m4_comment([$Id: dup.so,v 10.28 2003/10/18 19:15:54 bostic Exp $]) m4_ref_title(Access Methods, Duplicate data items, @duplicate data items, am_conf/byteorder, am_conf/malloc) m4_p([dnl The Btree and Hash access methods support the creation of multiple data items for a single key item. By default, multiple data items are not permitted, and each database store operation will overwrite any previous data item for that key. To configure m4_db for duplicate data items, call the m4_refT(dbh_set_flags) with the m4_ref(DB_DUP) flag. Only one copy of the key will be stored for each set of duplicate data items. If the Btree access method comparison routine returns that two keys compare equally, it is undefined which of the two keys will be stored and returned from future database operations.]) m4_p([dnl By default, m4_db stores duplicates in the order in which they were added, that is, each new duplicate data item will be stored after any already existing data items. This default behavior can be overridden by using the m4_refT(dbc_put) and one of the m4_ref(DB_AFTER), m4_ref(DB_BEFORE) m4_ref(DB_KEYFIRST) or m4_ref(DB_KEYLAST) flags. Alternatively, m4_db may be configured to sort duplicate data items.]) m4_p([dnl When stepping through the database sequentially, duplicate data items will be returned individually, as a key/data pair, where the key item only changes after the last duplicate data item has been returned. For this reason, duplicate data items cannot be accessed using the m4_refT(dbh_get), as it always returns the first of the duplicate data items. Duplicate data items should be retrieved using a m4_db cursor interface such as the m4_refT(dbc_get).]) m4_p([dnl There is a flag that permits applications to request the following data item only if it m4_bold(is) a duplicate data item of the current entry, see m4_ref(DB_NEXT_DUP) for more information. There is a flag that permits applications to request the following data item only if it m4_bold(is not) a duplicate data item of the current entry, see m4_ref(DB_NEXT_NODUP) and m4_ref(DB_PREV_NODUP) for more information.]) m4_p([dnl It is also possible to maintain duplicate records in sorted order. Sorting duplicates will significantly increase performance when searching them and performing equality joins, common operations when using secondary indices. To configure m4_db to sort duplicate data items, the application must call the m4_refT(dbh_set_flags) with the m4_ref(DB_DUPSORT) flag (in addition to the m4_ref(DB_DUP) flag). In addition, a custom comparison function may be specified using the m4_refT(dbh_set_dup_compare). If the m4_ref(DB_DUPSORT) flag is given, but no comparison routine is specified, then m4_db defaults to the same lexicographical sorting used for Btree keys, with shorter items collating before longer items.]) m4_p([dnl If the duplicate data items are unsorted, applications may store identical duplicate data items, or, for those that just like the way it sounds, m4_italic(duplicate duplicates).]) m4_p([dnl m4_bold([In this release it is an error to attempt to store identical duplicate data items when duplicates are being stored in a sorted order.]) This restriction is expected to be lifted in a future release. There is a flag that permits applications to disallow storing duplicate data items when the database has been configured for sorted duplicates, see m4_ref(DB_NODUPDATA) for more information. Applications not wanting to permit duplicate duplicates in databases configured for sorted duplicates should begin using the m4_ref(DB_NODUPDATA) flag immediately.]) m4_p([dnl For further information on how searching and insertion behaves in the presence of duplicates (sorted or not), see the m4_ref(dbh_get), m4_ref(dbh_put), m4_ref(dbc_get) and m4_ref(dbc_put) documentation.]) m4_page_footer
