psql: 致命錯誤: 用戶 "postgres" Ident 認證失敗
1安裝好postgresql數據庫後需要初始化及一些配置rails項目才能連接postgresql
安裝好postgresql數據後（yum命令直接安裝的）
第一步：初始化數據庫
#service postgresql initdb
（說明：初始化後默認postgresql數據庫有一個默認的用戶postgres（密碼爲空）和一個默認創建的postgres數據庫）
第二步：啓動數據庫
#service postgresql start
如果這個命令不能使用請使用：#systemctl start postgresql.service
來啓動數據庫，爲修改密碼條件
第三步：切換用戶修改密碼
#su postgres
#psql
#alter user postgres with password 'kuange' ;
(修改postgres用戶密碼爲kuange,這個密碼隨你自己修改)
# \q
第四步：修改認證文件/var/lib/pgsql/data/pg_hba.conf，登陸使用密碼。
#vi /var/lib/pgsql/data/pg_hba.conf
把這個配置文件中的認證 METHOD的ident修改爲trust，可以實現用賬戶和密碼來訪問數據庫，
即解決psql: 致命錯誤: 用戶 "postgres" Ident 認證失敗 這個問題）
第五步：重啓postgresql服務器使設置生效
#service postgresql restart
或者
#systemctl restart postgresql.service
這樣問題就解決了，數據庫可以正常被rails項目訪問了
pg_hba.conf這個文件的完整代碼如下：
#=========================================
# PostgreSQL Client Authentication Configuration File
# ===================================================
#
# Refer to the "Client Authentication" section in the PostgreSQL
# documentation for a complete description of this file. A short
# synopsis follows.
#
# This file controls: which hosts are allowed to connect, how clients
# are authenticated, which PostgreSQL user names they can use, which
# databases they can access. Records take one of these forms:
#
# local DATABASE USER METHOD [OPTIONS]
# host DATABASE USER ADDRESS METHOD [OPTIONS]
# hostssl DATABASE USER ADDRESS METHOD [OPTIONS]
# hostnossl DATABASE USER ADDRESS METHOD [OPTIONS]
#
# (The uppercase items must be replaced by actual values.)
#
# The first field is the connection type: "local" is a Unix-domain
# socket, "host" is either a plain or SSL-encrypted TCP/IP socket,
# "hostssl" is an SSL-encrypted TCP/IP socket, and "hostnossl" is a
# plain TCP/IP socket.
#
# DATABASE can be "all", "sameuser", "samerole", "replication", a
# database name, or a comma-separated list thereof. The "all"
# keyword does not match "replication". Access to replication
# must be enabled in a separate record (see example below).
#
# USER can be "all", a user name, a group name prefixed with "+", or a
# comma-separated list thereof. In both the DATABASE and USER fields
# you can also write a file name prefixed with "@" to include names
# from a separate file.
#
# ADDRESS specifies the set of hosts the record matches. It can be a
# host name, or it is made up of an IP address and a CIDR mask that is
# an integer (between 0 and 32 (IPv4) or 128 (IPv6) inclusive) that
# specifies the number of significant bits in the mask. A host name
# that starts with a dot (.) matches a suffix of the actual host name.
# Alternatively, you can write an IP address and netmask in separate
# columns to specify the set of hosts. Instead of a CIDR-address, you
# can write "samehost" to match any of the server's own IP addresses,
# or "samenet" to match any address in any subnet that the server is
# directly connected to.
#
# METHOD can be "trust", "reject", "md5", "password", "gss", "sspi",
# "krb5", "ident", "peer", "pam", "ldap", "radius" or "cert". Note that
# "password" sends passwords in clear text; "md5" is preferred since
# it sends encrypted passwords.
#
# OPTIONS are a set of options for the authentication in the format
# NAME=VALUE. The available options depend on the different
# authentication methods -- refer to the "Client Authentication"
# section in the documentation for a list of which options are
# available for which authentication methods.
#
# Database and user names containing spaces, commas, quotes and other
# special characters must be quoted. Quoting one of the keywords
# "all", "sameuser", "samerole" or "replication" makes the name lose
# its special character, and just match a database or username with
# that name.
#
# This file is read on server startup and when the postmaster receives
# a SIGHUP signal. If you edit the file on a running system, you have
# to SIGHUP the postmaster for the changes to take effect. You can
# use "pg_ctl reload" to do that.
# Put your actual configuration here
# ----------------------------------
#
# If you want to allow non-local connections, you need to add more
# "host" records. In that case you will also need to make PostgreSQL
# listen on a non-local interface via the listen_addresses
# configuration parameter, or via the -i or -h command line switches.
# TYPE DATABASE USER ADDRESS METHOD
# "local" is for Unix domain socket connections only
local all all trust
# IPv4 local connections:
host all all 127.0.0.1/32 trust
# IPv6 local connections:
host all all ::1/128 trust
# Allow replication connections from localhost, by a user with the
# replication privilege.
#local replication postgres peer
#host replication postgres 127.0.0.1/32 ident
#host replication postgres ::1/128 ident
#=========================================