T5 Log Queries

t5-query-log.php

<?php
/**
 * Plugin Name: T5 Log Queries
 * Description: Writes all queries to '/query-log.sql'.
 * Plugin URI:  http://wordpress.stackexchange.com/a/70853/73
 * Version:     2012.11.04
 * Author:      Thomas Scholz
 * Author URI:  http://toscho.de
 * Licence:     MIT
 */

add_filter( 'query', 't5_log_queries' );

/**
 * Write the SQL to a file.
 *
 * @wp-hook query
 * @param   string $query
 * @return  string Unchanged query
 */
function t5_log_queries( $query )
{
	static $first = TRUE;
	// Change the path here.
	$log_path = apply_filters(
		't5_log_queries_path',
		ABSPATH . 'query-log.sql'
	);
	$header = '';

	if ( $first )
	{
		$time    = date( 'Y-m-d H:i:s' );
		$request = $_SERVER['REQUEST_URI'];
		$header  = "\n\n# -- Request URI: $request, Time: $time ------------\n";
		$first   = FALSE;
	}

	file_put_contents( $log_path, "$header\n$query", FILE_APPEND | LOCK_EX );

	return $query;
}

Track it with tail:

$ tail -f query-log.sql -n 50