Redmine

• pg 1.0.0
• postgresql 9.5.3

I confirmed that all the tests are green under the above conditions.

diff --git a/Gemfile b/Gemfile
index a3f99a1ec..94dc31301 100644
--- a/Gemfile
+++ b/Gemfile
@@ -62,7 +62,7 @@ if File.exist?(database_file)
       when 'mysql2'
         gem "mysql2", "~> 0.4.6", :platforms => [:mri, :mingw, :x64_mingw]
       when /postgresql/
-        gem "pg", "~> 0.18.1", :platforms => [:mri, :mingw, :x64_mingw]
+        gem "pg", "~> 1.0.0", :platforms => [:mri, :mingw, :x64_mingw]
       when /sqlite3/
         gem "sqlite3", "~>1.3.12", :platforms => [:mri, :mingw, :x64_mingw]
       when /sqlserver/

pg 1.0.0 supports PostgreSQL 9.2 and later. If we update pg gem to 1.0.0, Redmine cannot run on some operating systems such as CentOS 6 (PostgreSQL is 8.4).

It may be too early to update to pg 1.0.0.

pg 1.0.0 supports PostgreSQL 9.2 and later. If we update pg gem to 1.0.0, Redmine cannot run on some operating systems such as CentOS 6 (PostgreSQL is 8.4).

It may be too early to update to pg 1.0.0.

PostgreSQL 8.4 is not a supported version in the PG community, so to update pg 1.0.0 is NOT too early in terms of security.
https://www.postgresql.org/support/versioning/

Anyway, CentOS 6 and 7 does not support Ruby 2.2 or higher as standard, but Redmine 4.0 will require Ruby 2.2 or higher. It means in any case Redmine 4.0 does not work on CentOS with only standard repository. I think you should consider the OS version that supports Ruby 2.2 or higher as a criteria and present PostrgreSQL's workaround same as Ruby installation if they use an OS like CentOS.

"Maintenance Support 1 Phase" for RHEL 6 and "Full Updates phase" for CentOS 6 ended on May 10, 2017. RHEL / CentOS 6 users should prioritize to update the operating system rather than Redmine.

https://access.redhat.com/support/policy/updates/errata#Life_Cycle_Dates
https://wiki.centos.org/About/Product