WordPress at a glance
filter is not described

posts_orderby filter-hook . WP 1.5.1

Filters the ORDER BY clause of the query.

Usage

add_filter( 'posts_orderby', 'filter_function_name_4600', 10, 2 );
function filter_function_name_4600( $orderby, $this ){
	// filter...

	return $orderby;
}
$orderby(string)
The ORDER BY clause of the query.
$this(WP_Query)
The WP_Query instance (passed by reference).

Where the hook is called

WP_Query::get_posts()
wp-includes/class-wp-query.php 2539
$orderby = apply_filters_ref_array( 'posts_orderby', array( $orderby, &$this ) );

Where the hook is used (in WP core)

Does not used.