WordPress at a glance
filter is not described

posts_fields filter-hook . WP 2.1.0

Filters the SELECT clause of the query.

Usage

add_filter( 'posts_fields', 'filter_function_name_4952', 10, 2 );
function filter_function_name_4952( $fields, $this ){
	// filter...

	return $fields;
}
$fields(string)
The SELECT 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 2569
$fields = apply_filters_ref_array( 'posts_fields', array( $fields, &$this ) );

Where the hook is used (in WP core)

Does not used.