database user "postgres" is not the install user

Question

I'm trying to upgrade postgres from 9.5 to 9.6. brew upgrade postgresql succeeds, but when running

pg_upgrade -b /usr/local/Cellar/postgresql/9.5.3/bin/ -B /usr/local/Cellar/postgresql/9.6.1/bin/ -d /usr/local/var/postgres -D /usr/local/var/postgres9.6 -U postgres

I get an error

Performing Consistency Checks
-----------------------------
Checking cluster versions                                   ok
Checking database user is the install user
database user "postgres" is not the install user
Failure, exiting

when trying without -U postgres at the end it gets even weirder

Performing Consistency Checks
-----------------------------
Checking cluster versions                                   ok
Checking database user is the install user                  ok
Checking database connection settings                       ok
Checking for prepared transactions                          ok
Checking for reg* system OID user data types                ok
Checking for contrib/isn with bigint-passing mismatch       ok
Checking for roles starting with 'pg_'                      ok
Creating dump of global objects                             ok
Creating dump of database schemas
                                                            ok
Checking for presence of required libraries                 ok
Checking database user is the install user
database user "dimid" is not the install user

So how come

Checking database user is the install user                  ok

Answer 1

The old PostgreSQL cluster was obviously created with

initdb -U dimid

but the new cluster was istalled with a different superuser.

You have to create the new cluster with the same superuser name as the old one.

Answer 2

Specifically for homebrew postgresql installations, the default user for initdb is $USER -- so if you just followed the instructions initially and did something like

initdb /usr/local/var/postgres -E utf8

the install user is your Unix username. In my case it is 'rob', so just adding '-U rob' to the pg_upgrade works as well:

pg_upgrade -b /usr/local/Cellar/postgresql/9.5.4_1/bin -B /usr/local/Cellar/postgresql/9.6.2/bin -d /usr/local/var/postgres95 -D /usr/local/var/postgres -U rob