[ticket/11469] Add SQL insert buffer allowing easier handling of multi inserts.

1. Tries to prevent going over max packet size by flushing to the database
   after a certain number of rows have been added.
2. Because of 1., it is less likely to reach a connection timeout when
   inserting a huge number of rows.
3. By flushing the buffer when a certain size is reached, memory usage should
   be lower compared to building the whole insert row set first.

PHPBB3-11469

phpBB/includes/db/sql_insert_buffer.php

@@ -0,0 +1,111 @@
+<?php
+/**
+*
+* @package dbal
+* @copyright (c) 2013 phpBB Group
+* @license http://opensource.org/licenses/gpl-2.0.php GNU General Public License v2
+*
+*/
+
+/**
+* @ignore
+*/
+if (!defined('IN_PHPBB'))
+{
+	exit;
+}
+
+/**
+* Collects rows for insert into a database until the buffer size is reached.
+* Then flushes the buffer to the database and starts over again.
+*
+* @package dbal
+*/
+class phpbb_db_sql_insert_buffer
+{
+	/** @var phpbb_db_driver */
+	protected $db;
+
+	/** @var bool */
+	protected $db_supports_multi_insert;
+
+	/** @var string */
+	protected $table_name;
+
+	/** @var int */
+	protected $max_buffered_rows;
+
+	/** @var array */
+	protected $buffer = array();
+
+	/**
+	* @param phpbb_db_driver $db
+	* @param string          $table_name
+	* @param int             $max_buffered_rows
+	*/
+	public function __construct(phpbb_db_driver $db, $table_name, $max_buffered_rows = 500)
+	{
+		$this->db = $db;
+		$this->db_supports_multi_insert = $db->multi_insert;
+		$this->table_name = $table_name;
+		$this->max_buffered_rows = $max_buffered_rows;
+	}
+
+	/**
+	* Inserts a single row into the buffer if multi insert is supported by the
+	* database (otherwise an insert query is sent immediately). Then flushes
+	* the buffer if the number of rows in the buffer is now greater than or
+	* equal to $max_buffered_rows.
+	*
+	* @param array $row
+	*
+	* @return null
+	*/
+	public function insert(array $row)
+	{
+		if (!$this->db_supports_multi_insert)
+		{
+			$this->db->sql_multi_insert($this->table_name, array($row));
+		}
+
+		$this->buffer[] = $row;
+
+		if (sizeof($this->buffer) >= $this->max_buffered_rows)
+		{
+			$this->flush();
+		}
+	}
+
+	/**
+	* Inserts a row set, i.e. an array of rows, by calling insert().
+	*
+	* Please note that it is in most cases better to use insert() instead of
+	* first building a huge rowset. Or at least sizeof($rows) should be kept
+	* small.
+	*
+	* @param array $rows 
+	*
+	* @return null
+	*/
+	public function insert_all(array $rows)
+	{
+		foreach ($rows as $row)
+		{
+			$this->insert($row);
+		}
+	}
+
+	/**
+	* Flushes the buffer content to the DB and clears the buffer.
+	*
+	* @return null
+	*/
+	public function flush()
+	{
+		if (!empty($this->buffer))
+		{
+			$this->db->sql_multi_insert($this->table_name, $this->buffer);
+			$this->buffer = array();
+		}
+	}
+}