UB-Tree Space Management

Currently, Astore indexes depend on AutoVacuum and FSM for space management. The space may not be recycled in a timely manner. However, Ustore indexes use the UB-tree recycle queue (URQ) to manage idle index space. The URQ contains two circular queues: potential empty page queue and available empty page queue. Completing space management of indexes in a DML process can effectively alleviate the sharp space expansion caused during the DML process. Index recycle queues are separately stored in FSM files corresponding to the B-tree indexes.

As shown in the preceding figure, the index page flow in the URQ is as follows:

1. Index empty page > Potential queue

   The index page tail column records the number of active tuples (activeTupleCount) on the page. During the DML process, all tuples on a page are deleted, that is, when activeTupleCount is set to 0, the index page is placed in the potential queue.

2. Potential queue > Available queue

   The flow from a potential queue to an available queue mainly achieves an income and expense balance for the potential queue and ensure that pages are available for the available queue. That is, after an index empty page is used up in an available queue, at least one index page is transferred from a potential queue to the available queue. Besides, if a new index page is added to a potential queue, at least one index page can be removed from the potential queue and inserted into the available queue.

3. Available queue > Index empty page

   When an empty index page is obtained during index splitting, the system first searches an available queue for an index page that can be reused. If such index page is found, it is directly reused. If no index page can be reused, physical page expansion is performed.