oci-set-prefetch.xml

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<refentry xmlns="http://docbook.org/ns/docbook" xml:id="function.oci-set-prefetch">
 <refnamediv>
  <refname>oci_set_prefetch</refname>
  <refpurpose>Sets number of rows to be prefetched by queries</refpurpose>
 </refnamediv>
 
 <refsect1 role="description">
  &reftitle.description;
  <methodsynopsis>
   <type>bool</type><methodname>oci_set_prefetch</methodname>
   <methodparam><type>resource</type><parameter>statement</parameter></methodparam>
   <methodparam><type>int</type><parameter>rows</parameter></methodparam>
  </methodsynopsis>
  <para>
   Sets the number of rows to be buffered by the Oracle Client
   libraries after a successful query call
   to <function>oci_execute</function> and for each subsequent
   internal fetch request to the database.  For queries returning a
   large number of rows, performance can be significantly improved by
   increasing the prefetch count above the
   default <link linkend="ini.oci8.default-prefetch">oci8.default_prefetch</link>
   value.
  </para>
  <para>
   Prefetching is Oracle's efficient way of returning more than one
   data row from the database in each network request.  This can
   result in better network and CPU utilization.  The buffering of
   rows is internal to OCI8 and the behavior of OCI8 fetching
   functions is unchanged regardless of the prefetch count. For
   example, <function>oci_fetch_row</function> will always return one
   row.  The prefetch buffer is per-statement and is not used by
   re-executed statements or by other connections.
  </para>
  <para>
   Call <function>oci_set_prefetch</function> before
   calling <function>oci_execute</function>.
  </para>
  <para>
   A tuning goal is to set the prefetch value to a reasonable size for
   the network and database to handle.  For queries returning a very
   large number of rows, overall system efficiency might be better if
   rows are retrieved from the database in several chunks (i.e set the
   prefetch value smaller than the number of rows).  This allows the
   database to handle other users' statements while the PHP script is
   processing the current set of rows.
  </para>
  <para>
   Query prefetching was introduced in Oracle 8<emphasis>i</emphasis>.  REF CURSOR
   prefetching was introduced in Oracle 11<emphasis>g</emphasis>R2 and occurs when PHP is
   linked with Oracle 11<emphasis>g</emphasis>R2 (or later) Client libraries.
   Nested cursor prefetching was
   introduced in Oracle 11<emphasis>g</emphasis>R2 and requires both the Oracle Client
   libraries and the database to be version 11<emphasis>g</emphasis>R2 or greater.  
  </para>
  <para>
   Prefetching is not supported when queries contain LONG or LOB
   columns.  The prefetch value is ignored and single-row fetches will
   be used in all the situations when prefetching is not supported.
  </para>
  <para>
   When using Oracle Database 12<emphasis>c</emphasis>, the prefetch
   value set by PHP can be overridden by Oracle's
   client <literal>oraaccess.xml</literal> configuration file.  Refer
   to Oracle documentation for more detail.
  </para>
 </refsect1>

 <refsect1 role="parameters">
  &reftitle.parameters;
  <para>
   <variablelist>
    <varlistentry>
     <term><parameter>statement</parameter></term>
     <listitem>
       &oci.arg.statement.id;
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><parameter>rows</parameter></term>
     <listitem>
      <para>
       The number of rows to be prefetched, &gt;= 0
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>
 </refsect1>

 <refsect1 role="returnvalues">
  &reftitle.returnvalues;
  <para>
   &return.success;
  </para>
 </refsect1>

 <refsect1 role="examples">
  &reftitle.examples;
  <para>
   <example>
    <title>Changing the default prefetch value for a query</title>
    <programlisting role="php">
<![CDATA[
<?php

$conn = oci_connect('hr', 'welcome', 'localhost/XE');

$stid = oci_parse($conn, 'SELECT * FROM myverybigtable');
oci_set_prefetch($stid, 300);  // Set before calling oci_execute()
oci_execute($stid);

echo "<table border='1'>\n";
while ($row = oci_fetch_array($stid, OCI_ASSOC+OCI_RETURN_NULLS)) {
    echo "<tr>\n";
    foreach ($row as $item) {
        echo "    <td>".($item !== null ? htmlentities($item, ENT_QUOTES) : "&nbsp;")."</td>\n";
    }
    echo "</tr>\n";
}
echo "</table>\n";

oci_free_statement($stid);
oci_close($conn);

?>
]]>
    </programlisting>
   </example>
  </para>
  <para>
   <example>
    <title>Changing the default prefetch for a REF CURSOR fetch</title>
    <programlisting role="php">
<![CDATA[
<?php
/*
  Create the PL/SQL stored procedure as:

  CREATE OR REPLACE PROCEDURE myproc(p1 OUT SYS_REFCURSOR) AS
  BEGIN
    OPEN p1 FOR SELECT * FROM all_objects WHERE ROWNUM < 5000;
  END;
*/

$conn = oci_connect('hr', 'welcome', 'localhost/XE');

$stid = oci_parse($conn, 'BEGIN myproc(:rc); END;');
$refcur = oci_new_cursor($conn);
oci_bind_by_name($stid, ':rc', $refcur, -1, OCI_B_CURSOR);
oci_execute($stid);

// Change the prefetch before executing the cursor.
// REF CURSOR prefetching works when PHP is linked with Oracle 11gR2 or later Client libraries
oci_set_prefetch($refcur, 200);
oci_execute($refcur);

echo "<table border='1'>\n";
while ($row = oci_fetch_array($refcur, OCI_ASSOC+OCI_RETURN_NULLS)) {
    echo "<tr>\n";
    foreach ($row as $item) {
        echo "    <td>".($item !== null ? htmlentities($item, ENT_QUOTES) : "&nbsp;")."</td>\n";
    }
    echo "</tr>\n";
}
echo "</table>\n";

oci_free_statement($refcur);
oci_free_statement($stid);
oci_close($conn);

?> 
]]>
    </programlisting>
   </example>
  </para>
  <para>
   If PHP OCI8 fetches from a REF CURSOR and then passes the REF
   CURSOR back to a second PL/SQL procedure for further processing,
   then set the REF CURSOR prefetch count to <literal>0</literal> to
   avoid rows being "lost" from the result set.  The prefetch value is
   the number of extra rows fetched in each OCI8 internal request to
   the database, so setting it to <literal>0</literal> means only
   fetch one row at a time.
   <example>
    <title>Setting the prefetch value when passing a REF CURSOR back to Oracle</title>
    <programlisting role="php">
<![CDATA[
<?php

$conn = oci_connect('hr', 'welcome', 'localhost/orcl');

// get the REF CURSOR
$stid = oci_parse($conn, 'BEGIN myproc(:rc_out); END;');
$refcur = oci_new_cursor($conn);
oci_bind_by_name($stid, ':rc_out', $refcur, -1, OCI_B_CURSOR);
oci_execute($stid);

// Display two rows, but don't prefetch any extra rows otherwise
// those extra rows would not be passed back to myproc_use_rc().
oci_set_prefetch($refcur, 0);
oci_execute($refcur);
$row = oci_fetch_array($refcur);
var_dump($row);
$row = oci_fetch_array($refcur);
var_dump($row);

// pass the REF CURSOR to myproc_use_rc() to do more data processing
// with the result set
$stid = oci_parse($conn, 'begin myproc_use_rc(:rc_in); end;'); 
oci_bind_by_name($stid, ':rc_in', $refcur, -1, OCI_B_CURSOR);
oci_execute($stid);

?>
]]>
    </programlisting>
   </example>
  </para>
 </refsect1>

 <refsect1 role="seealso">
  &reftitle.seealso;
  <para>
   <simplelist>
    <member>
     <link linkend="ini.oci8.default-prefetch">oci8.default_prefetch</link>
     ini option
    </member>
   </simplelist>
  </para>
 </refsect1>

</refentry>

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