Ticket #16706: 16706.2.diff

Index: wp-includes/taxonomy.php
===================================================================
--- wp-includes/taxonomy.php    (revision 17950)
+++ wp-includes/taxonomy.php    (working copy)
@@ -636,7 +636,26 @@
                $where = array();
                $i = 0;
 
-               foreach ( $this->queries as $query ) {
+               // convert AND queries to IN ones for more efficient SQL
+               if ( 'AND' == $this->relation ) {
+                       $final_queries = array();
+                       foreach ( $this->queries as $query ) {
+                               if ( 'AND' == $query['operator'] && count( $query['terms'] ) <= 10 ) {
+                                       foreach ( $query['terms'] as $term ) {
+                                               $final_queries[] = array_merge( $query, array(
+                                                       'terms' => $term,
+                                                       'operator' => 'IN'
+                                               ) );
+                                       }
+                               } else {
+                                       $final_queries[] = $query;
+                               }
+                       }
+               } else {
+                       $final_queries = $this->queries;
+               }
+
+               foreach ( $final_queries as $query ) {
                        $this->clean_query( $query );
 
                        if ( is_wp_error( $query ) ) {