j-tax-query
$tax_query = array( 'relation' => 'OR', array( 'taxonomy' => 'category', 'terms' => 'mycat', 'field' => 'slug', 'include_children' => true, ), array( 'taxonomy' => 'tag', 'terms' => 'mytag', 'field' => 'slug', 'operator' => 'IN', ), );
$tax_query = array( 'relation' => 'OR', array( 'taxonomy' => 'category', 'terms' => 'mycat', 'field' => 'slug', 'include_children' => true, ), array( 'taxonomy' => 'tag', 'terms' => 'mytag', 'field' => 'slug', 'operator' => 'IN', ), );