Make SQL selects less expensive in memory for some cases where we fetch big data chunks

program/lib/Roundcube/rcube_cache.php

@@ -308,6 +308,8 @@ class rcube_cache
                     $data = $this->unserialize($sql_arr['data']);
                 }
 
+                $this->db->reset();
+
                 if ($nostore) {
                     return $data;
                 }

program/lib/Roundcube/rcube_cache_shared.php

@@ -302,6 +302,8 @@ class rcube_cache_shared
                     $data = $this->unserialize($sql_arr['data']);
                 }
 
+                $this->db->reset();
+
                 if ($nostore) {
                     return $data;
                 }

program/lib/Roundcube/rcube_db.php

@@ -773,6 +773,16 @@ class rcube_db
         return $this->last_result = $this->dbh->rollBack();
     }
 
+    /**
+     * Release resources related to the last query result.
+     * When we know we don't need to access the last query result we can destroy it
+     * and release memory. Usefull especially if the query returned big chunk of data.
+     */
+    public function reset()
+    {
+        $this->last_result = null;
+    }
+
     /**
      * Formats input so it can be safely used in a query
      *

program/lib/Roundcube/rcube_imap_cache.php

@@ -353,6 +353,8 @@ class rcube_imap_cache
                 }
             }
 
+            $this->db->reset();
+
             $msgs = array_flip($msgs);
         }
 

program/lib/Roundcube/rcube_session_db.php

@@ -106,6 +106,8 @@ class rcube_session_db extends rcube_session
             $this->vars      = base64_decode($sql_arr['vars']);
             $this->key       = $key;
 
+            $this->db->reset();
+
             return !empty($this->vars) ? (string) $this->vars : '';
         }