<!-- doc/src/sgml/tablesample-method.sgml -->

<chapter id="tablesample-method">
 <title>Writing a Table Sampling Method</title>

 <indexterm zone="tablesample-method">
  <primary>table sampling method</primary>
 </indexterm>

 <indexterm zone="tablesample-method">
  <primary><literal>TABLESAMPLE</literal> method</primary>
 </indexterm>

 <para>
  <productname>PostgreSQL</productname>'s implementation of the <literal>TABLESAMPLE</literal>
  clause supports custom table sampling methods, in addition to
  the <literal>BERNOULLI</literal> and <literal>SYSTEM</literal> methods that are required
  by the SQL standard.  The sampling method determines which rows of the
  table will be selected when the <literal>TABLESAMPLE</literal> clause is used.
 </para>

 <para>
  At the SQL level, a table sampling method is represented by a single SQL
  function, typically implemented in C, having the signature
<programlisting>
method_name(internal) RETURNS tsm_handler
</programlisting>
  The name of the function is the same method name appearing in the
  <literal>TABLESAMPLE</literal> clause.  The <type>internal</type> argument is a dummy
  (always having value zero) that simply serves to prevent this function from
  being called directly from an SQL command.
  The result of the function must be a palloc'd struct of
  type <type>TsmRoutine</type>, which contains pointers to support functions for
  the sampling method.  These support functions are plain C functions and
  are not visible or callable at the SQL level.  The support functions are
  described in <xref linkend="tablesample-support-functions"/>.
 </para>

 <para>
  In addition to function pointers, the <type>TsmRoutine</type> struct must
  provide these additional fields:
 </para>

 <variablelist>
  <varlistentry>
   <term><literal>List *parameterTypes</literal></term>
   <listitem>
    <para>
     This is an OID list containing the data type OIDs of the parameter(s)
     that will be accepted by the <literal>TABLESAMPLE</literal> clause when this
     sampling method is used.  For example, for the built-in methods, this
     list contains a single item with value <literal>FLOAT4OID</literal>, which
     represents the sampling percentage.  Custom sampling methods can have
     more or different parameters.
    </para>
   </listitem>
  </varlistentry>

  <varlistentry>
   <term><literal>bool repeatable_across_queries</literal></term>
   <listitem>
    <para>
     If <literal>true</literal>, the sampling method can deliver identical samples
     across successive queries, if the same parameters
     and <literal>REPEATABLE</literal> seed value are supplied each time and the
     table contents have not changed.  When this is <literal>false</literal>,
     the <literal>REPEATABLE</literal> clause is not accepted for use with the
     sampling method.
    </para>
   </listitem>
  </varlistentry>

  <varlistentry>
   <term><literal>bool repeatable_across_scans</literal></term>
   <listitem>
    <para>
     If <literal>true</literal>, the sampling method can deliver identical samples
     across successive scans in the same query (assuming unchanging
     parameters, seed value, and snapshot).
     When this is <literal>false</literal>, the planner will not select plans that
     would require scanning the sampled table more than once, since that
     might result in inconsistent query output.
    </para>
   </listitem>
  </varlistentry>
 </variablelist>

 <para>
  The <type>TsmRoutine</type> struct type is declared
  in <filename>src/include/access/tsmapi.h</filename>, which see for additional
  details.
 </para>

 <para>
  The table sampling methods included in the standard distribution are good
  references when trying to write your own.  Look into
  the <filename>src/backend/access/tablesample</filename> subdirectory of the source
  tree for the built-in sampling methods, and into the <filename>contrib</filename>
  subdirectory for add-on methods.
 </para>

 <sect1 id="tablesample-support-functions">