Re: pg_dump versus ancient server versions

On Fri, Oct 22, 2021 at 6:42 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> So my first thought was just to revert 92316a458 and give up on it as
> a bad idea. However ... does anyone actually still care about being
> able to dump from such ancient servers?

I think I recently heard about an 8.4 server still out there in the
wild, but AFAICR it's been a long time since I've heard about anything
older.

It seems to me that if you're upgrading by a dozen server versions in
one shot, it's not a totally crazy idea that you might want to do it
in steps, or use the pg_dump for the version you have and then hack
the dump. I kind of wonder if there's really any hope of a pain-free
upgrade across that many versions anyway. There are things that can
bite you despite all the work we've put into pg_dump, like having
objects that depend on system objects whose definition has changed
over the years, plus implicit casting differences, operator precedence
changes, => getting deprecated, lots of GUC changes, etc. You are
going to be able to upgrade in the end, but it's probably going to
take some work. So I'm not really sure that giving up pg_dump
compatibility for versions that old is losing as much as it may seem.

Another thing to think about in that regard: how likely is that
PostgreSQL 7.4 and PostgreSQL 15 both compile and run on the same
operating system? I suspect the answer is "not very." I seem to recall
Greg Stark trying to compile really old versions of PostgreSQL for a
conference talk some years ago, and he got back to a point where it
just became impossible to make work on modern toolchains even with a
decent amount of hackery. One tends to think of C as about as static a
thing as can be, but that's kind of missing the point. On my laptop
for example, my usual configure invocation fails on 7.4 with:

checking for SSL_library_init in -lssl... no
configure: error: library 'ssl' is required for OpenSSL

In fact, I get that same failure on every branch older than 9.2. I
expect I could work around that by disabling SSL or finding an older
version of OpenSSL that works the way those branches expect, but that
might not be the only problem, either. Now I understand you could
have PostgreSQL 15 on a new box and PostgreSQL 7.x on an ancient one
and connect via the network, and it would in all fairness be cool if
that Just Worked. But I suspect that even if that did happen in the
lab, reality wouldn't often be so kind.

--
Robert Haas
EDB: http://www.enterprisedb.com