oci_set_prefetch

(PHP 5, PHP 7, PHP 8, PECL OCI8 >= 1.1.0)

oci_set_prefetch — 设置查询预读取的行数

说明

oci_set_prefetch(resource $statement, int $rows): bool

在调用 oci_execute() 成功查询以及对数据库的每个后续内部读取请求之后，设置 Oracle 客户端库缓冲的行数。对于返回大量行的查询，可以通过在默认 oci8.default_prefetch 值上增加预读取数来显着提高性能。

预读取是 Oracle 在每个网络请求中从数据库返回多个数据行的有效方式。这可以提高网络和 CPU 利用率。行缓冲是 OCI8 的内部操作，无论预取数如何，OCI8 提取函数的行为都不会改变。例如，oci_fetch_row() 将始终返回一行。预读取缓冲区是针对单个语句的，不会被重新执行的语句或其他连接使用。

在调用 oci_execute() 之前调用 oci_set_prefetch()。

调整目标是将预读取值设置为网络和数据库要处理的合理大小。对于返回大量行的查询，如果以多个块的形式从数据库中检索行（即设置预读取值小于行数），则整体系统效率可能会更好。这允许数据库在 PHP 脚本处理行的当前集合的同时处理其他用户的语句。

Oracle 8i 中引入了查询预读取。REF CURSOR 预读取是在 Oracle 11gR2 中引入的，当 PHP 与 Oracle 11gR2（或更高版本）客户端库链接时发生。嵌套游标预读取是在 Oracle 11gR2 中引入的，它要求 Oracle Client 库和数据库都是 11gR2 或更高版本。

当查询包含 LONG 或 LOB 列时，不支持预读取。预读取值将会忽略，并且在不支持预读取的所有情况下都将使用单行获取。

使用 Oracle Database 12c 时，PHP 设置的预读取值可以被 Oracle 的客户端 oraaccess.xml 配置文件覆盖。有关详细信息，请参阅 Oracle 文档。

参数

statement: 有效的 OCI8 报表标识符由 oci_parse() 创建，被 oci_execute() 或 REF CURSOR statement 标识执行。
rows: 要预读取的行数，>= 0

返回值

成功时返回 true，或者在失败时返回 false。

示例

示例 #1 更改查询的默认预读取值

<?php

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

$stid = oci_parse($conn, 'SELECT * FROM myverybigtable');
oci_set_prefetch($stid, 300);  // 调用 oci_execute() 之前设置
oci_execute($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);

?>

示例 #2 更改 REF CURSOR 获取的默认预获取值

<?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);

?>

如果 PHP OCI8 从 REF CURSOR 读取，然后将 REF CURSOR 传递回第二个 PL/SQL 过程以进行进一步处理，则将 REF CURSOR 预读取数设置为 0 以避免行从结果集中“丢失”。预读取值是在每个 OCI8 内部请求数据库中提取的额外行数，因此将其设置为 0 意味着一次只提取一行。

示例 #3 将 REF CURSOR 传回 Oracle 时设置读取值

<?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().
// A prefetch value of 0 is allowed in PHP 5.3.2 and PECL OCI8 1.4
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);

?>

参见

oci8.default_prefetch ini 选项

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bmichael at goldparrot dot com ¶

21 years ago

If your are using Oracle's OCI libraries, on any project, which PHP does, you can use this limit.



I have done network level testing on the effect of this parameter.  It does improved efficiency.  Big Time.



Oracle uses SQL*Net as the transport mechanism for data between your connection and the database.  That is why you must setup Oracle properly.



This parameter tells SQL*NET to Buffer more results.  When SQL*NET (at the server) gets a request for data, it bundles up X rows (1,2,3,1000, etc) for transport.  It sends the appropriate SQL*NET headers back to the client, Waits for an ACK then begins sending data in MTU sized chunks (ethernet is something like 1500 bytes and ATM's WANS are around 1000 bytes).  The chunk size can also be tuned in SQL*NET, but with much less improvements.



TCP/IP then takes the data across the wire, breaking it up into multiple TCP/IP packets.



Once the exchange is done, the SQL*NET client sends an

ACK back to the SQL*NET Listener (the Oracle Server) and the transaction is complete.



Each round trip, SQL*NET looks up inside the server memory (UGA - user global area) to find the query results. It then grabs the rows necessary to send.  If it is one row, versus 1000 rows.  The process is the same. 



There is much I could tell you on how the Database itself reacts.  If you can significantly lessen the amount of round trips you are making... WOW.  



For more info on Oracle OCI go to http://otn.oracle.com