Db::associate(DbTxn *txnid, Db *secondary,
int (*callback)(Db *, const Dbt *, const Dbt *, Dbt *),
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 Secondary indices for
The associate method called should be a method off a database handle for
the primary database that is to be indexed.
The secondary argument 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 DB_THREAD 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.
If the operation is to be transaction-protected (other than by specifying
the DB_AUTO_COMMIT flag), the txnid parameter is a
transaction handle returned from DbEnv::txn_begin; otherwise, NULL.
The callback argument should refer to a callback function that
creates a secondary key from a given primary key and data pair. When
called, the first argument will be the secondary Db handle; the
second and third arguments will be Dbts containing a primary
key and datum respectively; and the fourth argument will be a zeroed
DBT in which the callback function should fill in data and
size fields that describe the secondary key.
If the callback function needs to allocate memory for the data
field rather than simply pointing into the primary key or datum, the
flags field of the returned Dbt should be set to
DB_DBT_APPMALLOC, which indicates that Berkeley DB should free the
memory when it is done with it.
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 any other integer error
code in case of failure; the error code will be returned from the Berkeley DB
interface call that initiated the callback. Note that 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
The callback argument may be NULL if and only if both the primary and
secondary database handles were opened with the DB_RDONLY flag.
The flags value must be set to 0 or
the following value:
- If the secondary database is empty, walk through the primary and create
an index to it in the empty secondary. This operation is potentially
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.
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.
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.
In addition, the following flag may be set by
bitwise inclusively OR'ing it into the flags parameter:
- Enclose the Db::associate call within a transaction. If the call succeeds,
changes made by the operation will be recoverable. If the call fails,
the operation will have made no changes.
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.
The Db::associate method may fail and throw an exception or return a non-zero error for the following conditions:
- An invalid flag value or parameter was specified.
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.
The Db::associate method may fail and throw an exception or return a non-zero error for errors specified for other Berkeley DB and C library or system methods.
If a catastrophic error has occurred, the Db::associate method may fail and
either return DB_RUNRECOVERY or throw a
in which case all subsequent Berkeley DB calls will fail in the same way.
Databases and Related Methods
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