ref/am_misc/stability.html

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<title>Berkeley DB Reference Guide: Cursor stability</title>
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<td><b><dl><dt>Berkeley DB Reference Guide:<dd>Access Methods</dl></b></td>
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<p align=center><b>Cursor stability</b></p>
<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.</p>
<p>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_c/dbc_get.html#DB_CURRENT">DB_CURRENT</a> call
on the cursor is guaranteed to return the same record as was returned
on the last call to the cursor.</p>
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<p>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_c/db_cursor.html#DB_READ_COMMITTED">DB_READ_COMMITTED</a> flag.  Finally, Berkeley DB provides degree 1 isolation
when the <a href="../../api_c/db_open.html#DB_READ_UNCOMMITTED">DB_READ_UNCOMMITTED</a> flag is specified; that is, reads
may see data modified in transactions which have not yet committed.</p>
<p>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.</p>
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