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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>22.1. 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 V1.79.1" /><link rel="prev" href="managing-databases.html" title="Chapter 22. Managing Databases" /><link rel="next" href="manage-ag-createdb.html" title="22.2. Creating a Database" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">22.1. Overview</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="managing-databases.html" title="Chapter 22. Managing Databases">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="managing-databases.html" title="Chapter 22. Managing Databases">Up</a></td><th width="60%" align="center">Chapter 22. Managing Databases</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 12.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="manage-ag-createdb.html" title="22.2. Creating a Database">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="MANAGE-AG-OVERVIEW"><div class="titlepage"><div><div><h2 class="title" style="clear: both">22.1. Overview</h2></div></div></div><a id="id-1.6.9.4.2" 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 in
- its connection request the name of the database it wants to connect
- to. It is not possible to access more than one database per
- connection. However, an application is not restricted in the number of
- connections it opens to the same or other databases. Databases are
- physically separated and access control is managed at the
- connection level. If one <span class="productname">PostgreSQL</span> server
- instance is to house projects or users that should be separate and
- for the most part unaware of each other, it is therefore
- recommended to put them into separate databases. If the projects
- or users are interrelated and should be able to use each other's
- resources, they should be put in the same database but possibly
- into separate schemas. Schemas are a purely logical structure and who can
- access what is managed by the privilege system. More information about
- managing schemas is in <a class="xref" href="ddl-schemas.html" title="5.9. Schemas">Section 5.9</a>.
- </p><p>
- Databases are created with the <code class="command">CREATE DATABASE</code> command
- (see <a class="xref" href="manage-ag-createdb.html" title="22.2. Creating a Database">Section 22.2</a>) and destroyed with the
- <code class="command">DROP DATABASE</code> command
- (see <a class="xref" href="manage-ag-dropdb.html" title="22.5. Destroying a Database">Section 22.5</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">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="managing-databases.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="manage-ag-createdb.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter 22. Managing Databases </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> 22.2. Creating a Database</td></tr></table></div></body></html>
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