tell mysql to ignore the sort index for search queries

Signed-off-by: Robin Appelman <robin@icewind.nl>

Author: icewind1991

lib/private/DB/QueryBuilder/QueryBuilder.php

@@ -1113,7 +1113,7 @@ public function orderBy($sort, $order = null) {
 	/**
 	 * Adds an ordering to the query results.
 	 *
-	 * @param string $sort The ordering expression.
+	 * @param string|ILiteral|IParameter|IQueryFunction $sort The ordering expression.
 	 * @param string $order The ordering direction.
 	 *
 	 * @return $this This QueryBuilder instance.

lib/private/Files/Cache/SearchBuilder.php

@@ -232,6 +232,17 @@ public function addSearchOrdersToQuery(IQueryBuilder $query, array $orders) {
 			if ($field === 'fileid') {
 				$field = 'file.fileid';
 			}
+
+			// Mysql really likes to pick an index for sorting if it can't fully satisfy the where
+			// filter with an index, since search queries pretty much never are fully filtered by index
+			// mysql often picks an index for sorting instead of the much more useful index for filtering.
+			//
+			// By changing the order by to an expression, mysql isn't smart enough to see that it could still
+			// use the index, so it instead picks an index for the filtering
+			if ($field === 'mtime') {
+				$field = $query->func()->add($field, $query->createNamedParameter(0));
+			}
+
 			$query->addOrderBy($field, $order->getDirection());
 		}
 	}