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<!--$Id: db_join.so,v 10.57 2005/02/10 20:02:28 bostic Exp $-->
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<h3>DB->join</h3>
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<h3><pre>
#include <db.h>
<p>
int
DB->join(DB *primary,
DBC **curslist, DBC **dbcp, u_int32_t flags);
</pre></h3>
<hr size=1 noshade>
<h3>Description: DB->join</h3>
<p>The DB->join method creates a specialized join 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
<a href="../ref/am/join.html">Equality join</a>.</p>
<p>The DB->join method method is called using the <a href="../api_c/db_class.html">DB</a> handle of
the primary database.</p>
<p>The join cursor supports only the <a href="../api_c/dbc_get.html">DBcursor->c_get</a> and <b>dbc_close</b>
cursor functions:</p>
<dl compact>
<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
<b>curslist</b> was initialized. Any data value that appears in all
items specified by the <b>curslist</b> parameter is then used as a key
into the <b>primary</b>, and the key/data pair found in the
<b>primary</b> is returned.
The <b>flags</b> parameter must be set to 0 or
the following value:
<dl compact>
<dt><a name="DB_JOIN_ITEM">DB_JOIN_ITEM</a><dd>Do not use the data value found in all the cursors as a lookup key for
the <b>primary</b>, but simply return it in the key parameter instead.
The data parameter is left unchanged.
</dl>
In addition, the following flag may be set by
bitwise inclusively <b>OR</b>'ing it into the <b>flags</b> parameter:
<dl compact>
<dt><a name="DB_READ_UNCOMMITTED">DB_READ_UNCOMMITTED</a><dd>Read modified but not yet committed data. Silently ignored if the
<a href="../api_c/db_open.html#DB_READ_UNCOMMITTED">DB_READ_UNCOMMITTED</a> flag was not specified when the underlying
database was opened.
<dt><a name="DB_RMW">DB_RMW</a><dd>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.
</dl>
<dt><a href="../api_c/dbc_close.html">DBcursor->c_close</a><dd>Close the returned cursor and release all resources. (Closing the cursors
in <b>curslist</b> is the responsibility of the caller.)
</dl>
<p>The DB->join method
returns a non-zero error value on failure
and 0 on success.
</p>
<h3>Parameters</h3>
<dl compact>
<dt><b>curslist</b><dd>The <b>curslist</b> parameter 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 <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
cursors have been passed as part of a <b>curslist</b>, they should not
be accessed or modified until the newly created join cursor has been closed,
or else inconsistent results may be returned.
<p>Joined values are retrieved by doing a sequential iteration over the first
cursor in the <b>curslist</b> parameter, and a nested iteration over each
secondary cursor in the order they are specified in the <b>curslist</b>
parameter. 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.</p>
<p>For the returned join cursor to be used in a transaction-protected manner,
the cursors listed in <b>curslist</b> must have been created within the
context of the same transaction.</p>
<dt><b>dbcp</b><dd>The newly created join cursor is returned in the memory location to
which <b>dbcp</b> refers.
<dt><b>flags</b><dd>The <b>flags</b> parameter must be set to 0 or
the following value:
<dl compact>
<dt><a name="DB_JOIN_NOSORT">DB_JOIN_NOSORT</a><dd>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.
</dl>
</dl>
<h3>Errors</h3>
<p>The DB->join method
may fail and return one of the following non-zero errors:</p>
<dl compact>
<dt>DB_REP_HANDLE_DEAD<dd>The database handle has been invalidated because a replication election
unrolled a committed transaction.
<dt>DB_REP_LOCKOUT<dd>The operation was blocked by client/master synchronization.
</dl>
<dl compact>
<dt>DB_SECONDARY_BAD<dd>A secondary index references a nonexistent primary key.
</dl>
<dl compact>
<dt>EINVAL<dd>If cursor methods other than <a href="../api_c/dbc_get.html">DBcursor->c_get</a> or <a href="../api_c/dbc_close.html">DBcursor->c_close</a> were
called; or if an
invalid flag value or parameter was specified.
</dl>
<hr size=1 noshade>
<h3>Class</h3>
<a href="../api_c/db_class.html">DB</a>
<h3>See Also</h3>
<a href="../api_c/db_list.html">Databases and Related Methods</a>
</tt>
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