# DB->join |

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    #include <db.h>

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

# Description

The DB->join function creates a specialized cursor for use in performing joins on secondary indexes. For information on how to organize your data to use this functionality, see Logical join.

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 cursors. Each cursor 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. 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 argument, and a nested iteration over each secondary cursor in the order they are specified in the curslist argument. 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 references the least number of data items to the one that references the most. By default, DB->join does this sort on behalf of its caller.

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 they reference. If the data are structured such 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 DB->join.

A newly created cursor is returned in the memory location referenced by dbcp and has the standard cursor functions:

DBcursor->c_get
  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:

  DB_JOIN_ITEM
    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 flag may be set by bitwise inclusively OR'ing it into the flags parameter:

  DB_RMW
    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.

DBcursor->c_put
  Returns EINVAL.

DBcursor->c_del
  Returns EINVAL.

DBcursor->c_close
  Close the returned cursor and release all resources. (Closing the cursors in curslist is the responsibility of the 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 DB->join function returns a non-zero error value on failure and 0 on success.

# Errors

The DB->join function may fail and return a non-zero error for the following conditions:

EINVAL
  An invalid flag value or parameter was specified.

  The DBcursor->c_put or DBcursor->c_del functions were called.

The DB->join function may fail and return a non-zero error for errors specified for other Berkeley DB and C library or system functions. If a catastrophic error has occurred, the DB->join function may fail and return DB_RUNRECOVERY, in which case all subsequent Berkeley DB calls will fail in the same way.

# See Also

db_create, DB->close, DB->cursor, DB->del, DB->err, DB->fd, DB->get, DB->get_byteswapped, DB->get_type, DB->join, DB->key_range, DB->open, DB->put, DB->remove, DB->set_bt_compare, DB->set_bt_minkey, DB->set_bt_prefix, DB->set_cachesize, DB->set_dup_compare, DB->set_errcall, DB->set_errfile, DB->set_errpfx, DB->set_flags, DB->set_h_ffactor, DB->set_h_hash, DB->set_h_nelem, DB->set_lorder, DB->set_malloc, DB->set_pagesize, DB->set_paniccall, DB->set_q_extentsize, DB->set_realloc, DB->set_re_delim, DB->set_re_len, DB->set_re_pad, DB->set_re_source, DB->stat, DB->sync, DB->upgrade and DB->verify.`

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