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Berkeley DB Reference Guide:
  Access Methods

### Cursor stability

In the absence of locking, no guarantees are made about the stability of cursors in different threads of control. However, the Btree, Queue and Recno access methods guarantee that cursor operations, interspersed with any other operation in the same thread of control will always return keys in order and will return each non-deleted key/data pair exactly once. Because the Hash access method uses a dynamic hashing algorithm, it cannot guarantee any form of stability in the presence of inserts and deletes unless transactional locking is performed.

If locking was specified when the Berkeley DB environment was opened, but transactions are not in effect, the access methods provide repeatable reads with respect to the cursor. That is, a DB_CURRENT call on the cursor is guaranteed to return the same record as was returned on the last call to the cursor.

In the presence of transactions, the Btree, Hash and Recno access methods provide degree 3 isolation (serializable transactions). The Queue access method provides degree 3 isolation with the exception that it permits phantom records to appear between calls. That is, deleted records are not locked, therefore another transaction may replace a deleted record between two calls to retrieve it. The record would not appear in the first call but would be seen by the second call. For readers not enclosed in transactions, all access method calls provide degree 2 isolation, that is, reads are not repeatable. A transaction may be declared to run with degree 2 isolation by specifying the DB_DEGREE_2 flag. Finally, Berkeley DB provides degree 1 isolation when the DB_DIRTY_READ flag is specified; that is, reads may see data modified in transactions which have not yet committed.

For all access methods, a cursor scan of the database performed within the context of a transaction is guaranteed to return each key/data pair once and only once, except in the following case. If, while performing a cursor scan using the Hash access method, the transaction performing the scan inserts a new pair into the database, it is possible that duplicate key/data pairs will be returned.

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