Почему пакет `postgresql` отсутствует в списке при выполнении поиска apt-cache postgresql`?

Мне просто нужно было установить сервер postgresql на мою машину, поэтому я сделал:

apt-cache search postgresql

, и в нем перечислены пакеты, связанные с postgresql:

[...]
postgis - Geographic objects support for PostgreSQL -- common files
postgresql-8.4 - object-relational SQL database, version 8.4 server
postgresql-9.1-debversion - Debian version number type for PostgreSQL
postgresql-9.1-ip4r - IPv4 and IPv4 range index types for PostgreSQL 9.1
postgresql-9.1-orafce - Oracle support functions for PostgreSQL 9.1
[...]
postgresql-server-dev-8.4 - development files for PostgreSQL 8.4 server-side programming
postgresql-server-dev-all - extension build tool for multiple PostgreSQL versions
proftpd-mod-pgsql - Versatile, virtual-hosting FTP daemon - PostgreSQL module
ptop - PostgreSQL performance monitoring tool akin to top
[...]

$ apt-cache show postgresql
Package: postgresql
Priority: optional
Section: database
Installed-Size: 63
Maintainer: Martin Pitt <martin.pitt@ubuntu.com>
Original-Maintainer: Martin Pitt <mpitt@debian.org>
Architecture: all
Source: postgresql-common (129ubuntu1)
Version: 9.1+129ubuntu1
Depends: postgresql-9.1
Filename: pool/main/p/postgresql-common/postgresql_9.1+129ubuntu1_all.deb
Size: 5552
MD5sum: 0bee83a4dab3e1d236fb4de1abcbf5c2
SHA1: f58d51b7f855c10959078b6ed8c3ea22dd3293ac
SHA256: ec0a12dabb52fee21f8576d5bd0990df6827e551dcd9b24f950d3ca4564d2761
Description-en: object-relational SQL database (supported version)
 This metapackage always depends on the currently supported PostgreSQL
 database server version.
 .
 PostgreSQL is a fully featured object-relational database management
 system.  It supports a large part of the SQL standard and is designed
 to be extensible by users in many aspects.  Some of the features are:
 ACID transactions, foreign keys, views, sequences, subqueries,
 triggers, user-defined types and functions, outer joins, multiversion
 concurrency control.  Graphical user interfaces and bindings for many
 programming languages are available as well.
Description-md5: bdff2d6e5b2a1dd00e72b3ed8729d9ac
Bugs: https://bugs.launchpad.net/ubuntu/+filebug
Origin: Ubuntu
Supported: 5y
Task: postgresql-server

Package: postgresql
Priority: optional
Section: database
Installed-Size: 63
Maintainer: Martin Pitt <martin.pitt@ubuntu.com>
Original-Maintainer: Martin Pitt <mpitt@debian.org>
Architecture: all
Source: postgresql-common (129)
Version: 9.1+129
Depends: postgresql-9.1
Filename: pool/main/p/postgresql-common/postgresql_9.1+129_all.deb
Size: 5508
MD5sum: 1ea1455e036c0a058145f19c6e92ef3f
SHA1: 8e73a616a686ec1130dc01858ee07e7ce8eb1caa
SHA256: a21adb533516e79145d93d57279db69260e644d259c39bb06f7e17d26721ac4b
Description-en: object-relational SQL database (supported version)
 This metapackage always depends on the currently supported PostgreSQL
 database server version.
 .
 PostgreSQL is a fully featured object-relational database management
 system.  It supports a large part of the SQL standard and is designed
 to be extensible by users in many aspects.  Some of the features are:
 ACID transactions, foreign keys, views, sequences, subqueries,
 triggers, user-defined types and functions, outer joins, multiversion
 concurrency control.  Graphical user interfaces and bindings for many
 programming languages are available as well.
Description-md5: bdff2d6e5b2a1dd00e72b3ed8729d9ac
Bugs: https://bugs.launchpad.net/ubuntu/+filebug
Origin: Ubuntu
Supported: 5y
Task: postgresql-server

Мне удалось найти его с помощью Google, как установить postgresql на Ubuntu (apt-get install postgresql), но мне все еще интересно, почему он не был указан в apt-cache search.