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+<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>23.1. Overview</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="managing-databases.html" title="Chapter 23. Managing Databases" /><link rel="next" href="manage-ag-createdb.html" title="23.2. Creating a Database" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">23.1. Overview</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="managing-databases.html" title="Chapter 23. Managing Databases">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="managing-databases.html" title="Chapter 23. Managing Databases">Up</a></td><th width="60%" align="center">Chapter 23. Managing Databases</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 15.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="manage-ag-createdb.html" title="23.2. Creating a Database">Next</a></td></tr></table><hr /></div><div class="sect1" id="MANAGE-AG-OVERVIEW"><div class="titlepage"><div><div><h2 class="title" style="clear: both">23.1. Overview</h2></div></div></div><a id="id-1.6.10.4.2" class="indexterm"></a><p>
+   A small number of objects, like role, database, and tablespace
+   names, are defined at the cluster level and stored in the
+   <code class="literal">pg_global</code> tablespace.  Inside the cluster are
+   multiple databases, which are isolated from each other but can access
+   cluster-level objects.  Inside each database are multiple schemas,
+   which contain objects like tables and functions.  So the full hierarchy
+   is: cluster, database, schema, table (or some other kind of object,
+   such as a function).
+  </p><p>
+   When connecting to the database server, a client must specify the
+   database name in its connection request.
+   It is not possible to access more than one database per
+   connection. However, clients can open multiple connections to
+   the same database, or different databases.
+   Database-level security has two components: access control
+   (see <a class="xref" href="auth-pg-hba-conf.html" title="21.1. The pg_hba.conf File">Section 21.1</a>), managed at the
+   connection level, and authorization control
+   (see <a class="xref" href="ddl-priv.html" title="5.7. Privileges">Section 5.7</a>), managed via the grant system.
+   Foreign data wrappers (see <a class="xref" href="postgres-fdw.html" title="F.38. postgres_fdw">postgres_fdw</a>)
+   allow for objects within one database to act as proxies for objects in
+   other database or clusters.
+   The older dblink module (see <a class="xref" href="dblink.html" title="F.12. dblink">dblink</a>) provides a similar capability.
+   By default, all users can connect to all databases using all connection methods.
+  </p><p>
+   If one <span class="productname">PostgreSQL</span> server cluster is planned to contain
+   unrelated projects or users that should be, for the most part, unaware
+   of each other, it is recommended to put them into separate databases and
+   adjust authorizations and access controls accordingly.
+   If the projects or users are interrelated, and thus should be able to use
+   each other's resources, they should be put in the same database but probably
+   into separate schemas;  this provides a modular structure with namespace
+   isolation and authorization control.
+   More information about managing schemas is in <a class="xref" href="ddl-schemas.html" title="5.9. Schemas">Section 5.9</a>.
+  </p><p>
+   While multiple databases can be created within a single cluster, it is advised
+   to consider carefully whether the benefits outweigh the risks and limitations.
+   In particular, the impact that having a shared WAL (see <a class="xref" href="wal.html" title="Chapter 30. Reliability and the Write-Ahead Log">Chapter 30</a>)
+   has on backup and recovery options. While individual databases in the cluster
+   are isolated when considered from the user's perspective, they are closely bound
+   from the database administrator's point-of-view.
+  </p><p>
+   Databases are created with the <code class="command">CREATE DATABASE</code> command
+   (see <a class="xref" href="manage-ag-createdb.html" title="23.2. Creating a Database">Section 23.2</a>) and destroyed with the
+   <code class="command">DROP DATABASE</code> command
+   (see <a class="xref" href="manage-ag-dropdb.html" title="23.5. Destroying a Database">Section 23.5</a>).
+   To determine the set of existing databases, examine the
+   <code class="structname">pg_database</code> system catalog, for example
+</p><pre class="synopsis">
+SELECT datname FROM pg_database;
+</pre><p>
+   The <a class="xref" href="app-psql.html" title="psql"><span class="refentrytitle"><span class="application">psql</span></span></a> program's <code class="literal">\l</code> meta-command
+   and <code class="option">-l</code> command-line option are also useful for listing the
+   existing databases.
+  </p><div class="note"><h3 class="title">Note</h3><p>
+    The <acronym class="acronym">SQL</acronym> standard calls databases <span class="quote">“<span class="quote">catalogs</span>”</span>, but there
+    is no difference in practice.
+   </p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="managing-databases.html" title="Chapter 23. Managing Databases">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="managing-databases.html" title="Chapter 23. Managing Databases">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="manage-ag-createdb.html" title="23.2. Creating a Database">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter 23. Managing Databases </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 15.5 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 23.2. Creating a Database</td></tr></table></div></body></html>
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