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Green_GoodERP18_FSJ_Standard/PostgreSQL/doc/postgresql/html/manage-ag-templatedbs.html

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  1. <?xml version="1.0" encoding="UTF-8" standalone="no"?>

  2. <!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>22.3. Template Databases</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="manage-ag-createdb.html" title="22.2. Creating a Database" /><link rel="next" href="manage-ag-config.html" title="22.4. Database Configuration" /></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">22.3. Template Databases</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="manage-ag-createdb.html" title="22.2. Creating a Database">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-config.html" title="22.4. Database Configuration">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="MANAGE-AG-TEMPLATEDBS"><div class="titlepage"><div><div><h2 class="title" style="clear: both">22.3. Template Databases</h2></div></div></div><p>

  3.    <code class="command">CREATE DATABASE</code> actually works by copying an existing

  4.    database.  By default, it copies the standard system database named

  5.    <code class="literal">template1</code>.<a id="id-1.6.9.6.2.3" class="indexterm"></a> Thus that

  6.    database is the <span class="quote">“<span class="quote">template</span>”</span> from which new databases are

  7.    made.  If you add objects to <code class="literal">template1</code>, these objects

  8.    will be copied into subsequently created user databases.  This

  9.    behavior allows site-local modifications to the standard set of

  10.    objects in databases.  For example, if you install the procedural

  11.    language <span class="application">PL/Perl</span> in <code class="literal">template1</code>, it will

  12.    automatically be available in user databases without any extra

  13.    action being taken when those databases are created.

  14.   </p><p>

  15.    There is a second standard system database named

  16.    <code class="literal">template0</code>.<a id="id-1.6.9.6.3.2" class="indexterm"></a> This

  17.    database contains the same data as the initial contents of

  18.    <code class="literal">template1</code>, that is, only the standard objects

  19.    predefined by your version of

  20.    <span class="productname">PostgreSQL</span>.  <code class="literal">template0</code>

  21.    should never be changed after the database cluster has been

  22.    initialized.  By instructing

  23.    <code class="command">CREATE DATABASE</code> to copy <code class="literal">template0</code> instead

  24.    of <code class="literal">template1</code>, you can create a <span class="quote">“<span class="quote">virgin</span>”</span> user

  25.    database that contains none of the site-local additions in

  26.    <code class="literal">template1</code>.  This is particularly handy when restoring a

  27.    <code class="literal">pg_dump</code> dump: the dump script should be restored in a

  28.    virgin database to ensure that one recreates the correct contents

  29.    of the dumped database, without conflicting with objects that

  30.    might have been added to <code class="literal">template1</code> later on.

  31.   </p><p>

  32.    Another common reason for copying <code class="literal">template0</code> instead

  33.    of <code class="literal">template1</code> is that new encoding and locale settings

  34.    can be specified when copying <code class="literal">template0</code>, whereas a copy

  35.    of <code class="literal">template1</code> must use the same settings it does.

  36.    This is because <code class="literal">template1</code> might contain encoding-specific

  37.    or locale-specific data, while <code class="literal">template0</code> is known not to.

  38.   </p><p>

  39.    To create a database by copying <code class="literal">template0</code>, use:

  40. </p><pre class="programlisting">

  41. CREATE DATABASE <em class="replaceable"><code>dbname</code></em> TEMPLATE template0;

  42. </pre><p>

  43.    from the SQL environment, or:

  44. </p><pre class="programlisting">

  45. createdb -T template0 <em class="replaceable"><code>dbname</code></em>

  46. </pre><p>

  47.    from the shell.

  48.   </p><p>

  49.    It is possible to create additional template databases, and indeed

  50.    one can copy any database in a cluster by specifying its name

  51.    as the template for <code class="command">CREATE DATABASE</code>.  It is important to

  52.    understand, however, that this is not (yet) intended as

  53.    a general-purpose <span class="quote">“<span class="quote"><code class="command">COPY DATABASE</code></span>”</span> facility.

  54.    The principal limitation is that no other sessions can be connected to

  55.    the source database while it is being copied.  <code class="command">CREATE

  56.    DATABASE</code> will fail if any other connection exists when it starts;

  57.    during the copy operation, new connections to the source database

  58.    are prevented.

  59.   </p><p>

  60.    Two useful flags exist in <code class="literal">pg_database</code><a id="id-1.6.9.6.7.2" class="indexterm"></a> for each

  61.    database: the columns <code class="literal">datistemplate</code> and

  62.    <code class="literal">datallowconn</code>.  <code class="literal">datistemplate</code>

  63.    can be set to indicate that a database is intended as a template for

  64.    <code class="command">CREATE DATABASE</code>.  If this flag is set, the database can be

  65.    cloned by any user with <code class="literal">CREATEDB</code> privileges; if it is not set,

  66.    only superusers and the owner of the database can clone it.

  67.    If <code class="literal">datallowconn</code> is false, then no new connections

  68.    to that database will be allowed (but existing sessions are not terminated

  69.    simply by setting the flag false).  The <code class="literal">template0</code>

  70.    database is normally marked <code class="literal">datallowconn = false</code> to prevent its modification.

  71.    Both <code class="literal">template0</code> and <code class="literal">template1</code>

  72.    should always be marked with <code class="literal">datistemplate = true</code>.

  73.   </p><div class="note"><h3 class="title">Note</h3><p>

  74.     <code class="literal">template1</code> and <code class="literal">template0</code> do not have any special

  75.     status beyond the fact that the name <code class="literal">template1</code> is the default

  76.     source database name for <code class="command">CREATE DATABASE</code>.

  77.     For example, one could drop <code class="literal">template1</code> and recreate it from

  78.     <code class="literal">template0</code> without any ill effects.  This course of action

  79.     might be advisable if one has carelessly added a bunch of junk in

  80.     <code class="literal">template1</code>. (To delete <code class="literal">template1</code>,

  81.     it must have <code class="literal">pg_database.datistemplate = false</code>.)

  82.    </p><p>

  83.     The <code class="literal">postgres</code> database is also created when a database

  84.     cluster is initialized.  This database is meant as a default database for

  85.     users and applications to connect to. It is simply a copy of

  86.     <code class="literal">template1</code> and can be dropped and recreated if necessary.

  87.    </p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="manage-ag-createdb.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-config.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">22.2. Creating a Database </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> 22.4. Database Configuration</td></tr></table></div></body></html>
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