181+deb9u1) is not available, but a newer one is (181+deb9u2). We redirected you there.
/etc/postgresql-common/user_clusters is in postgresql-client-common 181+deb9u2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 | # This file maps users against the database clusters to which they
# will connect by default. Any user may create ~/.postgresqlrc which
# will supersede the defaults stored here. If a database is
# specified, that will be the one connected to by client tools if none
# is specified on the command line. If the database specified here is
# "*", this is interpreted as the database whose name is the same as
# the user's login. (Setting the database to "*" will provide the
# current default upstream behaviour for command line tools.)
#
# When pg_wrapper scans this file, the first matching line is used.
# It is a good idea to provide a default explicitly, with a final line
# where both user and group are set to "*". If there is no default,
# the implicit default is to connect to the cluster listening on
# port 5432 and to the database matching the user's login name.
#
# In the context of this file, user and group refer to the Unix login
# or group, not to PostgreSQL users and groups.
#
# Please see user_clusters(5) and postgresqlrc(5) for more information.
#
# USER GROUP VERSION CLUSTER DATABASE
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