Db::join()
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## Db::join()

    #include <db_cxx.h>

    int
    Db::join(Dbc **curslist, Dbc **dbcp, u_int32_t flags);

The `Db::join()` method creates a specialized join cursor for use in performing equality or natural joins on secondary indices. For information on how to organize your data to use this functionality, see Equality join.

The `Db::join()` method is called using the Db handle of the primary database.

The join cursor supports only the Dbc::get() and Dbc::close() cursor functions:

- Dbc::get()

  Iterates over the values associated with the keys to which each item in curslist was initialized. Any data value that appears in all items specified by the curslist parameter is then used as a key into the primary, and the key/data pair found in the primary is returned. The flags parameter must be set to 0 or the following value:

  - `DB_JOIN_ITEM`

    Do not use the data value found in all the cursors as a lookup key for the primary, but simply return it in the key parameter instead. The data parameter is left unchanged.

  In addition, the following flag may be set by bitwise inclusively OR'ing it into the flags parameter:

  - `DB_READ_UNCOMMITTED`

    Configure a transactional join operation to have degree 1 isolation, reading modified but not yet committed data. Silently ignored if the DB_READ_UNCOMMITTED flag was not specified when the underlying database was opened.

  - `DB_RMW`

    Acquire write locks instead of read locks when doing the read, if locking is configured. Setting this flag can eliminate deadlock during a read-modify-write cycle by acquiring the write lock during the read part of the cycle so that another thread of control acquiring a read lock for the same item, in its own read-modify-write cycle, will not result in deadlock.

- Dbc::close()

  Close the returned cursor and release all resources. (Closing the cursors in curslist is the responsibility of the caller.)

The `Db::join()` 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.

### Parameters

#### curslist

The curslist parameter contains a NULL terminated array of cursors. Each cursor must have been initialized to refer to the key on which the underlying database should be joined. Typically, this initialization is done by a Dbc::get() call with the DB_SET flag specified. Once the cursors have been passed as part of a curslist, they should not be accessed or modified until the newly created join cursor has been closed, or else inconsistent results may be returned.

Joined values are retrieved by doing a sequential iteration over the first cursor in the curslist parameter, and a nested iteration over each secondary cursor in the order they are specified in the curslist parameter. This requires database traversals to search for the current datum in all the cursors after the first. For this reason, the best join performance normally results from sorting the cursors from the one that refers to the least number of data items to the one that refers to the most. By default, `Db::join()` does this sort on behalf of its caller.

For the returned join cursor to be used in a transaction-protected manner, the cursors listed in curslist must have been created within the context of the same transaction.

#### dbcp

The newly created join cursor is returned in the memory location to which dbcp refers.

#### flags

The flags parameter must be set to 0 or the following value:

- `DB_JOIN_NOSORT`

  Do not sort the cursors based on the number of data items to which they refer. If the data are structured so that cursors with many data items also share many common elements, higher performance will result from listing those cursors before cursors with fewer data items; that is, a sort order other than the default. The DB_JOIN_NOSORT flag permits applications to perform join optimization prior to calling the `Db::join()` method.

### Errors

The `Db::join()` method may fail and throw a DbException exception, encapsulating one of the following non-zero errors, or return one of the following non-zero errors:

#### DbRepHandleDeadException or DB_REP_HANDLE_DEAD

When a client synchronizes with the master, it is possible for committed transactions to be rolled back. This invalidates all the database and cursor handles opened in the replication environment. Once this occurs, an attempt to use such a handle will throw a DbRepHandleDeadException (if your application is configured to throw exceptions), or return `DB_REP_HANDLE_DEAD`. The application will need to discard the handle and open a new one in order to continue processing.

#### DbDeadlockException or DB_REP_LOCKOUT

The operation was blocked by client/master synchronization.

DbDeadlockException is thrown if your Berkeley DB API is configured to throw exceptions. Otherwise, `DB_REP_LOCKOUT` is returned.

#### DB_SECONDARY_BAD

A secondary index references a nonexistent primary key.

#### EINVAL

If cursor methods other than Dbc::get() or Dbc::close() were called; or if an invalid flag value or parameter was specified.

### Class

Db

### See Also

Database and Related Methods

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