1<?xml version="1.0" encoding="UTF-8" standalone="no"?> 2<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> 3<html xmlns="http://www.w3.org/1999/xhtml"> 4 <head> 5 <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /> 6 <title>Db::join()</title> 7 <link rel="stylesheet" href="apiReference.css" type="text/css" /> 8 <meta name="generator" content="DocBook XSL Stylesheets V1.73.2" /> 9 <link rel="start" href="index.html" title="Berkeley DB C++ API Reference" /> 10 <link rel="up" href="db.html" title="Chapter 2. The Db Handle" /> 11 <link rel="prev" href="dbget_type.html" title="Db::get_type()" /> 12 <link rel="next" href="dbkey_range.html" title="Db::key_range()" /> 13 </head> 14 <body> 15 <div class="navheader"> 16 <table width="100%" summary="Navigation header"> 17 <tr> 18 <th colspan="3" align="center">Db::join()</th> 19 </tr> 20 <tr> 21 <td width="20%" align="left"><a accesskey="p" href="dbget_type.html">Prev</a> </td> 22 <th width="60%" align="center">Chapter 2. 23 The Db Handle 24 </th> 25 <td width="20%" align="right"> <a accesskey="n" href="dbkey_range.html">Next</a></td> 26 </tr> 27 </table> 28 <hr /> 29 </div> 30 <div class="sect1" lang="en" xml:lang="en"> 31 <div class="titlepage"> 32 <div> 33 <div> 34 <h2 class="title" style="clear: both"><a id="dbjoin"></a>Db::join()</h2> 35 </div> 36 </div> 37 </div> 38 <pre class="programlisting">#include <db_cxx.h> 39 40int 41Db::join(Dbc **curslist, Dbc **dbcp, u_int32_t flags);</pre> 42 <p> 43 The <code class="methodname">Db::join()</code> method creates a specialized join cursor for use in 44 performing equality or natural joins on secondary indices. For 45 information on how to organize your data to use this functionality, 46 see <a href="../../programmer_reference/am_cursor.html#am_join" class="olink">Equality join</a>. 47 </p> 48 <p> 49 The <code class="methodname">Db::join()</code> method is called using the 50 <a class="link" href="db.html" title="Chapter 2. The Db Handle">Db</a> handle of the primary 51 database. 52 </p> 53 <p> 54 The join cursor supports only the <a class="xref" href="dbcget.html" title="Dbc::get()">Dbc::get()</a> and 55 <a class="xref" href="dbcclose.html" title="Dbc::close()">Dbc::close()</a> cursor functions: 56 </p> 57 <div class="itemizedlist"> 58 <ul type="disc"> 59 <li> 60 <p> 61 <span class="bold"><strong> <a class="xref" href="dbcget.html" title="Dbc::get()">Dbc::get()</a> </strong></span> 62 </p> 63 <p> 64 Iterates over the values associated with the keys to which each item 65 in <span class="bold"><strong>curslist</strong></span> was initialized. Any 66 data value that appears in all items specified by the <span class="bold"><strong>curslist</strong></span> parameter is then used as a key into 67 the <span class="bold"><strong>primary</strong></span>, and the key/data pair 68 found in the <span class="bold"><strong>primary</strong></span> is returned. The 69 <span class="bold"><strong>flags</strong></span> parameter must be set to 0 or 70 the following value: 71 72 </p> 73 <div class="itemizedlist"> 74 <ul type="circle"> 75 <li> 76 <p> 77 <span class="bold"><strong><code class="literal">DB_JOIN_ITEM</code></strong></span> 78 </p> 79 <p> 80 Do not use the data value found in all the cursors as a lookup key for 81 the <span class="bold"><strong>primary</strong></span>, but simply return it in 82 the key parameter instead. The data parameter is left unchanged. 83 </p> 84 </li> 85 </ul> 86 </div> 87 <p> 88 In addition, the following flag may be set by bitwise inclusively 89 <span class="bold"><strong>OR</strong></span>'ing it into the <span class="bold"><strong>flags</strong></span> parameter: 90 </p> 91 <div class="itemizedlist"> 92 <ul type="circle"> 93 <li> 94 <p> 95 <span class="bold"><strong> <code class="literal">DB_READ_UNCOMMITTED</code></strong></span> 96 </p> 97 <p> 98 Configure a transactional join operation to have degree 1 isolation, 99 reading modified but not yet committed data. Silently ignored if the 100 <a class="link" href="dbopen.html#dbopen_DB_READ_UNCOMMITTED">DB_READ_UNCOMMITTED</a> 101 flag was not specified when the underlying database was opened. 102 </p> 103 </li> 104 <li> 105 <p> 106 <span class="bold"><strong><code class="literal">DB_RMW</code></strong></span> 107 </p> 108 <p> 109 Acquire write locks instead of read locks when doing the read, if 110 locking is configured. Setting this flag can eliminate deadlock 111 during a read-modify-write cycle by acquiring the write lock during 112 the read part of the cycle so that another thread of control acquiring 113 a read lock for the same item, in its own read-modify-write cycle, 114 will not result in deadlock. 115 </p> 116 </li> 117 </ul> 118 </div> 119 </li> 120 <li> 121 <p> 122 <span class="bold"><strong> <a class="xref" href="dbcclose.html" title="Dbc::close()">Dbc::close()</a> </strong></span> 123 </p> 124 <p> 125 Close the returned cursor and release all resources. (Closing the 126 cursors in <span class="bold"><strong>curslist</strong></span> is the 127 responsibility of the caller.) 128 </p> 129 </li> 130 </ul> 131 </div> 132 <p> 133 The <code class="methodname">Db::join()</code> <span> 134 135 <span> 136 method either returns a non-zero error value or throws an 137 exception that encapsulates a non-zero error value on 138 failure, and returns 0 on success. 139 </span> 140 </span> 141 </p> 142 <div class="sect2" lang="en" xml:lang="en"> 143 <div class="titlepage"> 144 <div> 145 <div> 146 <h3 class="title"><a id="id1639144"></a>Parameters</h3> 147 </div> 148 </div> 149 </div> 150 <div class="sect3" lang="en" xml:lang="en"> 151 <div class="titlepage"> 152 <div> 153 <div> 154 <h4 class="title"><a id="id1638877"></a>curslist</h4> 155 </div> 156 </div> 157 </div> 158 <p> 159 The <span class="bold"><strong>curslist</strong></span> parameter contains a 160 NULL terminated array of cursors. Each cursor must have been 161 initialized to refer to the key on which the underlying database 162 should be joined. Typically, this initialization is done by a 163 <a class="xref" href="dbcget.html" title="Dbc::get()">Dbc::get()</a> call with the 164 <a class="link" href="dbcget.html#dbcget_DB_SET">DB_SET</a> flag 165 specified. Once the cursors have been passed as part of a <span class="bold"><strong>curslist</strong></span>, they should not be accessed or 166 modified until the newly created join cursor has been closed, or else 167 inconsistent results may be returned. 168 </p> 169 <p> 170 Joined values are retrieved by doing a sequential iteration over the 171 first cursor in the <span class="bold"><strong>curslist</strong></span> 172 parameter, and a nested iteration over each secondary cursor in the 173 order they are specified in the <span class="bold"><strong>curslist</strong></span> parameter. This requires database 174 traversals to search for the current datum in all the cursors after 175 the first. For this reason, the best join performance normally 176 results from sorting the cursors from the one that refers to the least 177 number of data items to the one that refers to the most. By default, 178 <code class="methodname">Db::join()</code> does this sort on behalf of its caller. 179 </p> 180 <p> 181 For the returned join cursor to be used in a transaction-protected 182 manner, the cursors listed in <span class="bold"><strong>curslist</strong></span> must have been created within the 183 context of the same transaction. 184 </p> 185 </div> 186 <div class="sect3" lang="en" xml:lang="en"> 187 <div class="titlepage"> 188 <div> 189 <div> 190 <h4 class="title"><a id="id1638370"></a>dbcp</h4> 191 </div> 192 </div> 193 </div> 194 <p> 195 The newly created join cursor is returned in the memory location to 196 which <span class="bold"><strong>dbcp</strong></span> refers. 197 </p> 198 </div> 199 <div class="sect3" lang="en" xml:lang="en"> 200 <div class="titlepage"> 201 <div> 202 <div> 203 <h4 class="title"><a id="id1639064"></a>flags</h4> 204 </div> 205 </div> 206 </div> 207 <p> 208 The <span class="bold"><strong>flags</strong></span> parameter must be set to 0 209 or the following value: 210 </p> 211 <div class="itemizedlist"> 212 <ul type="disc"> 213 <li> 214 <p><a id="join_DB_JOIN_NOSORT"></a> 215 <code class="literal">DB_JOIN_NOSORT</code> 216 </p> 217 <p> 218 Do not sort the cursors based on the number of data items to which 219 they refer. If the data are structured so that cursors with many data 220 items also share many common elements, higher performance will result 221 from listing those cursors before cursors with fewer data items; that 222 is, a sort order other than the default. The DB_JOIN_NOSORT flag 223 permits applications to perform join optimization prior to calling the 224 <code class="methodname">Db::join()</code> method. 225 </p> 226 </li> 227 </ul> 228 </div> 229 </div> 230 </div> 231 <div class="sect2" lang="en" xml:lang="en"> 232 <div class="titlepage"> 233 <div> 234 <div> 235 <h3 class="title"><a id="id1638984"></a>Errors</h3> 236 </div> 237 </div> 238 </div> 239 <p> 240 The <code class="methodname">Db::join()</code> <span> 241 242 <span> 243 method may fail and throw a <a class="link" href="dbexception.html" title="Chapter 6. The DbException Class">DbException</a> 244 exception, encapsulating one of the following non-zero errors, or return one 245 of the following non-zero errors: 246 </span> 247 </span> 248 </p> 249 <div class="sect3" lang="en" xml:lang="en"> 250 <div class="titlepage"> 251 <div> 252 <div> 253 <h4 class="title"><a id="id1638820"></a><span>DbRepHandleDeadException or</span> DB_REP_HANDLE_DEAD</h4> 254 </div> 255 </div> 256 </div> 257 <p> 258 When a client synchronizes with the master, it is possible for committed 259 transactions to be rolled back. This invalidates all the database and cursor 260 handles opened in the replication environment. Once this occurs, an attempt to use 261 such a handle will 262 <span> 263 throw a <a class="xref" href="dbrephandledead.html" title="DbRepHandleDeadException">DbRepHandleDeadException</a> (if 264 your application is configured to throw exceptions), or 265 </span> 266 return <code class="literal">DB_REP_HANDLE_DEAD</code>. 267 The application will need to discard the handle and open a new one in order to 268 continue processing. 269 </p> 270 </div> 271 <div class="sect3" lang="en" xml:lang="en"> 272 <div class="titlepage"> 273 <div> 274 <div> 275 <h4 class="title"><a id="id1639412"></a><span>DbDeadlockException or </span>DB_REP_LOCKOUT</h4> 276 </div> 277 </div> 278 </div> 279 <p> 280 The operation was blocked by client/master synchronization. 281 </p> 282 <p> 283 <a class="xref" href="dbdeadlock.html" title="DbDeadlockException">DbDeadlockException</a> is thrown if 284 your Berkeley DB API is configured to throw exceptions. 285 Otherwise, <code class="literal">DB_REP_LOCKOUT</code> is returned. 286 </p> 287 </div> 288 <div class="sect3" lang="en" xml:lang="en"> 289 <div class="titlepage"> 290 <div> 291 <div> 292 <h4 class="title"><a id="id1639067"></a>DB_SECONDARY_BAD</h4> 293 </div> 294 </div> 295 </div> 296 <p> 297 A secondary index references a nonexistent primary key. 298 </p> 299 </div> 300 <div class="sect3" lang="en" xml:lang="en"> 301 <div class="titlepage"> 302 <div> 303 <div> 304 <h4 class="title"><a id="id1639168"></a>EINVAL</h4> 305 </div> 306 </div> 307 </div> 308 <p> 309 If cursor methods other than <a class="xref" href="dbcget.html" title="Dbc::get()">Dbc::get()</a> 310 or <a class="xref" href="dbcclose.html" title="Dbc::close()">Dbc::close()</a> were called; 311 or if an invalid flag value or parameter was specified. 312 </p> 313 </div> 314 </div> 315 <div class="sect2" lang="en" xml:lang="en"> 316 <div class="titlepage"> 317 <div> 318 <div> 319 <h3 class="title"><a id="id1639111"></a>Class</h3> 320 </div> 321 </div> 322 </div> 323 <p> 324 <a class="link" href="db.html" title="Chapter 2. The Db Handle">Db</a> 325 </p> 326 </div> 327 <div class="sect2" lang="en" xml:lang="en"> 328 <div class="titlepage"> 329 <div> 330 <div> 331 <h3 class="title"><a id="id1638707"></a>See Also</h3> 332 </div> 333 </div> 334 </div> 335 <p> 336 <a class="xref" href="db.html#dblist" title="Database and Related Methods">Database and Related Methods</a> 337 </p> 338 </div> 339 </div> 340 <div class="navfooter"> 341 <hr /> 342 <table width="100%" summary="Navigation footer"> 343 <tr> 344 <td width="40%" align="left"><a accesskey="p" href="dbget_type.html">Prev</a> </td> 345 <td width="20%" align="center"> 346 <a accesskey="u" href="db.html">Up</a> 347 </td> 348 <td width="40%" align="right"> <a accesskey="n" href="dbkey_range.html">Next</a></td> 349 </tr> 350 <tr> 351 <td width="40%" align="left" valign="top">Db::get_type() </td> 352 <td width="20%" align="center"> 353 <a accesskey="h" href="index.html">Home</a> 354 </td> 355 <td width="40%" align="right" valign="top"> Db::key_range()</td> 356 </tr> 357 </table> 358 </div> 359 </body> 360</html> 361