POSTGRES was developed on SunOS (and its close cousin 4.3BSD or 4.2?). It's not so hard to support Solaris, as it was so influential that Linux uses a lot of similar things, including for example the ELF format and linker details, the sort of thing that developers have to maintain. So the burden is lower than (say) AIX. There is also someone from the Solaris community who feeds and waters a modern Solaris build farm animal (buildfarm.postgresql.org). Same goes for illumos (= forked from OpenSolaris, like Solaris 10), people run build farm animals and help test occasionally as required etc, which makes a big difference. That's my take, anyway (speaking as someone who works on PostgreSQL portability).
I'm not that shocked to see Solaris, but I am surprised to see Solaris and not AIX, which is also actively developed and I thought IBM specifically made an effort to keep assorted FOSS working on it.
Solaris runs on commodity x86-64 hardware, Oracle offers installers for download and a free license for development use, and there is an open source fork (Illumos) which is 99% compatible. Given all of this, it isn’t hard for open source developers to setup CI/dev/test environments to support Solaris x86-64 as part of their project. SPARC is harder (hardware availability), but if it builds on Solaris SPARC and tests fine on Solaris x86-64, it is probably going to run on Solaris SPARC too. There are some potential gotchas around memory model differences and endianness, but one can just punt those issues until a user reports them, and then say “we can’t fix it unless you let us use your hardware”
AIX only runs on non-mainstream hardware (POWER), plus IBM isn’t as open as Oracle is with providing access to it. This makes it significantly harder to support than Solaris is
Added to that, Solaris and Linux are closer to begin with. Historically, Linux copied a number of its APIs from Solaris; AIX was far less of an influence. Years ago, I had to port some psuedoterminal management code from Linux to both Solaris and AIX. I only had to make some minor changes to the Linux code to make it work on Solaris; the changes I had to make to get it to work on AIX were much more extensive
I've been able to debug PostgreSQL issues reported by [closed] Solaris users by booting illumos inside a virtual machine on my laptop in minutes...
I want to keep AIX support, for, well mainly irrational nostalgic reasons -- PostgreSQL used to run on pretty much the entire Unix family tree, and I wrote lots of code on AIX for a decade. But they make it hard. I don't know why an OS vendor wouldn't make an image of an OS available to developers as conveniently as possible (it is even possible to boot recent AIX versions on QEMU if you have the patience... the hypothesis is that they might have done work to make that possible, 'cause it didn't work in earlier versions; but you can't get an image of OS, so shrug).
The easiest legal way to get an AIX box is via IBM Cloud. You can get a tiny AIX box (s922 in Dallas, 0.25 cores, 2GB RAM, 10GB disk) for about US$50 a month. About US$8/month of that is AIX licensing and the rest is the hardware cost. Still a lot cheaper than IBM i licensing, which is US$350/month even for such a low-end system, or IBM Z (which is around US$1600/month for a low-end system).
IBM Cloud does offer some free credits – at the moment US$1000 free VPC credits – but I'm not sure if you can use them on AIX (which isn't part of their main VPC offering, POWER is a separate offering), and even if you can, they aren't going to last forever.
If one is an open source developer doing this on one's own time, as opposed to a business – why spend US$50/month to get an AIX environment? Maybe, if a user of the project really wants AIX support, they could donate that to the project.
We (the denizens of pgsql-hackers@postgresql.org) didn't see any AIX users or developers or other interested parties in our community... There was a group in France, but they disbanded... However, after AIX was dropped in the PostgreSQL 17 development cycle, suddenly we heard from IBM who run products on it. So perhaps it will make a comeback in 18? That was the latest idea, anyway, let's see.
Re: AIX. Why won't IBM pay for it? For example, from the case I know, IBM is paying for AIX support for Rust. I would think PostgreSQL deserves the same.
After it was dropped from PostgreSQL, a team from IBM showed up on the mailing list (see one of my other answers for link), so perhaps that is now going to happen! I think it's a case that they were using it, but didn't realise that the project was on the verge of dropping it for years due to lack of interested maintainer & resources. Open source is funny like that. Deleting it was certainly one way to get their attention.
