WordPress at a glance
filter is not described

posts_groupby filter-hook . WP 2.0.0

Filters the GROUP BY clause of the query.

Usage

add_filter( 'posts_groupby', 'filter_function_name_441', 10, 2 );
function filter_function_name_441( $groupby, $this ){
	// filter...

	return $groupby;
}
$groupby(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 2517
$groupby = apply_filters_ref_array( 'posts_groupby', array( $groupby, &$this ) );

Where the hook is used (in WP core)

Does not used.