I’ve email previously on this, but the eclasses have been through a bit
of a change since then.

postgres.eclass has some common functions and initializes some variables
that are used in many of the PostgreSQL-related packages.

postgres-multi.eclass makes it possible to install one package (e.g.,
dev-db/postgis) to all dev-db/postgresql slots. This will eliminate one
pain point of migrating PostgreSQL (e.g., from 9.6 to 10).

A lot of the packages in my overlay[1] have been modified to use this
eclass, notably dev-db/postgis and dev-db/pgpool2.

Naturally, I’ll want to add a USE_EXPAND for POSTGRES_TARGETS as the
eclasses use postgres_targets* use flags to control which slots to build
for.

1: https://github.com/titanofold/titanofold-gentoo-x86/tree/pgsql-eclass