string $join, WP_Query $query )
Filters the JOIN clause of the query.
When you use the
wp_query object to run a query, not all tables are queried by default. For example, a query on the blog archive will only query the posts table. If you wanted to display posts that have specific meta value you will have to alter the
wp_query object to include the required meta key.
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$join = apply_filters_ref_array( 'posts_join', array( $join, &$this ) );