is there any risk?

Yes, there is the risk that the restore will fail.
Downgrading is not supported.

That's why the best way to do it is a plain format dump: you can edit the
dump file to manually fix any errors.
I am surprised that you think that restoring a plain format dump would
be significantly slower than restoring a different format (unless you are
talking about parallel restore with -j).

> I tried a small test with a bunch of tables and it worked, but am wondering
> about the pitfalls. I am restoring from the directory format dump.

Anything can happen...

> When I do dump/restore like this for a test table, I get the following errors
> during restore but the table gets restored fine.
>
> pg_restore: error: while PROCESSING TOC:
>  error: pg_restore:  error:   pg_restore:  from TOC entry 17168; 1259 58572315 TABLE pkgs s14
> pg_restore: error: pg_restore: pg_restore: pg_restore:   from TOC entry 17168; 1259 58572315 TABLE pkgs s14
> pg_restore:   error: pg_restore: from TOC entry 17168; 1259 58572315 TABLE pkgs s14
> pg_restore: error: pg_restore: from TOC entry 17168; 1259 58572315 TABLE pkgs s14
> error: from TOC entry 17168; 1259 58572315 TABLE pkgs s14
> pg_restore: warning: errors ignored on restore: 2
>
> pkgs is the table and s14 is my database

There should be more: the actual error messages. These will give you a clue.

Yours,
Laurenz Albe