As a Gentoo and Arch user, SLOTs are really overstated as a feature; a near-identical result can be produced by using multiple package names, possibly combined with a "provides" type feature. As a concrete example, Gentoo has fuse:2 and fuse:3 whereas Arch has fuse2 and fuse3. Searching for sys-fs/fuse:* in (what used to be called) gentoo-x86 yields only lxd, so the SLOT is providing little value here.
Categories are a similar issue in Portage: emerge graphviz asks if I want dev-python/graphviz or media-gfx/graphviz, but obviously the default should be the latter. Arch instead has python-graphviz and graphviz, which is much better design. Categories fix naming collisions in exactly the wrong way: dumping it on the user and giving no sensible default.
For PostgreSQL specifically, Arch provides postgresql and postgresql-old-upgrade, so as long as you don't wait years to upgrade your PostgreSQL, you'll be fine. It's a reduction in flexibility but brings with it a significant reduction in complexity: nobody can properly maintain half a dozen Postgres versions anyways, so just keep the latest two around.
I'll agree that slots are useful... for whatever reason on arm, uboot and ATF won't compile with the latest binutils and gcc. Slotting lets me switch back to the older stuff to build new firmware, while keeping the rest of the system current.
Categories are a similar issue in Portage: emerge graphviz asks if I want dev-python/graphviz or media-gfx/graphviz, but obviously the default should be the latter. Arch instead has python-graphviz and graphviz, which is much better design. Categories fix naming collisions in exactly the wrong way: dumping it on the user and giving no sensible default.
For PostgreSQL specifically, Arch provides postgresql and postgresql-old-upgrade, so as long as you don't wait years to upgrade your PostgreSQL, you'll be fine. It's a reduction in flexibility but brings with it a significant reduction in complexity: nobody can properly maintain half a dozen Postgres versions anyways, so just keep the latest two around.