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 <chapter id="manage-ag">
  <title>Managing a Database</title>

  <para>
   If the <productname>Postgres</productname>
   <application>postmaster</application> is up and running we can create
   some databases  to  experiment  with.  Here, we describe the
   basic commands for managing a database.
  </para>

  <sect1>
   <title>Creating a Database</title>

   <para>
    Let's say you want to create  a  database  named  mydb.
    You can do this with the following command:

    <programlisting>
% createdb <replaceable class="parameter">dbname</replaceable>
    </programlisting>

    <productname>Postgres</productname>  allows  you to create 
    any number of databases
    at a  given  site  and  you  automatically  become  the
    database  administrator  of  the database you just created.  
    Database names must  have  an  alphabetic  first
    character and are limited to 31 characters in length.
    Not  every  user has authorization to become a database
    administrator.  If <productname>Postgres</productname> 
    refuses to create databases
    for you, then the site administrator needs to grant you
    permission to  create  databases.   Consult  your  site
    administrator if this occurs.
   </para>
  </sect1>

  <sect1>
   <title>Accessing a Database</title>

   <para>
    Once you have constructed a database, you can access it
    by:

    <itemizedlist spacing="compact" mark="bullet">
     <listitem>
      <para>
       running the <productname>Postgres</productname>  terminal  monitor  program 
       (<application>psql</application>) which allows you to interactively
       enter, edit, and execute <acronym>SQL</acronym> commands.
      </para>
     </listitem>

     <listitem>
      <para>
       writing a  C  program  using  the  <literal>libpq</literal>  subroutine
       library.   This  allows  you  to submit <acronym>SQL</acronym> commands
       from C and get answers and status messages  back  to
       your  program.   This interface is discussed further
       in the <citetitle>PostgreSQL Programmer's Guide</citetitle>.
      </para>
     </listitem>
    </itemizedlist>

    You might want to start up <application>psql</application>, 
    to try out  the  examples  in  this manual. It can be activated for the
    <replaceable class="parameter">dbname</replaceable> database by typing the command:

<programlisting>
psql <replaceable class="parameter">dbname</replaceable>
</programlisting>

    You will be greeted with the following message:

<programlisting>
Welcome to psql, the PostgreSQL interactive terminal.

Type:  \copyright for distribution terms
       \h for help with SQL commands
       \? for help on internal slash commands
       \g or terminate with semicolon to execute query
       \q to quit

<replaceable>dbname</replaceable>=&gt;
</programlisting>
   </para>

   <para>
    This prompt indicates that the terminal monitor is listening  
    to you and that you can type <acronym>SQL</acronym> queries into a
    workspace maintained by the terminal monitor.
    The <application>psql</application> program responds to escape
    codes  that  begin
    with  the  backslash  character, "\".  For example, you
    can get help on the syntax of various 
    <productname>Postgres</productname> <acronym>SQL</acronym> commands by typing:

    <programlisting>
<replaceable>dbname</replaceable>=> \h
    </programlisting>

    Once  you  have finished entering your queries into the
    workspace, you can pass the contents of  the  workspace
    to the <productname>Postgres</productname> server by typing:

    <programlisting>
<replaceable>dbname</replaceable>=> \g
    </programlisting>

    This  tells  the  server  to process the query.  If you
    terminate your query with a semicolon, the  backslash-g is  not
    necessary.   <application>psql</application> will automatically 
    process semicolon terminated queries.
    To read queries from a file,  instead  of
    entering them interactively, type:

    <programlisting>
<replaceable>dbname</replaceable>=> \i <replaceable class="parameter">filename</replaceable>
    </programlisting>

    To get out of <application>psql</application> and return to Unix, type

    <programlisting>
<replaceable>dbname</replaceable>=&gt; \q
    </programlisting>

    and  <application>psql</application>  will  quit  and  return  
    you to your command shell. (For more escape codes, type
    backslash-h at  the  monitor prompt.)
    White  space  (i.e.,  spaces, tabs and newlines) may be
    used freely in <acronym>SQL</acronym> queries.  
    Single-line comments  are  denoted  by two dashes
    ("<literal>--</literal>").   Everything  after the dashes up to the end of the
    line is ignored. Multiple-line comments, and comments within a line,
    are denoted by "<literal>/* ... */</literal>", a convention borrowed
    from <productname>Ingres</productname>.
   </para>
  </sect1>
     
  <sect1>
   <title>Destroying a Database</title>

   <para>
    If you are the database administrator for the  database
    mydb,  you can destroy it using the following Unix command:

    <programlisting>
% dropdb <replaceable class="parameter">dbname</replaceable>
    </programlisting>

    This action physically removes all of  the  Unix  files
    associated  with  the database and cannot be undone, so
    this should only be done with a  great  deal  of  forethought.
   </para>

   <para>
    It is also possible to destroy a database from within an
    <acronym>SQL</acronym> session by using

    <programlisting>
&gt; drop database <replaceable class="parameter">dbname</replaceable>
    </programlisting>
   </para>
  </sect1>

  <sect1>
   <title>Backup and Restore</title>

   <caution>
    <para>
     Every database should be backed up on a regular basis. Since
     <productname>Postgres</productname> manages it's own files in the
     file system, it is <emphasis>not advisable</emphasis> to rely on
     system backups of your file system for your database backups;
     there is no guarantee that the files will be in a usable,
     consistant state after restoration.
    </para>
   </caution>

   <para>
    <productname>Postgres</productname> provides two utilities to
    backup your system: <application>pg_dump</application> to backup
    individual databases and
    <application>pg_dumpall</application> to backup your installation
    in one step.
   </para>

   <para>
    An individual database can be backed up using the following
    command:

    <programlisting>
% pg_dump <replaceable class="parameter">dbname</replaceable> &gt; <replaceable class="parameter">dbname</replaceable>.pgdump
    </programlisting>

    and can be restored using

    <programlisting>
cat <replaceable class="parameter">dbname</replaceable>.pgdump | psql <replaceable class="parameter">dbname</replaceable>
    </programlisting>
   </para>

   <para>
    This technique can be used to move databases to new
    locations, and to rename existing databases.
   </para>

   <sect2>
    <title>Large Databases</title>

    <note>
     <title>Author</title>
     <para>
      Written by <ulink url="hannu@trust.ee">Hannu Krosing</ulink> on
      1999-06-19.
     </para>
    </note>

    <para>
     Since <productname>Postgres</productname> allows tables larger
     than the maximum file size on your system, it can be problematic
     to dump the table to a file, since the resulting file will likely 
     be larger than the maximum size allowed by your system.</para>

    <para>
     As <application>pg_dump</application> writes to stdout,
     you can just use standard *nix tools 
     to work around this possible problem:

     <itemizedlist>
      <listitem>
       <para>
	Use compressed dumps:

	<programlisting>
% pg_dump <replaceable class="parameter">dbname</replaceable> | gzip > <replaceable class="parameter">filename</replaceable>.dump.gz
	</programlisting>

	reload with

	<programlisting>
% createdb <replaceable class="parameter">dbname</replaceable>
% gunzip -c <replaceable class="parameter">filename</replaceable>.dump.gz | psql <replaceable class="parameter">dbname</replaceable>
	</programlisting>

or

	<programlisting>
% cat <replaceable class="parameter">filename</replaceable>.dump.gz | gunzip | psql <replaceable class="parameter">dbname</replaceable>
	</programlisting>
       </para>
      </listitem>

      <listitem>
       <para>
	Use split:

	<programlisting>
% pg_dump <replaceable class="parameter">dbname</replaceable> | split -b 1m - <replaceable class="parameter">filename</replaceable>.dump.
	</programlisting>

reload with

	<programlisting>
% createdb <replaceable class="parameter">dbname</replaceable>
% cat <replaceable class="parameter">filename</replaceable>.dump.* | pgsql <replaceable class="parameter">dbname</replaceable>
	</programlisting>
       </para>
      </listitem>
     </itemizedlist>
    </para>

    <para>
     Of course, the name of the file
     (<replaceable class="parameter">filename</replaceable>) and the
     content of the <application>pg_dump</application> output need not 
     match the name of the database. Also, the restored database can
     have an arbitrary new name, so this mechanism is also suitable
     for renaming databases.
    </para>
   </sect2>
  </sect1>
</chapter>

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