string $orderby, WP_Query $query )
Filters the ORDER BY clause of the query.
This filter is applied before a post-retrieving SQL statement is executed. Use it to make custom modifications to the orderby. WP_Query is versatile but there may be situations where you need to orderby a value that is in a separate database, a custom equation, etc.
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$orderby = apply_filters_ref_array( 'posts_orderby', array( $orderby, &$this ) );