<!-- doc/src/sgml/contrib-spi.sgml -->

<sect1 id="contrib-spi" xreflabel="spi">
 <title>spi &mdash; Server Programming Interface features/examples</title>

 <indexterm zone="contrib-spi">
  <primary>SPI</primary>
  <secondary>examples</secondary>
 </indexterm>

 <para>
  The <application>spi</application> module provides several workable examples
  of using the <link linkend="spi">Server Programming Interface</link>
  (<acronym>SPI</acronym>) and triggers.  While these functions are of
  some value in
  their own right, they are even more useful as examples to modify for
  your own purposes.  The functions are general enough to be used
  with any table, but you have to specify table and field names (as described
  below) while creating a trigger.
 </para>

 <para>
  Each of the groups of functions described below is provided as a
  separately-installable extension.
 </para>

 <sect2 id="contrib-spi-refint">
  <title>refint &mdash; Functions for Implementing Referential Integrity</title>

  <para>
   <function>check_primary_key()</function> and
   <function>check_foreign_key()</function> are used to check foreign key constraints.
   (This functionality is long since superseded by the built-in foreign
   key mechanism, of course, but the module is still useful as an example.)
  </para>

  <para>
   <function>check_primary_key()</function> checks the referencing table.
   To use, create an <literal>AFTER INSERT OR UPDATE</literal> trigger using this
   function on a table referencing another table. Specify as the trigger
   arguments: the referencing table's column name(s) which form the foreign
   key, the referenced table name, and the column names in the referenced table
   which form the primary/unique key.  To handle multiple foreign
   keys, create a trigger for each reference.
  </para>

  <para>
   <function>check_foreign_key()</function> checks the referenced table.
   To use, create an <literal>AFTER DELETE OR UPDATE</literal> trigger using this
   function on a table referenced by other table(s).  Specify as the trigger
   arguments: the number of referencing tables for which the function has to
   perform checking, the action if a referencing key is found
   (<literal>cascade</literal> &mdash; to delete the referencing row,
   <literal>restrict</literal> &mdash; to abort transaction if referencing keys
   exist, <literal>setnull</literal> &mdash; to set referencing key fields to null),
   the triggered table's column names which form the primary/unique key, then
   the referencing table name and column names (repeated for as many
   referencing tables as were specified by first argument).  Note that the
   primary/unique key columns should be marked NOT NULL and should have a
   unique index.
  </para>

  <para>
   Note that if these triggers are executed from
   another <literal>BEFORE</literal> trigger, they can fail unexpectedly. For
   example, if a user inserts row1 and then the <literal>BEFORE</literal>
   trigger inserts row2 and calls a trigger with the
   <function>check_foreign_key()</function>,
   the <function>check_foreign_key()</function>
   function will not see row1 and will fail.
  </para>

  <para>
   There are examples in <filename>refint.example</filename>.
  </para>
 </sect2>

 <sect2 id="contrib-spi-autoinc">
  <title>autoinc &mdash; Functions for Autoincrementing Fields</title>

  <para>
   <function>autoinc()</function> is a trigger that stores the next value of
   a sequence into an integer field.  This has some overlap with the
   built-in <quote>serial column</quote> feature, but it is not the same.
   The trigger will replace the field's value only if that value is
   initially zero or null (after the action of the SQL statement that
   inserted or updated the row).  Also, if the sequence's next value is
   zero, <function>nextval()</function> will be called a second time in
   order to obtain a non-zero value.
  </para>

  <para>
   To use, create a <literal>BEFORE INSERT</literal> (or optionally <literal>BEFORE
   INSERT OR UPDATE</literal>) trigger using this function.  Specify two
   trigger arguments: the name of the integer column to be modified,
   and the name of the sequence object that will supply values.
   (Actually, you can specify any number of pairs of such names, if
   you'd like to update more than one autoincrementing column.)
  </para>

  <para>
   There is an example in <filename>autoinc.example</filename>.
  </para>

 </sect2>

 <sect2 id="contrib-spi-insert-username">
  <title>insert_username &mdash; Functions for Tracking Who Changed a Table</title>

  <para>
   <function>insert_username()</function> is a trigger that stores the current
   user's name into a text field.  This can be useful for tracking
   who last modified a particular row within a table.
  </para>

  <para>
   To use, create a <literal>BEFORE INSERT</literal> and/or <literal>UPDATE</literal>
   trigger using this function.  Specify a single trigger
   argument: the name of the text column to be modified.
  </para>

  <para>
   There is an example in <filename>insert_username.example</filename>.
  </para>

 </sect2>

 <sect2 id="contrib-spi-moddatetime">
  <title>moddatetime &mdash; Functions for Tracking Last Modification Time</title>

  <para>
   <function>moddatetime()</function> is a trigger that stores the current
   time into a <type>timestamp</type> field.  This can be useful for tracking
   the last modification time of a particular row within a table.
  </para>

  <para>
   To use, create a <literal>BEFORE UPDATE</literal>
   trigger using this function.  Specify a single trigger
   argument: the name of the column to be modified.
   The column must be of type <type>timestamp</type> or <type>timestamp with
   time zone</type>.
  </para>

  <para>
   There is an example in <filename>moddatetime.example</filename>.
  </para>

 </sect2>

</sect1>
