#include <db.h>

DB->join(DB *primary,
    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 DBcursor->get() and DBcursor->close() cursor functions:

The DB->join() method returns a non-zero error value on failure and 0 on success.



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 DBcursor->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.


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


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


    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.


The DB->join() method may fail and return one of the following non-zero errors:


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 return DB_REP_HANDLE_DEAD. The application will need to discard the handle and open a new one in order to continue processing.


The operation was blocked by client/master synchronization.


A secondary index references a nonexistent primary key.


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



See Also

Database and Related Methods