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<!--$Id: db_join.so,v 10.30 2000/12/20 15:34:50 bostic Exp $-->
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<!--Copyright 1997, 1998, 1999, 2000 by Sleepycat Software, Inc.-->
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<!--All rights reserved.-->
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<html>
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<head>
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<title>Berkeley DB: DB->join</title>
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<meta name="description" content="Berkeley DB: An embedded database programmatic toolkit.">
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<meta name="keywords" content="embedded,database,programmatic,toolkit,b+tree,btree,hash,hashing,transaction,transactions,locking,logging,access method,access methods,java,C,C++">
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</head>
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<body bgcolor=white>
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<a name="2"><!--meow--></a>
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<table><tr valign=top>
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<td>
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<h1>DB->join</h1>
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</td>
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<td width="1%">
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<a href="../api_c/c_index.html"><img src="../images/api.gif" alt="API"></a><a href="../ref/toc.html"><img src="../images/ref.gif" alt="Ref"></a>
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</td></tr></table>
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<hr size=1 noshade>
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<tt>
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<h3><pre>
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#include <db.h>
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<p>
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int
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DB->join(DB *primary,
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DBC **curslist, DBC **dbcp, u_int32_t flags);
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</pre></h3>
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<h1>Description</h1>
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<p>The DB->join function creates a specialized cursor for use in performing
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joins on secondary indexes. For information on how to organize your data
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to use this functionality, see <a href="../ref/am/join.html">Logical
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join</a>.
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<p>The <b>primary</b> argument contains the DB handle of the primary
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database, which is keyed by the data values found in entries in the
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<b>curslist</b>.
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<p>The <b>curslist</b> argument contains a NULL terminated array of cursors.
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Each cursor must have been initialized to reference the key on which the
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underlying database should be joined. Typically, this initialization is done
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by a <a href="../api_c/dbc_get.html">DBcursor->c_get</a> call with the <a href="../api_c/dbc_get.html#DB_SET">DB_SET</a> flag specified. Once the
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cursors have been passed as part of a <b>curslist</b>, they should not
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be accessed or modified until the newly created join cursor has been closed,
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or else inconsistent results may be returned.
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<p>Joined values are retrieved by doing a sequential iteration over the first
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cursor in the <b>curslist</b> argument, and a nested iteration over each
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secondary cursor in the order they are specified in the <b>curslist</b>
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argument. This requires database traversals to search for the current
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datum in all the cursors after the first. For this reason, the best join
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performance normally results from sorting the cursors from the one that
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references the least number of data items to the one that references the
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most. By default, DB->join does this sort on behalf of its caller.
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<p>The <b>flags</b> parameter must be set to 0 or the following value:
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<p><dl compact>
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<p><dt><a name="DB_JOIN_NOSORT">DB_JOIN_NOSORT</a><dd>Do not sort the cursors based on the number of data items they reference.
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If the data are structured such that cursors with many data items also
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share many common elements, higher performance will result from listing
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those cursors before cursors with fewer data items, that is, a sort order
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other than the default. The DB_JOIN_NOSORT flag permits
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applications to perform join optimization prior to calling DB->join.
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</dl>
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<p>A newly created cursor is returned in the memory location referenced by
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<b>dbcp</b> and has the standard cursor functions:
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<p><dl compact>
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<p><dt><a href="../api_c/dbc_get.html">DBcursor->c_get</a><dd>Iterates over the values associated with the keys to which each item in
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<b>curslist</b> has been initialized. Any data value which appears in
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all items specified by the <b>curslist</b> argument is then used as a
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key into the <b>primary</b>, and the key/data pair found in the
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<b>primary</b> is returned.
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<p>The <b>flags</b> parameter must be set to 0 or the following value:
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<p><dl compact>
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<p><dt><a name="DB_JOIN_ITEM">DB_JOIN_ITEM</a><dd>Do not use the data value found in all of the cursors as a lookup
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key for the <b>primary</b>, but simply return it in the key parameter
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instead. The data parameter is left unchanged.
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</dl>
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<p>In addition, the following flag may be set by bitwise inclusively <b>OR</b>'ing it into the
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<b>flags</b> parameter:
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<p><dl compact>
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<p><dt><a name="DB_RMW">DB_RMW</a><dd>Acquire write locks instead of read locks when doing the retrieval.
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Setting this flag may decrease the likelihood of deadlock during a
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read-modify-write cycle by immediately acquiring the write lock during
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the read part of the cycle so that another thread of control acquiring
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a read lock for the same item, in its own read-modify-write cycle, will
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not result in deadlock.
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</dl>
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<p><dt><a href="../api_c/dbc_put.html">DBcursor->c_put</a><dd>Returns EINVAL.
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<p><dt><a href="../api_c/dbc_del.html">DBcursor->c_del</a><dd>Returns EINVAL.
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<p><dt><a href="../api_c/dbc_close.html">DBcursor->c_close</a><dd>Close the returned cursor and release all resources. (Closing the cursors
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in <b>curslist</b> is the responsibility of the caller.)
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</dl>
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<p>For the returned join cursor to be used in a transaction protected manner,
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the cursors listed in <b>curslist</b> must have been created within the
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context of the same transaction.
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<p>The DB->join function returns a non-zero error value on failure and 0 on success.
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<h1>Errors</h1>
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<p>The DB->join function may fail and return a non-zero error for the following conditions:
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<p><dl compact>
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<p><dt>EINVAL<dd>An invalid flag value or parameter was specified.
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<p>The <a href="../api_c/dbc_put.html">DBcursor->c_put</a> or <a href="../api_c/dbc_del.html">DBcursor->c_del</a> functions were called.
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</dl>
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<p>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.
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If a catastrophic error has occurred, the DB->join function may fail and return
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<a href="../ref/program/errorret.html#DB_RUNRECOVERY">DB_RUNRECOVERY</a>, in which case all subsequent Berkeley DB calls will fail
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in the same way.
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<h1>See Also</h1>
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<a href="../api_c/db_create.html">db_create</a>,
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<a href="../api_c/db_close.html">DB->close</a>,
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<a href="../api_c/db_cursor.html">DB->cursor</a>,
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<a href="../api_c/db_del.html">DB->del</a>,
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<a href="../api_c/db_err.html">DB->err</a>,
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<a href="../api_c/db_fd.html">DB->fd</a>,
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<a href="../api_c/db_get.html">DB->get</a>,
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<a href="../api_c/db_get_byteswapped.html">DB->get_byteswapped</a>,
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<a href="../api_c/db_get_type.html">DB->get_type</a>,
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<a href="../api_c/db_join.html">DB->join</a>,
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<a href="../api_c/db_key_range.html">DB->key_range</a>,
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<a href="../api_c/db_open.html">DB->open</a>,
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<a href="../api_c/db_put.html">DB->put</a>,
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<a href="../api_c/db_remove.html">DB->remove</a>,
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<a href="../api_c/db_set_bt_compare.html">DB->set_bt_compare</a>,
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<a href="../api_c/db_set_bt_minkey.html">DB->set_bt_minkey</a>,
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<a href="../api_c/db_set_bt_prefix.html">DB->set_bt_prefix</a>,
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<a href="../api_c/db_set_cachesize.html">DB->set_cachesize</a>,
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<a href="../api_c/db_set_dup_compare.html">DB->set_dup_compare</a>,
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<a href="../api_c/db_set_errcall.html">DB->set_errcall</a>,
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<a href="../api_c/db_set_errfile.html">DB->set_errfile</a>,
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<a href="../api_c/db_set_errpfx.html">DB->set_errpfx</a>,
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<a href="../api_c/db_set_flags.html">DB->set_flags</a>,
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<a href="../api_c/db_set_h_ffactor.html">DB->set_h_ffactor</a>,
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<a href="../api_c/db_set_h_hash.html">DB->set_h_hash</a>,
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<a href="../api_c/db_set_h_nelem.html">DB->set_h_nelem</a>,
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<a href="../api_c/db_set_lorder.html">DB->set_lorder</a>,
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<a href="../api_c/db_set_malloc.html">DB->set_malloc</a>,
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<a href="../api_c/db_set_pagesize.html">DB->set_pagesize</a>,
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<a href="../api_c/db_set_paniccall.html">DB->set_paniccall</a>,
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<a href="../api_c/db_set_q_extentsize.html">DB->set_q_extentsize</a>,
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<a href="../api_c/db_set_realloc.html">DB->set_realloc</a>,
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<a href="../api_c/db_set_re_delim.html">DB->set_re_delim</a>,
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<a href="../api_c/db_set_re_len.html">DB->set_re_len</a>,
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<a href="../api_c/db_set_re_pad.html">DB->set_re_pad</a>,
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<a href="../api_c/db_set_re_source.html">DB->set_re_source</a>,
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<a href="../api_c/db_stat.html">DB->stat</a>,
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<a href="../api_c/db_sync.html">DB->sync</a>,
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<a href="../api_c/db_upgrade.html">DB->upgrade</a>
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and
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<a href="../api_c/db_verify.html">DB->verify</a>.
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</tt>
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<table><tr><td><br></td><td width="1%">
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<a href="../api_c/c_index.html"><img src="../images/api.gif" alt="API"></a><a href="../ref/toc.html"><img src="../images/ref.gif" alt="Ref"></a>
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</td></tr></table>
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<p><font size=1><a href="http://www.sleepycat.com">Copyright Sleepycat Software</a></font>
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