DATABASE ERROR: CONNECTION FAILED! (pgsql)

It may be a problem of documentation.
I have installed with aptitude roundcube and roundcube-pgsql, I connect to www.my.server/roundcube
And I get this error : DATABASE ERROR: CONNECTION FAILED.
I have tried to create manually the DB using /usr/share/dbconfig-common/data/roundcube/install/pgsql
I get the same message.
Incidently, in the README of roundcube-pgsql there is a reference to an INSTALL which does not exists in the same directory.
Maybe some .php file sohld be modified, but it is not clear which ones.

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: roundcube-pgsql 0.7.1-2
ProcVersionSignature: Ubuntu 3.2.0-41.66-generic 3.2.42
Uname: Linux 3.2.0-41-generic x86_64
ApportVersion: 2.0.1-0ubuntu17.2
Architecture: amd64
Date: Tue May 14 10:47:59 2013
InstallationMedia: Ubuntu-Server 12.04 LTS "Precise Pangolin" - Release amd64 (20120424.1)
MarkForUpload: True
PackageArchitecture: all
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 LANG=fr_FR.UTF-8
 SHELL=/bin/bash
SourcePackage: roundcube
UpgradeStatus: No upgrade log present (probably fresh install)