# Db::join |

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    #include <db_cxx.h>

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

# Description

The Db::join method creates a specialized 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 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 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 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 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.

The flags value 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 Db::join.

A newly created cursor is returned in the memory location to which dbcp refers. It 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 argument is then used as a key into the primary, and the key/data pair found in the primary is returned.

  The flags value 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_DIRTY_READ
    Read modified but not yet committed data. Silently ignored if the DB_DIRTY_READ flag was not specified when the underlying database was opened.

  DB_RMW
    Acquire write locks instead of read locks when doing the retrieval. 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.)

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 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.

# Errors

The Db::join method may fail and throw an exception or return a non-zero error for the following conditions:

DB_SECONDARY_BAD
  A secondary index references a nonexistent primary key.

EINVAL
  An invalid flag value or parameter was specified.

  Cursor functions other than Dbc::get or Dbc::close were called.

The Db::join 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::join method may fail and either return DB_RUNRECOVERY or throw a DbRunRecoveryException, in which case all subsequent Berkeley DB calls will fail in the same way.

# Class

Db

# See Also

Databases and Related Methods`

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