$ mix deps.get --target postgres
Umbrella Targets: :host
Elixir Targets: :host
Phoenix Targets: :host
Resolving Hex dependencies...

Failed to use "phoenix_ecto" (version 4.0.0) because
  apps/phx_targets_test/mix.exs requires ~> 3.0
  apps/phx_targets_test/mix.exs requires ~> 4.0

$ cd apps/
$ cd phx_targets_test/
$ mix deps.get --target postgres
Phoenix Targets: :host
warning: the dependency :phoenix_ecto is duplicated at the top level, please remove one of them
Resolving Hex dependencies...
Dependency resolution completed:
New:
  ...
  phoenix_ecto 4.0.0
  ...
  postgrex 0.14.1
  ...

$ mix deps.tree --target postgres
Phoenix Targets: :host
warning: the dependency :phoenix_ecto is duplicated at the top level, please remove one of them
warning: the dependency :phoenix_ecto is duplicated at the top level, please remove one of them
phx_targets_test
...
├── postgrex >= 0.0.0 (Hex package)
│   ├── connection ~> 1.0 (Hex package)
│   ├── db_connection ~> 2.0 (Hex package)
│   │   └── connection ~> 1.0.2 (Hex package)
│   ├── decimal ~> 1.5 (Hex package)
│   └── jason ~> 1.0 (Hex package)
...
└── phoenix_ecto ~> 4.0 (Hex package)
    ├── ecto ~> 3.0 (Hex package)
    ├── phoenix_html ~> 2.9 (Hex package)
    └── plug ~> 1.0 (Hex package)

$ MIX_TARGET=postgres mix deps.get
Phoenix Targets: :postgres
warning: the dependency :phoenix_ecto is duplicated at the top level, please remove one of them
Resolving Hex dependencies...

Failed to use "ecto" (version 3.0.7) because
  absinthe_ecto (version 0.1.3) requires >= 0.0.0
  ecto_sql (version 3.0.5) requires ~> 3.0.6
  mssql_ecto (version 1.2.0) requires ~> 2.2.0
  phoenix_ecto (version 4.0.0) requires ~> 3.0
  mix.lock specifies 3.0.7

** (Mix) Hex dependency resolution failed, change the version requirements of your dependencies or unlock them (by using mix deps.update or mix deps.unlock). If you are unable to resolve the conflicts you can try overriding with {:dependency, "~> 1.0", override: true}