set.sgml

关系型数据库 Postgresql 6.5.2

    
    <variablelist>
     <varlistentry>
      <term>
COST_HEAP
      </term>
      <listitem>
       <para>
	Sets the default cost of a heap scan for use by the optimizer.
	
	<variablelist>
	 <varlistentry>
	  <term>
<replaceable class="parameter">float4</replaceable>
	  </term>
	  <listitem>
	   <para>
	    Set the cost of a heap scan to the specified floating point value.
	   </para>
	  </listitem>
	 </varlistentry>
	 
	 <varlistentry>
	  <term>
DEFAULT
	  </term>
	  <listitem>
	   <para>
	    Sets the cost of a heap scan to the default value.
	   </para>
	  </listitem>
	 </varlistentry>
	</variablelist>
       </para>
       <para>
	The frontend may be initialized by setting the PGCOSTHEAP
	environment variable.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>
COST_INDEX
      </term>
      <listitem>
       <para>
	Sets the default cost of an index scan for use by the optimizer.

	<variablelist>     
	 <varlistentry>
	  <term>
<replaceable class="parameter">float4</replaceable>
	  </term>
	  <listitem>
	   <para>
	    Set the cost of an index scan to the specified floating point value.
	   </para>
	  </listitem>
	 </varlistentry>
	 
	 <varlistentry>
	  <term>
DEFAULT
	  </term>
	  <listitem>
	   <para>
	    Sets the cost of an index scan to the default value.
	   </para>
	  </listitem>
	 </varlistentry>
	</variablelist>
       </para>

       <para>
	The frontend may be initialized by setting the PGCOSTINDEX
	environment variable.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>
GEQO
      </term>
      <listitem>
       <para>
	Sets the threshold for using the genetic optimizer algorithm.
       
	<variablelist>
	 <varlistentry>
	  <term>
ON
	  </term>
	  <listitem>
	   <para>
	    enables the genetic optimizer algorithm
	    for statements with 6 or more tables.
	   </para>
	  </listitem>
	 </varlistentry>
	 <varlistentry>
	  <term>
ON=<replaceable class="parameter">#</replaceable>
	  </term>
	  <listitem>
	   <para>
	    Takes an integer argument to enable the genetic optimizer algorithm
	    for statements with <replaceable class="parameter">#</replaceable>
	    or more tables in the query.
	   </para>
	  </listitem>
	 </varlistentry>
	 <varlistentry>
	  <term>
OFF
	  </term>
	  <listitem>
	   <para>
	    disables the genetic optimizer algorithm.
	   </para>
	  </listitem>
	 </varlistentry>
	 <varlistentry>
	  <term>
DEFAULT
	  </term>
	  <listitem>
	   <para>
	    Equivalent to specifying <command>SET GEQO='ON'</command>
	   </para>
	  </listitem>
	 </varlistentry>
	</variablelist>
       </para>

       <para>
	This algorithm is on by default, which used GEQO for
	statements of eleven or more tables.
	(See the chapter on GEQO in the Programmer's Guide
	for more information).
       </para>
       <para>
	The frontend may be initialized by setting PGGEQO
	environment variable.
       </para>
       <para>
	It may be useful when joining big relations with
	small ones. This algorithm is off by default.
	It's not used by GEQO anyway.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>
KSQO
      </term>
      <listitem>
       <para>
	<firstterm>Key Set Query Optimizer</firstterm> forces the query optimizer
	to optimize repetative OR clauses such as generated by
	<productname>MicroSoft Access</productname>:
       
	<variablelist>
	 <varlistentry>
	  <term>
ON
	  </term>
	  <listitem>
	   <para>
	    enables this optimization.
	   </para>
	  </listitem>
	 </varlistentry>
	
	 <varlistentry>
	  <term>
OFF
	  </term>
	  <listitem>
	   <para>
	    disables this optimization.
	   </para>
	  </listitem>
	 </varlistentry>
	 
	 <varlistentry>
	  <term>
DEFAULT
	  </term>
	  <listitem>
	   <para>
	    Equivalent to specifying <command>SET KSQO='OFF'</command>.
	   </para>
	  </listitem>
	 </varlistentry>
	</variablelist>
       </para>

       <para>
	It may be useful when joining big relations with
	small ones. This algorithm is off by default.
	It's not used by GEQO anyway.
       </para>
       <para>
	The frontend may be initialized by setting the PGKSQO
	environment variable.
       </para>
      </listitem>
     </varlistentry>

    </variablelist>
   </para>
  </refsect2>

  <refsect2 id="R2-SQL-SET-2">
   <refsect2info>
    <date>1998-09-24</date>
   </refsect2info>
   <title>
    Outputs
   </title>
   <para>
    
    <variablelist>
     <varlistentry>
      <term>
<returnvalue>SET VARIABLE</returnvalue>
      </term>
      <listitem>
       <para>
	Message returned if successfully.
       </para>
      </listitem>
     </varlistentry>
     
     <varlistentry>
      <term>
<returnvalue>WARN:  Bad value for <replaceable class="parameter">variable</replaceable> (<replaceable class="parameter">value</replaceable>)</returnvalue>
      </term>
      <listitem>
       <para>
	If the command fails to set the specified variable.
       </para>
      </listitem>
     </varlistentry>
     
    </variablelist>
   </para>
  </refsect2>
 </refsynopsisdiv>
 
 <refsect1 id="R1-SQL-SET-1">
  <refsect1info>
   <date>1998-09-24</date>
  </refsect1info>
  <title>
   Description
  </title>
  <para>
   <command>SET</command> will modify configuration parameters for variable during
   a session.
  </para>
  <para>
   Current values can be obtained using <command>SHOW</command>, and values
   can be restored to the defaults using <command>RESET</command>.
   Parameters and values are case-insensitive. Note that the value
   field is always specified as a string, so is enclosed in
   single-quotes.
  </para>
  <para>
   <command>SET TIME ZONE</command> changes the session's
   default time zone offset.
   An SQL-session always begins with an initial default time zone
   offset.
   The <command>SET TIME ZONE</command> statement is used to change the default
   time zone offset for the current SQL session.
  </para>
  
  <refsect2 id="R2-SQL-SET-3">
   <refsect2info>
    <date>1998-09-24</date>
   </refsect2info>
   <title>
    Notes
   </title>
   <para>
    The <command>SET <replaceable class="parameter">variable</replaceable></command>
    statement is a <productname>Postgres</productname> language extension.
   </para>
   <para>
    Refer to <command>SHOW</command> and <command>RESET</command> to 
    display or reset the current values.
   </para>
  </refsect2>
 </refsect1>
 
 <refsect1 id="R1-SQL-SET-2">
  <title>
   Usage
  </title>
  <para>
   Set the style of date to ISO:
   
   <programlisting>
   SET DATESTYLE TO 'ISO';
   </programlisting>

   Enable GEQO for queries with 4 or more tables:

   <programlisting> 
   SET GEQO ON=4;
   </programlisting>

   Set GEQO to default:

   <programlisting> 
   SET GEQO = DEFAULT;
   </programlisting>

   Set the timezone for Berkeley, California:

   <programlisting> 
   SET TIME ZONE 'PST8PDT';
   SELECT CURRENT_TIMESTAMP AS today;
   
   today
   ----------------------
   1998-03-31 07:41:21-08
   </programlisting>

   Set the timezone for Italy:

   <programlisting> 
   SET TIME ZONE 'Europe/Rome';
   SELECT CURRENT_TIMESTAMP AS today;
   
   today
   ----------------------
   1998-03-31 17:41:31+02
   </programlisting>
  </para>
 </refsect1>

 <refsect1 id="R1-SQL-SET-3">
  <title>
   Compatibility
  </title>
  <para>
  </para>
  
  <refsect2 id="R2-SQL-SET-4">
   <refsect2info>
    <date>1998-09-24</date>
   </refsect2info>
   <title>
    SQL92
   </title>
   <para>
    There is no
    <command>SET <replaceable class="parameter">variable</replaceable></command>
    in <acronym>SQL92</acronym> (except for SET TRANSACTION ISOLATION LEVEL).
    
    The <acronym>SQL92</acronym> syntax for <command>SET TIME ZONE</command>
    is slightly different,
    allowing only a single integer value for time zone specification:
    
    <programlisting>
     SET TIME ZONE { interval_value_expression | LOCAL }
    </programlisting>
   </para>
  </refsect2>
 </refsect1>
</refentry>

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