Techniques are provided for efficiently collecting statistics for hierarchically-organized collections of data. A database system leverages container-level modification time stamps and stored subtree-level change information to gather statistical information from only those resources in a hierarchical collection for which the statistics have changed since the last time that statistics were gathered for the collection. A lockless data structure is also described for storing the subtree-level change information in which an identifier corresponding to each subtree in a collection containing a changed resource may be stored. This data structure may be a table that is distinct from one or more tables representing the collection. In one embodiment of the invention, the immediate parent resource of a particular modified resource may be omitted from the subtree table by leveraging modification time stamps while gathering statistics based on tracked subtree-level information.