diff --git a/wp-includes/link-template.php b/wp-includes/link-template.php
index 4735696425..108a40331d 100644
--- a/wp-includes/link-template.php
+++ b/wp-includes/link-template.php
@@ -359,11 +359,11 @@ function get_previous_post($in_same_cat = false, $excluded_categories = '') {
 
 	$join = '';
 	if ( $in_same_cat ) {
-		$join = " INNER JOIN $wpdb->post2cat ON $wpdb->posts.ID= $wpdb->post2cat.post_id ";
-		$cat_array = get_the_category($post->ID);
-		$join .= ' AND (category_id = ' . intval($cat_array[0]->term_id);
+		$join = " INNER JOIN $wpdb->term_relationships AS tr ON $wpdb->posts.ID = tr.object_id ";
+		$cat_array = get_object_terms($post->ID, 'category', 'fields=tt_ids');
+		$join .= ' AND (tr.term_taxonomy_id = ' . intval($cat_array[0]);
 		for ( $i = 1; $i < (count($cat_array)); $i++ ) {
-			$join .= ' OR category_id = ' . intval($cat_array[$i]->term_id);
+			$join .= ' OR tr.term_taxonomy_id = ' . intval($cat_array[$i]);
 		}
 		$join .= ')';
 	}
@@ -371,11 +371,7 @@ function get_previous_post($in_same_cat = false, $excluded_categories = '') {
 	$sql_exclude_cats = '';
 	if ( !empty($excluded_categories) ) {
 		$blah = explode(' and ', $excluded_categories);
-		foreach ( $blah as $category ) {
-			$category = intval($category);
-			$sql_cat_ids = " OR pc.category_ID = '$category'";
-		}
-		$posts_in_ex_cats = $wpdb->get_col("SELECT p.ID FROM $wpdb->posts p LEFT JOIN $wpdb->post2cat pc ON pc.post_id=p.ID WHERE 1 = 0 $sql_cat_ids GROUP BY p.ID");
+		$posts_in_ex_cats = get_objects_in_term($blah, 'category');
 		$posts_in_ex_cats_sql = 'AND ID NOT IN (' . implode($posts_in_ex_cats, ',') . ')';
 	}
 
@@ -396,11 +392,11 @@ function get_next_post($in_same_cat = false, $excluded_categories = '') {
 
 	$join = '';
 	if ( $in_same_cat ) {
-		$join = " INNER JOIN $wpdb->post2cat ON $wpdb->posts.ID= $wpdb->post2cat.post_id ";
-		$cat_array = get_the_category($post->ID);
-		$join .= ' AND (category_id = ' . intval($cat_array[0]->term_id);
+		$join = " INNER JOIN $wpdb->term_relationships AS tr ON $wpdb->posts.ID = tr.object_id ";
+		$cat_array = get_object_terms($post->ID, 'category', 'fields=tt_ids');
+		$join .= ' AND (tr.term_taxonomy_id = ' . intval($cat_array[0]);
 		for ( $i = 1; $i < (count($cat_array)); $i++ ) {
-			$join .= ' OR category_id = ' . intval($cat_array[$i]->term_id);
+			$join .= ' OR tr.term_taxonomy_id = ' . intval($cat_array[$i]);
 		}
 		$join .= ')';
 	}
@@ -408,11 +404,7 @@ function get_next_post($in_same_cat = false, $excluded_categories = '') {
 	$sql_exclude_cats = '';
 	if ( !empty($excluded_categories) ) {
 		$blah = explode(' and ', $excluded_categories);
-		foreach ( $blah as $category ) {
-			$category = intval($category);
-			$sql_cat_ids = " OR pc.category_ID = '$category'";
-		}
-		$posts_in_ex_cats = $wpdb->get_col("SELECT p.ID from $wpdb->posts p LEFT JOIN $wpdb->post2cat pc ON pc.post_id = p.ID WHERE 1 = 0 $sql_cat_ids GROUP BY p.ID");
+		$posts_in_ex_cats = get_objects_in_term($blah, 'category');
 		$posts_in_ex_cats_sql = 'AND ID NOT IN (' . implode($posts_in_ex_cats, ',') . ')';
 	}
 
diff --git a/wp-includes/post.php b/wp-includes/post.php
index 0983bab992..7161dee5f6 100644
--- a/wp-includes/post.php
+++ b/wp-includes/post.php
@@ -230,13 +230,13 @@ function get_posts($args) {
 		$exclusions .= ')';
 
 	$query  = "SELECT DISTINCT * FROM $wpdb->posts ";
-	$query .= empty( $category ) ? '' : ", $wpdb->post2cat "; 
+	$query .= empty( $category ) ? '' : ", $wpdb->term_relationships, $wpdb->term_taxonomy  "; 
 	$query .= empty( $meta_key ) ? '' : ", $wpdb->postmeta ";
 	$query .= " WHERE 1=1 ";
 	$query .= empty( $post_type ) ? '' : "AND post_type = '$post_type' ";
 	$query .= empty( $post_status ) ? '' : "AND post_status = '$post_status' ";
 	$query .= "$exclusions $inclusions " ;
-	$query .= empty( $category ) ? '' : "AND ($wpdb->posts.ID = $wpdb->post2cat.post_id AND $wpdb->post2cat.category_id = " . $category. ") ";
+	$query .= empty( $category ) ? '' : "AND ($wpdb->posts.ID = $wpdb->term_relationships.object_id AND $wpdb->term_relationships.term_taxonomy_id = $wpdb->term_taxonomy.term_taxonomy_id AND $wpdb->term_taxonomy.term_id = " . $category. ") ";
 	$query .= empty( $post_parent ) ? '' : "AND $wpdb->posts.post_parent = '$post_parent' ";
 	$query .= empty( $meta_key ) | empty($meta_value)  ? '' : " AND ($wpdb->posts.ID = $wpdb->postmeta.post_id AND $wpdb->postmeta.meta_key = '$meta_key' AND $wpdb->postmeta.meta_value = '$meta_value' )";
 	$query .= " GROUP BY $wpdb->posts.ID ORDER BY " . $orderby . ' ' . $order;
@@ -1695,7 +1695,7 @@ function clean_page_cache($id) {
 }
 
 function update_post_category_cache($post_ids) {
-	global $wpdb, $category_cache, $tag_cache, $blog_id;
+	global $wpdb, $term_cache, $blog_id;
 	// TODO
 	return;
 	if ( empty($post_ids) )
@@ -1708,30 +1708,26 @@ function update_post_category_cache($post_ids) {
 	$count = count( $post_id_array);
 	for ( $i = 0; $i < $count; $i++ ) {
 		$post_id = (int) $post_id_array[ $i ];
-		if ( isset( $category_cache[$blog_id][$post_id] ) ) {
+		if ( isset( $term_cache[$blog_id][$post_id] ) ) {
 			unset( $post_id_array[ $i ] );
 			continue;
 		}
 	}
 	if ( count( $post_id_array ) == 0 )
 		return;
-	$post_id_list = join( ',', $post_id_array ); // with already cached stuff removed
 
-	$dogs = $wpdb->get_results("SELECT post_id, category_id, rel_type FROM $wpdb->post2cat WHERE post_id IN ($post_id_list)");
+	$dogs = get_object_terms($post_id_array, array('category', 'post_tag'));
 
 	if ( empty($dogs) )
 		return;
 
 	foreach ($dogs as $catt) {
-		if ( 'category' == $catt->rel_type )
-			$category_cache[$blog_id][$catt->post_id][$catt->category_id] = &get_category($catt->category_id);
-		elseif ( 'tag' == $catt->rel_type )
-			$tag_cache[$blog_id][$catt->post_id][$catt->category_id] = &get_category($catt->category_id);
+		$term_cache[$blog_id][$catt->post_id][$catt->taxonomy][$catt->category_id] = &get_category($catt->category_id);
 	}
 }
 
 function update_post_caches(&$posts) {
-	global $post_cache, $category_cache, $post_meta_cache, $tag_cache;
+	global $post_cache;
 	global $wpdb, $blog_id;
 
 	// No point in doing all this work if we didn't match any posts.
diff --git a/wp-includes/query.php b/wp-includes/query.php
index 413ff7d062..58f32a3a01 100644
--- a/wp-includes/query.php
+++ b/wp-includes/query.php
@@ -849,6 +849,7 @@ class WP_Query {
 				$cat = intval($cat);
 				$in = (strpos($cat, '-') !== false) ? false : true;
 				$cat = trim($cat, '-');
+				// TODO make an array, not a string, for out_cats.  use get_term_children()
 				if ( $in )
 					$in_cats .= "$cat, " . get_category_children($cat, '', ', ');
 				else
@@ -860,7 +861,7 @@ class WP_Query {
 				$in_cats = " AND $wpdb->term_taxonomy.term_id IN ({$q['cat']}) ";
 			if ( strlen($out_cats) > 0 ) {
 				// TODO use get_objects_in_term
-				$ids = $wpdb->get_col("SELECT post_id FROM $wpdb->post2cat WHERE $wpdb->post2cat.category_id IN ($out_cats)");
+				$ids = get_objects_in_terms($out_cats, 'category');
 				if ( is_array($ids) && count($ids > 0) ) {
 					foreach ( $ids as $id )
 						$out_posts .= "$id, ";
diff --git a/wp-includes/taxonomy.php b/wp-includes/taxonomy.php
index 35d1943467..e3f29cac06 100644
--- a/wp-includes/taxonomy.php
+++ b/wp-includes/taxonomy.php
@@ -379,6 +379,8 @@ function get_objects_in_term( $terms, $taxonomies, $args = array() ) {
 	$args = wp_parse_args( $args, $defaults );
 	extract($args);
 
+	$terms = array_map('intval', $terms);
+
 	$taxonomies = "'" . implode("', '", $taxonomies) . "'";
 	$terms = "'" . implode("', '", $terms) . "'";