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<!--$Id: db_associate.html 63573 2008-05-23 21:43:21Z trent.nelson $-->
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<title>Berkeley DB: Db::associate</title>
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<td>
<b>Db::associate</b>
</td>
<td align=right>
<a href="../api_cxx/api_core.html"><img src="../images/api.gif" alt="API"></a>
<a href="../ref/toc.html"><img src="../images/ref.gif" alt="Ref"></a></td>
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<tt>
<b><pre>
#include &lt;db_cxx.h&gt;
<p>
int
Db::associate(DbTxn *txnid, Db *secondary,
int (*callback)(Db *secondary,
const Dbt *key, const Dbt *data, Dbt *result), u_int32_t flags);
</pre></b>
<hr size=1 noshade>
<b>Description: Db::associate</b>
<p>The Db::associate function is used to declare one database a
secondary index for a primary database. After a secondary database has
been "associated" with a primary database, all updates to the primary
will be automatically reflected in the secondary and all reads from the
secondary will return corresponding data from the primary. Note that
as primary keys must be unique for secondary indices to work, the
primary database must be configured without support for duplicate data
items. See <a href="../ref/am/second.html">Secondary indices</a> for
more information.</p>
<p>The Db::associate method
either returns a non-zero error value
or throws an exception that encapsulates a non-zero error value on
failure, and returns 0 on success.
</p>
<b>Parameters</b> <br>
<b>callback</b><ul compact><li>The <b>callback</b> parameter is a callback function that creates the
set of secondary keys corresponding to a given primary key and data
pair.
<p>The callback parameter may be NULL if both the primary and secondary
database handles were opened with the <a href="../api_cxx/db_open.html#DB_RDONLY">DB_RDONLY</a> flag.</p>
<p>The callback takes four arguments:</p>
<br>
<b><b>secondary</b></b><ul compact><li>The <b>secondary</b> parameter is the database handle for the secondary.</ul>
<b><b>key</b></b><ul compact><li>The <b>key</b> parameter is a <a href="../api_cxx/dbt_class.html">Dbt</a> referencing the primary key.</ul>
<b><b>data</b></b><ul compact><li>The <b>data</b> parameter is a <a href="../api_cxx/dbt_class.html">Dbt</a> referencing the primary data
item.</ul>
<b><b>result</b></b><ul compact><li>The <b>result</b> parameter is a zeroed <a href="../api_cxx/dbt_class.html">Dbt</a> in which the callback
function should fill in <b>data</b> and <b>size</b> fields that describe
the secondary key or keys.</ul>
<br>
<p>The callback optionally returns some special values:</p>
<br>
<b><a name="DB_DBT_APPMALLOC">DB_DBT_APPMALLOC</a></b><ul compact><li>If the callback function needs to allocate memory for the <b>result</b>
data field (rather than simply pointing into the primary key or datum),
DB_DBT_APPMALLOC should be set in the <b>flags</b> field of the
<b>result</b> <a href="../api_cxx/dbt_class.html">Dbt</a>, which indicates that Berkeley DB should free the
memory when it is done with it.</ul>
<b><a name="DB_DBT_MULTIPLE">DB_DBT_MULTIPLE</a></b><ul compact><li>To return multiple secondary keys, DB_DBT_MULTIPLE should be set
in the <b>flags</b> field of the <b>result</b> <a href="../api_cxx/dbt_class.html">Dbt</a>, which
indicates Berkeley DB should treat the <b>size</b> field as the number of
secondary keys (zero or more), and the <b>data</b> field as a pointer
to an array of that number of <a href="../api_cxx/dbt_class.html">Dbt</a>s describing the set of
secondary keys.
<p><b>When multiple secondary keys are returned, keys may not be repeated</b>.
In other words, there must be no repeated record numbers in the array
for Recno and Queue databases, and keys must not compare equally using
the secondary database's comparison function for Btree and Hash
databases. If keys are repeated, operations may fail and the secondary
may become inconsistent with the primary.</p>
<p>The DB_DBT_APPMALLOC flag may be set for any <a href="../api_cxx/dbt_class.html">Dbt</a> in the
array of returned <a href="../api_cxx/dbt_class.html">Dbt</a>'s to indicate that Berkeley DB should free the
memory referenced by that particular <a href="../api_cxx/dbt_class.html">Dbt</a>'s data field when it
is done with it.</p>
<p>The DB_DBT_APPMALLOC flag may be combined with
DB_DBT_MULTIPLE in the <b>result</b> <a href="../api_cxx/dbt_class.html">Dbt</a>'s <b>flag</b>
field to indicate that Berkeley DB should free the array once it is done with
all of the returned keys.</p></ul>
<b><a name="DB_DONOTINDEX">DB_DONOTINDEX</a></b><ul compact><li>If any key/data pair in the primary yields a null secondary key and
should be left out of the secondary index, the callback function may
optionally return DB_DONOTINDEX. Otherwise, the callback
function should return 0 in case of success or an error outside of the
Berkeley DB name space in case of failure; the error code will be returned
from the Berkeley DB call that initiated the callback.
<p>If the callback function returns DB_DONOTINDEX for any key/data
pairs in the primary database, the secondary index will not contain any
reference to those key/data pairs, and such operations as cursor
iterations and range queries will reflect only the corresponding subset
of the database. If this is not desirable, the application should
ensure that the callback function is well-defined for all possible
values and never returns DB_DONOTINDEX.</p>
<p>Returning DB_DONOTINDEX is equivalent to setting
DB_DBT_MULTIPLE on the <b>result</b> <a href="../api_cxx/dbt_class.html">Dbt</a> and setting
the <b>size</b> field to zero.</p></ul>
<br>
<p>Berkeley DB is not re-entrant. Callback functions should not attempt to make
library calls (for example, to release locks or close open handles).
Re-entering Berkeley DB is not guaranteed to work correctly, and the results
are undefined.</p></ul>
<b>flags</b><ul compact><li>The <b>flags</b> parameter must be set to 0 or by bitwise inclusively <b>OR</b>'ing together one
or more of the following values:
<br>
<b><a name="DB_CREATE">DB_CREATE</a></b><ul compact><li>If the secondary database is empty, walk through the primary and create
an index to it in the empty secondary. This operation is potentially
very expensive.
<p>If the secondary database has been opened in an environment configured
with transactions, each put necessary for its creation will be done in
the context of a transaction created for the purpose.</p>
<p>Care should be taken not to use a newly-populated secondary database in
another thread of control until the Db::associate call has
returned successfully in the first thread.</p>
<p>If transactions are not being used, care should be taken not to modify
a primary database being used to populate a secondary database, in
another thread of control, until the Db::associate call has
returned successfully in the first thread. If transactions are being
used, Berkeley DB will perform appropriate locking and the application need
not do any special operation ordering.</p></ul>
<b><a name="DB_IMMUTABLE_KEY">DB_IMMUTABLE_KEY</a></b><ul compact><li>Specifies the secondary key is immutable.
<p>This flag can be used to optimize updates when the secondary key in a
primary record will never be changed after the primary record is
inserted. For immutable secondary keys, a best effort is made to avoid
calling the secondary callback function when primary records are
updated. This optimization may reduce the overhead of update operations
significantly if the callback function is expensive.</p>
<p>Be sure to specify this flag only if the secondary key in the primary
record is never changed. If this rule is violated, the secondary index
will become corrupted, that is, it will become out of sync with the
primary.</p></ul>
<br></ul>
<b>primary</b><ul compact><li>The associate method called should be a method of a database handle for
the primary database that is to be indexed.</ul>
<b>secondary</b><ul compact><li>The <b>secondary</b> parameter should be an open database handle of
either a newly created and empty database that is to be used to store
a secondary index, or of a database that was previously associated with
the same primary and contains a secondary index. Note that it is not
safe to associate as a secondary database a handle that is in use by
another thread of control or has open cursors. If the handle was opened
with the <a href="../api_cxx/env_open.html#DB_THREAD">DB_THREAD</a> flag it is safe to use it in multiple threads
of control after the Db::associate method has returned. Note also
that either secondary keys must be unique or the secondary database must
be configured with support for duplicate data items.</ul>
<b>txnid</b><ul compact><li>If the operation is part of an application-specified transaction, the
<b>txnid</b> parameter is a transaction handle returned from
<a href="../api_cxx/txn_begin.html">DbEnv::txn_begin</a>; if the operation is part of a Berkeley DB Concurrent Data Store group, the
<b>txnid</b> parameter is a handle returned from
<a href="../api_cxx/env_cdsgroup_begin.html">DbEnv::cdsgroup_begin</a>; otherwise NULL.
If no transaction handle is
specified, but the
operation occurs in a transactional
database,
the operation will be implicitly transaction protected.
</ul>
<br>
<br><b>Errors</b>
<p>The Db::associate method
may fail and throw
<a href="../api_cxx/except_class.html">DbException</a>,
encapsulating one of the following non-zero errors, or return one of
the following non-zero errors:</p>
<br>
<b>DB_REP_HANDLE_DEAD</b><ul compact><li>The database handle has been invalidated because a replication election
unrolled a committed transaction.</ul>
<br>
<br>
<b>DB_REP_LOCKOUT</b><ul compact><li>The operation was blocked by client/master synchronization.</ul>
<br>
<br>
<b>EINVAL</b><ul compact><li>If the secondary database handle has already been associated with this or
another database handle; the secondary database handle is not open; the
primary database has been configured to allow duplicates; or if an
invalid flag value or parameter was specified.</ul>
<br>
<hr size=1 noshade>
<br><b>Class</b>
<a href="../api_cxx/db_class.html">Db</a>
<br><b>See Also</b>
<a href="../api_cxx/db_list.html">Databases and Related Methods</a>
</tt>
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