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<title>Berkeley DB Reference Guide: Cursor stability</title>
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<td><h3><dl><dt>Berkeley DB Reference Guide:<dd>Access Methods</dl></h3></td>
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<h1 align=center>Cursor stability</h1>
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<p>In the absence of locking, no guarantees are made about the stability
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of cursors in different threads of control. However, the Btree, Queue
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and Recno access methods guarantee that cursor operations, interspersed
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with any other operation in the same thread of control will always
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return keys in order and will return each non-deleted key/data pair
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exactly once. Because the Hash access method uses a dynamic hashing
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algorithm, it cannot guarantee any form of stability in the presence of
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inserts and deletes unless transactional locking is performed.
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<p>If locking was specified when the Berkeley DB environment was opened, but
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transactions are not in effect, the access methods provide repeatable
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reads with respect to the cursor. That is, a <a href="../../api_c/dbc_get.html#DB_CURRENT">DB_CURRENT</a> call
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on the cursor is guaranteed to return the same record as was returned
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on the last call to the cursor.
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<p>In the presence of transactions, the Btree, Hash and Recno access
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methods provide degree 3 isolation (serializable transactions). The
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Queue access method provides degree 3 isolation with the exception that
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it permits phantom records to appear between calls. That is, deleted
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records are not locked, therefore another transaction may replace a
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deleted record between two calls to retrieve it. The record would not
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appear in the first call but would be seen by the second call. For
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readers not enclosed in transactions, all access method calls provide
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degree 2 isolation, that is, reads are not repeatable. Finally, Berkeley DB
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provides degree 1 isolation when the <a href="../../api_c/db_open.html#DB_DIRTY_READ">DB_DIRTY_READ</a> flag is
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specified; that is, reads may see data modified in transactions which
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have not yet committed.
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<p>For all access methods, a cursor scan of the database performed within
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the context of a transaction is guaranteed to return each key/data pair
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once and only once, except in the following case. If, while performing
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a cursor scan using the Hash access method, the transaction performing
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the scan inserts a new pair into the database, it is possible that
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duplicate key/data pairs will be returned.
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