WordPress at a glance
filter is not described

comment_feed_groupby filter-hook . WP 2.2.0

Filters the GROUP BY clause of the comments feed query before sending.

Usage

add_filter( 'comment_feed_groupby', 'filter_function_name_103', 10, 2 );
function filter_function_name_103( $cgroupby, $this ){
	// filter...

	return $cgroupby;
}
$cgroupby(string)
The GROUP 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 2446
$cgroupby = apply_filters_ref_array( 'comment_feed_groupby', array( $cgroupby, &$this ) );
wp-includes/class-wp-query.php 2861
$cgroupby = apply_filters_ref_array( 'comment_feed_groupby', array( '', &$this ) );

Where the hook is used (in WP core)

Does not used.