Berkeley DB: Db->join
Полезная информация


#include <db.h>

int DB->join(DB *primary, DBC **curslist, u_int32_t flags, DBC **dbcp);


The Db->join function creates a specialized cursor for use in performing joins on secondary indices. Your data must be organized in the following manner in order to take advantage of this:

  1. The actual data should be stored in the database represented by the primary DB handle.

  2. Secondary indices should be stored in separate database files, whose keys are the values of the secondary indices and whose data items are the primary keys corresponding to the records having the designated secondary key value. It is acceptable (and expected) that there may be duplicate entries in the secondary indices. These duplicate entries should be sorted. For more information see the DB_INFO structure's DB_DUPSORT flag.

The primary argument contains the DB handle of the primary database, which is keyed by the data values found in entries in the curslist.

The curslist argument contains a NULL terminated array of DBC structures. Each DBC must have been initialized to reference the key on which the underlying database should be joined. Typically, this initialization is done by a DBcursor->c_get call with the DB_SET flag specified.

The flags parameter is currently unused, and must be set to 0.

The newly created cursor is returned in the memory location referenced by dbcp.

The returned cursor has the standard cursor functions, that behave as follows:

Iterates over the values associated with the keys to which each item in curslist has been initialized. Any data value which appears in all items specified by the curslist argument 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:

Do not use the data value found in all of 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 value may be set by logically OR'ing it into the flags parameter:

Acquire write locks instead of read locks when doing the retrieval. Setting this flag may decrease the likelihood of deadlock during a read-modify-write cycle by immediately 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.

Returns EINVAL.

Returns EINVAL.

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

In a transaction protected environment, all of the cursors listed in curslist must have been created within the same transaction.

The Db->join function returns the value of errno on failure, and 0 on success.


If a fatal error occurs in Berkeley DB, the Db->join function may fail and return DB_RUNRECOVERY, at which point all subsequent database calls will also return DB_RUNRECOVERY.

The Db->join function may fail and return errno for any of the errors specified for the following Berkeley DB and C library functions: fflush(3), fprintf(3), free(3), malloc(3), memset(3), vfprintf(3), and vsnprintf(3).

In addition, the Db->join function may fail and return errno for the following conditions:

An invalid flag value or parameter was specified.

The c_put or c_del functions were called.

See Also

db_appexit, db_appinit, db_version, DB->close, DB->cursor, DB->del, DB->fd, DB->get, Db->join, db_open, DB->put, DB->stat, DB->sync, DBcursor->c_close, DBcursor->c_del, DBcursor->c_get and DBcursor->c_put.