An error occurs when Capistrano deploys the Rails app to EC2.PG::ConnectionBad:FATAL:Peer authentication failed for user "postgres"

AW I am a beginner at AWS.Until now, I have only used Heroku.

AWS Configuration
1 EC2 instance
PostgreSQL on RDS, one

I'd like to deploy the Rails app to EC2 via Capistrano.
The following Capistrano command (deployment) results in an error:
vagrant@vagrant-ubuntu-trusty:~/sample_app$bundle exec cap production deploy

error statement
PG::ConnectionBad:FATAL:Peer authentication failed for user "postgres"
Tasks: TOP=>db:migrate
(See full trace by running task with --trace)

After SSH login to EC2,
sudo service postgresql start
sudo su-postgres
psql
postgres=#
postgres=#\du
List of roles
Role name | Attributes | Member of
-------------------------------------------------------------------------------------------------------- ec2-user | Create role, Create DB | {}
postgres | Superuser, Create role, Create DB, Replication | {}
root | Create role, Create DB | {}

postgres=#who ami
postgres-#\q
-bash-4.2$
can be run.

EC2 database
postgres=#\l
List of databases
Name | Owner | Encoding | Collate | Ctype | Access privileges
-------------------------------------------------------------------------------------------------------------------------
postgres | postgres | UTF8 | en_US.UTF-8 | en_US.UTF-8 |
sample_app_development | root | UTF8 | en_US.UTF-8 | en_US.UTF-8 |
sample_app_test | root | UTF8 | en_US.UTF-8 | en_US.UTF-8 |
template0|postgres|UTF8|en_US.UTF-8|en_US.UTF-8|=c/postgres+
| | | postgres=CTc/postgres
template1 | postgres | UTF8 | en_US.UTF-8 | en_US.UTF-8 | = c/postgres +
| | | postgres=CTc/postgres

Local (vagrant) database
vagrant@vagrant-ubuntu-trusty:~/hoge_app/config$railsdb
psql(9.5.3)
Type "help" for help.

hoge_app_development=#\l
List of databases
Name | Owner | Encoding | Collate | Ctype | Access privileges
-------------------------------------------------------------------------------------------------------------------------
fuga_development | vagrant | UTF8 | en_US.UTF-8 | en_US.UTF-8 |
fuga_test | vagrant | UTF8 | en_US.UTF-8 | en_US.UTF-8 |
postgres | postgres | UTF8 | en_US.UTF-8 | en_US.UTF-8 |
hoge_app_development | vagrant | UTF8 | en_US.UTF-8 | en_US.UTF-8 |
hoge_app_test | vagrant | UTF8 | en_US.UTF-8 | en_US.UTF-8 |
template0|postgres|UTF8|en_US.UTF-8|en_US.UTF-8|=c/postgres+
| | | postgres=CTc/postgres
template1 | postgres | UTF8 | en_US.UTF-8 | en_US.UTF-8 | = c/postgres +
| | | postgres=CTc/postgres
(7 rows)

What would you like to know
1. I understand that creating PostgreSQL with RDS creates a database, and after logging in to EC2, I can run the "sudo service postgressql start" command. Do I need to install PostgreSQL on EC2 by logging in to EC2 such as "sudo yum install-y postgreresql"?
2. Is the config/database.yml setting appropriate?

database.yml
production:
<<:*default
adapter:postgresql
host:32.873.61.427
encoding:utf8
database —Instance identifier when creating RDS
username —ec2-user
password —The password you set when you created the RDS.
pool:5
timeout —5000

Thank you for your cooperation.