Index: wp-includes/query.php
===================================================================
--- wp-includes/query.php	(revision 6812)
+++ wp-includes/query.php	(working copy)
@@ -969,10 +969,8 @@
 		}
 
 		if ( !empty($q['category__in']) ) {
-			$join = " INNER JOIN $wpdb->term_relationships ON ($wpdb->posts.ID = $wpdb->term_relationships.object_id) INNER JOIN $wpdb->term_taxonomy ON ($wpdb->term_relationships.term_taxonomy_id = $wpdb->term_taxonomy.term_taxonomy_id) ";
-			$whichcat .= " AND $wpdb->term_taxonomy.taxonomy = 'category' ";
 			$include_cats = "'" . implode("', '", $q['category__in']) . "'";
-			$whichcat .= " AND $wpdb->term_taxonomy.term_id IN ($include_cats) ";
+			$join .= " INNER JOIN $wpdb->term_relationships ON ($wpdb->posts.ID = $wpdb->term_relationships.object_id) INNER JOIN $wpdb->term_taxonomy ON ($wpdb->term_relationships.term_taxonomy_id = $wpdb->term_taxonomy.term_taxonomy_id AND $wpdb->term_taxonomy.taxonomy = 'category' AND $wpdb->term_taxonomy.term_id IN ($include_cats)) ";
 		}
 
 		if ( !empty($q['category__not_in']) ) {
@@ -1010,12 +1008,10 @@
 
 			$q['cat'] = $reqcat;
 
-			$join = " INNER JOIN $wpdb->term_relationships ON ($wpdb->posts.ID = $wpdb->term_relationships.object_id) INNER JOIN $wpdb->term_taxonomy ON ($wpdb->term_relationships.term_taxonomy_id = $wpdb->term_taxonomy.term_taxonomy_id) ";
-			$whichcat = " AND $wpdb->term_taxonomy.taxonomy = 'category' ";
 			$in_cats = array($q['cat']);
 			$in_cats = array_merge($in_cats, get_term_children($q['cat'], 'category'));
 			$in_cats = "'" . implode("', '", $in_cats) . "'";
-			$whichcat .= "AND $wpdb->term_taxonomy.term_id IN ($in_cats)";
+			$join .= " INNER JOIN $wpdb->term_relationships ON ($wpdb->posts.ID = $wpdb->term_relationships.object_id) INNER JOIN $wpdb->term_taxonomy ON ($wpdb->term_relationships.term_taxonomy_id = $wpdb->term_taxonomy.term_taxonomy_id AND $wpdb->term_taxonomy.taxonomy = 'category' AND $wpdb->term_taxonomy.term_id IN ($in_cats))";
 			$groupby = "{$wpdb->posts}.ID";
 		}
 
@@ -1045,20 +1041,16 @@
 		}
 
 		if ( !empty($q['tag__in']) ) {
-			$join = " INNER JOIN $wpdb->term_relationships ON ($wpdb->posts.ID = $wpdb->term_relationships.object_id) INNER JOIN $wpdb->term_taxonomy ON ($wpdb->term_relationships.term_taxonomy_id = $wpdb->term_taxonomy.term_taxonomy_id) ";
-			$whichcat .= " AND $wpdb->term_taxonomy.taxonomy = 'post_tag' ";
 			$include_tags = "'" . implode("', '", $q['tag__in']) . "'";
-			$whichcat .= " AND $wpdb->term_taxonomy.term_id IN ($include_tags) ";
+			$join .= " INNER JOIN $wpdb->term_relationships ON ($wpdb->posts.ID = $wpdb->term_relationships.object_id) INNER JOIN $wpdb->term_taxonomy ON ($wpdb->term_relationships.term_taxonomy_id = $wpdb->term_taxonomy.term_taxonomy_id AND $wpdb->term_taxonomy.taxonomy = 'post_tag' AND $wpdb->term_taxonomy.term_id IN ($include_tags)) ";
 			$reqtag = is_term( $q['tag__in'][0], 'post_tag' );
 			if ( !empty($reqtag) )
 				$q['tag_id'] = $reqtag['term_id'];
 		}
 
 		if ( !empty($q['tag_slug__in']) ) {
-			$join = " INNER JOIN $wpdb->term_relationships ON ($wpdb->posts.ID = $wpdb->term_relationships.object_id) INNER JOIN $wpdb->term_taxonomy ON ($wpdb->term_relationships.term_taxonomy_id = $wpdb->term_taxonomy.term_taxonomy_id) INNER JOIN $wpdb->terms ON ($wpdb->term_taxonomy.term_id = $wpdb->terms.term_id) ";
-			$whichcat .= " AND $wpdb->term_taxonomy.taxonomy = 'post_tag' ";
 			$include_tags = "'" . implode("', '", $q['tag_slug__in']) . "'";
-			$whichcat .= " AND $wpdb->terms.slug IN ($include_tags) ";
+			$join .= " INNER JOIN $wpdb->term_relationships ON ($wpdb->posts.ID = $wpdb->term_relationships.object_id) INNER JOIN $wpdb->term_taxonomy ON ($wpdb->term_relationships.term_taxonomy_id = $wpdb->term_taxonomy.term_taxonomy_id) INNER JOIN $wpdb->terms ON ($wpdb->term_taxonomy.term_id = $wpdb->terms.term_id AND $wpdb->term_taxonomy.taxonomy = 'post_tag' AND $wpdb->terms.slug IN ($include_tags)) ";
 			$reqtag = is_term( $q['tag_slug__in'][0], 'post_tag' );
 			if ( !empty($reqtag) )
 				$q['tag_id'] = $reqtag['term_id'];