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<h2 class="title" style="clear: both"><a id="am_misc_stability"></a>Cursor stability</h2>
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<p>
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.
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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 <a href="../api_reference/C/dbcget.html#dbcget_DB_CURRENT" class="olink">DB_CURRENT</a> call on the cursor is guaranteed to
return the same record as was returned on the last call to the
cursor.
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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 <a href="../api_reference/C/dbcget.html#dbcget_DB_READ_COMMITTED" class="olink">DB_READ_COMMITTED</a> flag. Finally,
Berkeley DB provides degree 1 isolation when the
<a href="../api_reference/C/dbopen.html#dbopen_DB_READ_UNCOMMITTED" class="olink">DB_READ_UNCOMMITTED</a> flag is specified; that is, reads may
see data modified in transactions which have not yet
committed.
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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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