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>Cursor operations</title> 7 <link rel="stylesheet" href="gettingStarted.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 Programmer's Reference Guide" /> 10 <link rel="up" href="am.html" title="Chapter��3.�� Access Method Operations" /> 11 <link rel="prev" href="am_foreign.html" title="Foreign key indexes" /> 12 <link rel="next" href="am_misc.html" title="Chapter��4.�� Access Method Wrapup" /> 13 </head> 14 <body> 15 <div class="navheader"> 16 <table width="100%" summary="Navigation header"> 17 <tr> 18 <th colspan="3" align="center">Cursor operations</th> 19 </tr> 20 <tr> 21 <td width="20%" align="left"><a accesskey="p" href="am_foreign.html">Prev</a>��</td> 22 <th width="60%" align="center">Chapter��3.�� 23 Access Method Operations 24 </th> 25 <td width="20%" align="right">��<a accesskey="n" href="am_misc.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="am_cursor"></a>Cursor operations</h2> 35 </div> 36 </div> 37 </div> 38 <div class="toc"> 39 <dl> 40 <dt> 41 <span class="sect2"> 42 <a href="am_cursor.html#am_curget">Retrieving records with a cursor</a> 43 </span> 44 </dt> 45 <dt> 46 <span class="sect2"> 47 <a href="am_cursor.html#am_curput">Storing records with a cursor</a> 48 </span> 49 </dt> 50 <dt> 51 <span class="sect2"> 52 <a href="am_cursor.html#am_curdel">Deleting records with a cursor</a> 53 </span> 54 </dt> 55 <dt> 56 <span class="sect2"> 57 <a href="am_cursor.html#am_curdup">Duplicating a cursor</a> 58 </span> 59 </dt> 60 <dt> 61 <span class="sect2"> 62 <a href="am_cursor.html#am_join">Equality Join</a> 63 </span> 64 </dt> 65 <dt> 66 <span class="sect2"> 67 <a href="am_cursor.html#am_count">Data item count</a> 68 </span> 69 </dt> 70 <dt> 71 <span class="sect2"> 72 <a href="am_cursor.html#am_curclose">Cursor close</a> 73 </span> 74 </dt> 75 </dl> 76 </div> 77 <p>A database cursor refers to a single key/data pair in the database. It 78supports traversal of the database and is the only way to access 79individual duplicate data items. Cursors are used for operating on 80collections of records, for iterating over a database, and for saving 81handles to individual records, so that they can be modified after they 82have been read.</p> 83 <p>The <a href="../api_reference/C/dbcursor.html" class="olink">DB->cursor()</a> method opens a cursor into a database. Upon return the 84cursor is uninitialized, cursor positioning occurs as part of the first 85cursor operation.</p> 86 <p>Once a database cursor has been opened, records may be retrieved 87(<a href="../api_reference/C/dbcget.html" class="olink">DBC->get()</a>), stored (<a href="../api_reference/C/dbcput.html" class="olink">DBC->put()</a>), and deleted (<a href="../api_reference/C/dbcdel.html" class="olink">DBC->del()</a>).</p> 88 <p>Additional operations supported by the cursor handle include duplication 89(<a href="../api_reference/C/dbcdup.html" class="olink">DBC->dup()</a>), equality join (<a href="../api_reference/C/dbjoin.html" class="olink">DB->join()</a>), and a count of 90duplicate data items (<a href="../api_reference/C/dbccount.html" class="olink">DBC->count()</a>). Cursors are eventually closed 91using <a href="../api_reference/C/dbcclose.html" class="olink">DBC->close()</a>.</p> 92 <div class="informaltable"> 93 <table border="1" width="80%"> 94 <colgroup> 95 <col /> 96 <col /> 97 </colgroup> 98 <thead> 99 <tr> 100 <th>Database Cursors and Related Methods</th> 101 <th>Description</th> 102 </tr> 103 </thead> 104 <tbody> 105 <tr> 106 <td> 107 <a href="../api_reference/C/dbcursor.html" class="olink">DB->cursor()</a> 108 </td> 109 <td>Create a cursor</td> 110 </tr> 111 <tr> 112 <td> 113 <a href="../api_reference/C/dbcclose.html" class="olink">DBC->close()</a> 114 </td> 115 <td>Close a cursor</td> 116 </tr> 117 <tr> 118 <td> 119 <a href="../api_reference/C/dbccount.html" class="olink">DBC->count()</a> 120 </td> 121 <td>Return count of duplicates</td> 122 </tr> 123 <tr> 124 <td> 125 <a href="../api_reference/C/dbcdel.html" class="olink">DBC->del()</a> 126 </td> 127 <td>Delete by cursor</td> 128 </tr> 129 <tr> 130 <td> 131 <a href="../api_reference/C/dbcdup.html" class="olink">DBC->dup()</a> 132 </td> 133 <td>Duplicate a cursor</td> 134 </tr> 135 <tr> 136 <td> 137 <a href="../api_reference/C/dbcget.html" class="olink">DBC->get()</a> 138 </td> 139 <td>Retrieve by cursor</td> 140 </tr> 141 <tr> 142 <td> 143 <a href="../api_reference/C/dbcput.html" class="olink">DBC->put()</a> 144 </td> 145 <td>Store by cursor</td> 146 </tr> 147 <tr> 148 <td> 149 <a href="../api_reference/C/dbcset_priority.html" class="olink">DBC->set_priority()</a> 150 </td> 151 <td>Set the cursor's cache priority</td> 152 </tr> 153 </tbody> 154 </table> 155 </div> 156 <div class="sect2" lang="en" xml:lang="en"> 157 <div class="titlepage"> 158 <div> 159 <div> 160 <h3 class="title"><a id="am_curget"></a>Retrieving records with a cursor</h3> 161 </div> 162 </div> 163 </div> 164 <p>The <a href="../api_reference/C/dbcget.html" class="olink">DBC->get()</a> method retrieves records from the database using a cursor. 165The <a href="../api_reference/C/dbcget.html" class="olink">DBC->get()</a> method takes a flag which controls how the cursor is 166positioned within the database and returns the key/data item associated 167with that positioning. Similar to <a href="../api_reference/C/dbget.html" class="olink">DB->get()</a>, <a href="../api_reference/C/dbcget.html" class="olink">DBC->get()</a> may 168also take a supplied key and retrieve the data associated with that key 169from the database. There are several flags that you can set to 170customize retrieval.</p> 171 <div class="sect3" lang="en" xml:lang="en"> 172 <div class="titlepage"> 173 <div> 174 <div> 175 <h4 class="title"><a id="id1594258"></a>Cursor position flags</h4> 176 </div> 177 </div> 178 </div> 179 <div class="variablelist"> 180 <dl> 181 <dt> 182 <span class="term"><a href="../api_reference/C/dbcget.html#dbcget_DB_FIRST" class="olink">DB_FIRST</a>, <a href="../api_reference/C/dbcget.html#dbcget_DB_LAST" class="olink">DB_LAST</a></span> 183 </dt> 184 <dd>Return the first (last) record in the database.</dd> 185 <dt> 186 <span class="term"><a href="../api_reference/C/dbcget.html#dbcget_DB_NEXT" class="olink">DB_NEXT</a>, <a href="../api_reference/C/dbcget.html#dbcget_DB_PREV" class="olink">DB_PREV</a></span> 187 </dt> 188 <dd>Return the next (previous) record in the database.</dd> 189 <dt> 190 <span class="term"> 191 <a href="../api_reference/C/dbcget.html#dbcget_DB_NEXT_DUP" class="olink">DB_NEXT_DUP</a> 192 </span> 193 </dt> 194 <dd>Return the next record in the database, if it is a duplicate data item 195for the current key.</dd> 196 <dt> 197 <span class="term"><a href="../api_reference/C/dbcget.html#dbcget_DB_NEXT_NODUP" class="olink">DB_NEXT_NODUP</a>, <a href="../api_reference/C/dbcget.html#dbcget_DB_PREV_NODUP" class="olink">DB_PREV_NODUP</a></span> 198 </dt> 199 <dd>Return the next (previous) record in the database that is not a 200duplicate data item for the current key.</dd> 201 <dt> 202 <span class="term"> 203 <a href="../api_reference/C/dbcget.html#dbcget_DB_CURRENT" class="olink">DB_CURRENT</a> 204 </span> 205 </dt> 206 <dd>Return the record from the database to which the cursor currently refers.</dd> 207 </dl> 208 </div> 209 </div> 210 <div class="sect3" lang="en" xml:lang="en"> 211 <div class="titlepage"> 212 <div> 213 <div> 214 <h4 class="title"><a id="id1594936"></a>Retrieving specific key/data pairs</h4> 215 </div> 216 </div> 217 </div> 218 <div class="variablelist"> 219 <dl> 220 <dt> 221 <span class="term"> 222 <a href="../api_reference/C/dbcget.html#dbcget_DB_SET" class="olink">DB_SET</a> 223 </span> 224 </dt> 225 <dd>Return the record from the database that matches the supplied key. In 226the case of duplicates the first duplicate is returned and the cursor 227is positioned at the beginning of the duplicate list. The user can then 228traverse the duplicate entries for the key.</dd> 229 <dt> 230 <span class="term"> 231 <a href="../api_reference/C/dbcget.html#dbcget_DB_SET_RANGE" class="olink">DB_SET_RANGE</a> 232 </span> 233 </dt> 234 <dd>Return the smallest record in the database greater than or equal to the 235supplied key. This functionality permits partial key matches and range 236searches in the Btree access method.</dd> 237 <dt> 238 <span class="term"> 239 <a href="../api_reference/C/dbcget.html#dbcget_DB_GET_BOTH" class="olink">DB_GET_BOTH</a> 240 </span> 241 </dt> 242 <dd>Return the record from the database that matches both the supplied key 243and data items. This is particularly useful when there are large 244numbers of duplicate records for a key, as it allows the cursor to 245easily be positioned at the correct place for traversal of some part of 246a large set of duplicate records.</dd> 247 <dt> 248 <span class="term"> 249 <a href="../api_reference/C/dbcget.html#dbcget_DB_GET_BOTH_RANGE" class="olink">DB_GET_BOTH_RANGE</a> 250 </span> 251 </dt> 252 <dd>Return the smallest record in the database greater than or equal to the 253supplied key and data items.</dd> 254 </dl> 255 </div> 256 </div> 257 <div class="sect3" lang="en" xml:lang="en"> 258 <div class="titlepage"> 259 <div> 260 <div> 261 <h4 class="title"><a id="id1594955"></a>Retrieving based on record numbers</h4> 262 </div> 263 </div> 264 </div> 265 <div class="variablelist"> 266 <dl> 267 <dt> 268 <span class="term"> 269 <a href="../api_reference/C/dbcget.html#dbcget_DB_SET_RECNO" class="olink">DB_SET_RECNO</a> 270 </span> 271 </dt> 272 <dd>If the underlying database is a Btree, and was configured so that it is 273possible to search it by logical record number, retrieve a specific 274record based on a record number argument.</dd> 275 <dt> 276 <span class="term"> 277 <a href="../api_reference/C/dbcget.html#dbcget_DB_GET_RECNO" class="olink">DB_GET_RECNO</a> 278 </span> 279 </dt> 280 <dd>If the underlying database is a Btree, and was configured so that it is 281possible to search it by logical record number, return the record number 282for the record to which the cursor refers.</dd> 283 </dl> 284 </div> 285 </div> 286 <div class="sect3" lang="en" xml:lang="en"> 287 <div class="titlepage"> 288 <div> 289 <div> 290 <h4 class="title"><a id="id1595030"></a>Special-purpose flags</h4> 291 </div> 292 </div> 293 </div> 294 <div class="variablelist"> 295 <dl> 296 <dt> 297 <span class="term"> 298 <a href="../api_reference/C/dbget.html#dbget_DB_CONSUME" class="olink">DB_CONSUME</a> 299 </span> 300 </dt> 301 <dd>Read-and-delete: the first record (the head) of the queue is returned and 302deleted. The underlying database must be a Queue.</dd> 303 <dt> 304 <span class="term"> 305 <a href="../api_reference/C/dbcget.html#dbcget_DB_RMW" class="olink">DB_RMW</a> 306 </span> 307 </dt> 308 <dd>Read-modify-write: acquire write locks instead of read locks during 309retrieval. This can enhance performance in threaded applications by 310reducing the chance of deadlock.</dd> 311 </dl> 312 </div> 313 <p>In all cases, the cursor is repositioned by a <a href="../api_reference/C/dbcget.html" class="olink">DBC->get()</a> operation 314to point to the newly-returned key/data pair in the database.</p> 315 <p>The following is a code example showing a cursor walking through a 316database and displaying the records it contains to the standard 317output:</p> 318 <pre class="programlisting">int 319display(database) 320 char *database; 321{ 322 DB *dbp; 323 DBC *dbcp; 324 DBT key, data; 325 int close_db, close_dbc, ret; 326 327 close_db = close_dbc = 0; 328 329 /* Open the database. */ 330 if ((ret = db_create(&dbp, NULL, 0)) != 0) { 331 fprintf(stderr, 332 "%s: db_create: %s\n", progname, db_strerror(ret)); 333 return (1); 334 } 335 close_db = 1; 336 337 /* Turn on additional error output. */ 338 dbp->set_errfile(dbp, stderr); 339 dbp->set_errpfx(dbp, progname); 340 341 /* Open the database. */ 342 if ((ret = dbp->open(dbp, NULL, database, NULL, 343 DB_UNKNOWN, DB_RDONLY, 0)) != 0) { 344 dbp->err(dbp, ret, "%s: DB->open", database); 345 goto err; 346 } 347 348 /* Acquire a cursor for the database. */ 349 if ((ret = dbp->cursor(dbp, NULL, &dbcp, 0)) != 0) { 350 dbp->err(dbp, ret, "DB->cursor"); 351 goto err; 352 } 353 close_dbc = 1; 354 355 /* Initialize the key/data return pair. */ 356 memset(&key, 0, sizeof(key)); 357 memset(&data, 0, sizeof(data)); 358 359 /* Walk through the database and print out the key/data pairs. */ 360 while ((ret = dbcp->c_get(dbcp, &key, &data, DB_NEXT)) == 0) 361 printf("%.*s : %.*s\n", 362 (int)key.size, (char *)key.data, 363 (int)data.size, (char *)data.data); 364 if (ret != DB_NOTFOUND) { 365 dbp->err(dbp, ret, "DBcursor->get"); 366 goto err; 367 } 368 369err: if (close_dbc && (ret = dbcp->c_close(dbcp)) != 0) 370 dbp->err(dbp, ret, "DBcursor->close"); 371 if (close_db && (ret = dbp->close(dbp, 0)) != 0) 372 fprintf(stderr, 373 "%s: DB->close: %s\n", progname, db_strerror(ret)); 374 return (0); 375}</pre> 376 </div> 377 </div> 378 <div class="sect2" lang="en" xml:lang="en"> 379 <div class="titlepage"> 380 <div> 381 <div> 382 <h3 class="title"><a id="am_curput"></a>Storing records with a cursor</h3> 383 </div> 384 </div> 385 </div> 386 <p>The <a href="../api_reference/C/dbcput.html" class="olink">DBC->put()</a> method stores records into the database using a cursor. In 387general, <a href="../api_reference/C/dbcput.html" class="olink">DBC->put()</a> takes a key and inserts the associated data 388into the database, at a location controlled by a specified flag.</p> 389 <p>There are several flags that you can set to customize storage:</p> 390 <div class="variablelist"> 391 <dl> 392 <dt> 393 <span class="term"> 394 <a href="../api_reference/C/dbcput.html#put_DB_AFTER" class="olink">DB_AFTER</a> 395 </span> 396 </dt> 397 <dd>Create a new record, immediately after the record to which the cursor 398refers.</dd> 399 <dt> 400 <span class="term"> 401 <a href="../api_reference/C/dbcput.html#put_DB_BEFORE" class="olink">DB_BEFORE</a> 402 </span> 403 </dt> 404 <dd>Create a new record, immediately before the record to which the cursor 405refers.</dd> 406 <dt> 407 <span class="term"> 408 <a href="../api_reference/C/dbcget.html#dbcget_DB_CURRENT" class="olink">DB_CURRENT</a> 409 </span> 410 </dt> 411 <dd>Replace the data part of the record to which the cursor refers.</dd> 412 <dt> 413 <span class="term"> 414 <a href="../api_reference/C/dbcput.html#put_DB_KEYFIRST" class="olink">DB_KEYFIRST</a> 415 </span> 416 </dt> 417 <dd>Create a new record as the first of the duplicate records for the 418supplied key.</dd> 419 <dt> 420 <span class="term"> 421 <a href="../api_reference/C/dbcput.html#put_DB_KEYLAST" class="olink">DB_KEYLAST</a> 422 </span> 423 </dt> 424 <dd>Create a new record, as the last of the duplicate records for the supplied 425key.</dd> 426 </dl> 427 </div> 428 <p>In all cases, the cursor is repositioned by a <a href="../api_reference/C/dbcput.html" class="olink">DBC->put()</a> operation 429to point to the newly inserted key/data pair in the database.</p> 430 <p>The following is a code example showing a cursor storing two data items 431in a database that supports duplicate data items:</p> 432 <pre class="programlisting">int 433store(dbp) 434 DB *dbp; 435{ 436 DBC *dbcp; 437 DBT key, data; 438 int ret; 439 440 /* 441 * The DB handle for a Btree database supporting duplicate data 442 * items is the argument; acquire a cursor for the database. 443 */ 444 if ((ret = dbp->cursor(dbp, NULL, &dbcp, 0)) != 0) { 445 dbp->err(dbp, ret, "DB->cursor"); 446 goto err; 447 } 448 449 /* Initialize the key. */ 450 memset(&key, 0, sizeof(key)); 451 key.data = "new key"; 452 key.size = strlen(key.data) + 1; 453 454 /* Initialize the data to be the first of two duplicate records. */ 455 memset(&data, 0, sizeof(data)); 456 data.data = "new key's data: entry #1"; 457 data.size = strlen(data.data) + 1; 458 459 /* Store the first of the two duplicate records. */ 460 if ((ret = dbcp->c_put(dbcp, &key, &data, DB_KEYFIRST)) != 0) 461 dbp->err(dbp, ret, "DB->cursor"); 462 463 /* Initialize the data to be the second of two duplicate records. */ 464 data.data = "new key's data: entry #2"; 465 data.size = strlen(data.data) + 1; 466 467 /* 468 * Store the second of the two duplicate records. No duplicate 469 * record sort function has been specified, so we explicitly 470 * store the record as the last of the duplicate set. 471 */ 472 if ((ret = dbcp->c_put(dbcp, &key, &data, DB_KEYLAST)) != 0) 473 dbp->err(dbp, ret, "DB->cursor"); 474 475err: if ((ret = dbcp->c_close(dbcp)) != 0) 476 dbp->err(dbp, ret, "DBcursor->close"); 477 478 return (0); 479}</pre> 480 </div> 481 <div class="sect2" lang="en" xml:lang="en"> 482 <div class="titlepage"> 483 <div> 484 <div> 485 <h3 class="title"><a id="am_curdel"></a>Deleting records with a cursor</h3> 486 </div> 487 </div> 488 </div> 489 <p>The <a href="../api_reference/C/dbcdel.html" class="olink">DBC->del()</a> method deletes records from the database using a cursor. 490The <a href="../api_reference/C/dbcdel.html" class="olink">DBC->del()</a> method deletes the record to which the cursor currently 491refers. In all cases, the cursor position is unchanged after a 492delete.</p> 493 </div> 494 <div class="sect2" lang="en" xml:lang="en"> 495 <div class="titlepage"> 496 <div> 497 <div> 498 <h3 class="title"><a id="am_curdup"></a>Duplicating a cursor</h3> 499 </div> 500 </div> 501 </div> 502 <p>Once a cursor has been initialized (for example, by a call to 503<a href="../api_reference/C/dbcget.html" class="olink">DBC->get()</a>), it can be thought of as identifying a particular 504location in a database. The <a href="../api_reference/C/dbcdup.html" class="olink">DBC->dup()</a> method permits an application to 505create a new cursor that has the same locking and transactional 506information as the cursor from which it is copied, and which optionally 507refers to the same position in the database.</p> 508 <p>In order to maintain a cursor position when an application is using 509locking, locks are maintained on behalf of the cursor until the cursor is 510closed. In cases when an application is using locking without 511transactions, cursor duplication is often required to avoid 512self-deadlocks. For further details, refer to 513<a class="xref" href="lock_am_conv.html" title="Berkeley DB Transactional Data Store locking conventions">Berkeley DB Transactional Data Store locking conventions</a>.</p> 514 </div> 515 <div class="sect2" lang="en" xml:lang="en"> 516 <div class="titlepage"> 517 <div> 518 <div> 519 <h3 class="title"><a id="am_join"></a>Equality Join</h3> 520 </div> 521 </div> 522 </div> 523 <p>Berkeley DB supports "equality" (also known as "natural"), joins on secondary 524indices. An equality join is a method of retrieving data from a primary 525database using criteria stored in a set of secondary indices. It 526requires the data be organized as a primary database which contains the 527primary key and primary data field, and a set of secondary indices. 528Each of the secondary indices is indexed by a different secondary key, 529and, for each key in a secondary index, there is a set of duplicate data 530items that match the primary keys in the primary database.</p> 531 <p>For example, let's assume the need for an application that will return 532the names of stores in which one can buy fruit of a given color. We 533would first construct a primary database that lists types of fruit as 534the key item, and the store where you can buy them as the data item:</p> 535 <div class="informaltable"> 536 <table border="1" width="80%"> 537 <colgroup> 538 <col /> 539 <col /> 540 </colgroup> 541 <thead> 542 <tr> 543 <th>Primary key:</th> 544 <th>Primary data:</th> 545 </tr> 546 </thead> 547 <tbody> 548 <tr> 549 <td align="left">apple</td> 550 <td align="left">Convenience Store</td> 551 </tr> 552 <tr> 553 <td align="left">blueberry</td> 554 <td align="left">Farmer's Market</td> 555 </tr> 556 <tr> 557 <td align="left">peach</td> 558 <td align="left">Shopway</td> 559 </tr> 560 <tr> 561 <td align="left">pear</td> 562 <td align="left">Farmer's Market</td> 563 </tr> 564 <tr> 565 <td align="left">raspberry</td> 566 <td align="left">Shopway</td> 567 </tr> 568 <tr> 569 <td align="left">strawberry</td> 570 <td align="left">Farmer's Market</td> 571 </tr> 572 </tbody> 573 </table> 574 </div> 575 <p>We would then create a secondary index with the key <span class="bold"><strong>color</strong></span>, and, 576as the data items, the names of fruits of different colors.</p> 577 <div class="informaltable"> 578 <table border="1" width="80%"> 579 <colgroup> 580 <col /> 581 <col /> 582 </colgroup> 583 <thead> 584 <tr> 585 <th>Secondary key:</th> 586 <th>Secondary data:</th> 587 </tr> 588 </thead> 589 <tbody> 590 <tr> 591 <td align="left">blue</td> 592 <td align="left">blueberry</td> 593 </tr> 594 <tr> 595 <td align="left">red</td> 596 <td align="left">apple</td> 597 </tr> 598 <tr> 599 <td align="left">red</td> 600 <td align="left">raspberry</td> 601 </tr> 602 <tr> 603 <td align="left">red</td> 604 <td align="left">strawberry</td> 605 </tr> 606 <tr> 607 <td align="left">yellow</td> 608 <td align="left">peach</td> 609 </tr> 610 <tr> 611 <td align="left">yellow</td> 612 <td align="left">pear</td> 613 </tr> 614 </tbody> 615 </table> 616 </div> 617 <p>This secondary index would allow an application to look up a color, and 618then use the data items to look up the stores where the colored fruit 619could be purchased. For example, by first looking up <span class="bold"><strong>blue</strong></span>, 620the data item <span class="bold"><strong>blueberry</strong></span> could be used as the lookup key in the 621primary database, returning <span class="bold"><strong>Farmer's Market</strong></span>.</p> 622 <p>Your data must be organized in the following manner in order to use the 623<a href="../api_reference/C/dbjoin.html" class="olink">DB->join()</a> method:</p> 624 <div class="orderedlist"> 625 <ol type="1"> 626 <li>The actual data should be stored in the database represented by the 627<a href="../api_reference/C/db.html" class="olink">DB</a> object used to invoke this method. Generally, this 628<a href="../api_reference/C/db.html" class="olink">DB</a> object is called the <span class="emphasis"><em>primary</em></span>.</li> 629 <li>Secondary indices should be stored in separate databases, whose keys 630are the values of the secondary indices and whose data items are the 631primary keys corresponding to the records having the designated 632secondary key value. It is acceptable (and expected) that there may be 633duplicate entries in the secondary indices. 634<p>These duplicate entries should be sorted for performance reasons, although 635it is not required. For more information see the <a href="../api_reference/C/dbset_flags.html#dbset_flags_DB_DUPSORT" class="olink">DB_DUPSORT</a> flag 636to the <a href="../api_reference/C/dbset_flags.html" class="olink">DB->set_flags()</a> method.</p></li> 637 </ol> 638 </div> 639 <p>What the <a href="../api_reference/C/dbjoin.html" class="olink">DB->join()</a> method does is review a list of secondary keys, and, 640when it finds a data item that appears as a data item for all of the 641secondary keys, it uses that data item as a lookup into the primary 642database, and returns the associated data item.</p> 643 <p>If there were another secondary index that had as its key the <span class="bold"><strong>cost</strong></span> 644of the fruit, a similar lookup could be done on stores where inexpensive 645fruit could be purchased:</p> 646 <div class="informaltable"> 647 <table border="1" width="80%"> 648 <colgroup> 649 <col /> 650 <col /> 651 </colgroup> 652 <thead> 653 <tr> 654 <th>Secondary key:</th> 655 <th>Secondary data:</th> 656 </tr> 657 </thead> 658 <tbody> 659 <tr> 660 <td align="left">expensive</td> 661 <td align="left">blueberry</td> 662 </tr> 663 <tr> 664 <td align="left">expensive</td> 665 <td align="left">peach</td> 666 </tr> 667 <tr> 668 <td align="left">expensive</td> 669 <td align="left">pear</td> 670 </tr> 671 <tr> 672 <td align="left">expensive</td> 673 <td align="left">strawberry</td> 674 </tr> 675 <tr> 676 <td align="left">inexpensive</td> 677 <td align="left">apple</td> 678 </tr> 679 <tr> 680 <td align="left">inexpensive</td> 681 <td align="left">pear</td> 682 </tr> 683 <tr> 684 <td align="left">inexpensive</td> 685 <td align="left">raspberry</td> 686 </tr> 687 </tbody> 688 </table> 689 </div> 690 <p>The <a href="../api_reference/C/dbjoin.html" class="olink">DB->join()</a> method provides equality join functionality. While not 691strictly cursor functionality, in that it is not a method off a cursor 692handle, it is more closely related to the cursor operations than to the 693standard <a href="../api_reference/C/db.html" class="olink">DB</a> operations.</p> 694 <p>It is also possible to do lookups based on multiple criteria in a single 695operation. For example, it is possible to look up fruits that are both 696red and expensive in a single operation. If the same fruit appeared as 697a data item in both the color and expense indices, then that fruit name 698would be used as the key for retrieval from the primary index, and would 699then return the store where expensive, red fruit could be purchased.</p> 700 <div class="sect3" lang="en" xml:lang="en"> 701 <div class="titlepage"> 702 <div> 703 <div> 704 <h4 class="title"><a id="id1595662"></a>Example</h4> 705 </div> 706 </div> 707 </div> 708 <p>Consider the following three databases:</p> 709 <div class="variablelist"> 710 <dl> 711 <dt> 712 <span class="term">personnel</span> 713 </dt> 714 <dd> 715 <div class="itemizedlist"> 716 <ul type="disc"> 717 <li>key = SSN</li> 718 <li>data = record containing name, address, phone number, job title</li> 719 </ul> 720 </div> 721 </dd> 722 <dt> 723 <span class="term">lastname</span> 724 </dt> 725 <dd> 726 <div class="itemizedlist"> 727 <ul type="disc"> 728 <li>key = lastname</li> 729 <li>data = SSN</li> 730 </ul> 731 </div> 732 </dd> 733 <dt> 734 <span class="term">jobs</span> 735 </dt> 736 <dd> 737 <div class="itemizedlist"> 738 <ul type="disc"> 739 <li>key = job title</li> 740 <li>data = SSN</li> 741 </ul> 742 </div> 743 </dd> 744 </dl> 745 </div> 746 <p>Consider the following query:</p> 747 <pre class="programlisting">Return the personnel records of all people named smith with the job 748title manager.</pre> 749 <p>This query finds are all the records in the primary database (personnel) 750for whom the criteria <span class="bold"><strong>lastname=smith and job title=manager</strong></span> is 751true.</p> 752 <p>Assume that all databases have been properly opened and have the 753handles: pers_db, name_db, job_db. We also assume that we have an 754active transaction to which the handle txn refers.</p> 755 <pre class="programlisting">DBC *name_curs, *job_curs, *join_curs; 756DBC *carray[3]; 757DBT key, data; 758int ret, tret; 759 760name_curs = NULL; 761job_curs = NULL; 762memset(&key, 0, sizeof(key)); 763memset(&data, 0, sizeof(data)); 764 765if ((ret = 766 name_db->cursor(name_db, txn, &name_curs, 0)) != 0) 767 goto err; 768key.data = "smith"; 769key.size = sizeof("smith"); 770if ((ret = 771 name_curs->c_get(name_curs, &key, &data, DB_SET)) != 0) 772 goto err; 773 774if ((ret = job_db->cursor(job_db, txn, &job_curs, 0)) != 0) 775 goto err; 776key.data = "manager"; 777key.size = sizeof("manager"); 778if ((ret = 779 job_curs->c_get(job_curs, &key, &data, DB_SET)) != 0) 780 goto err; 781 782carray[0] = name_curs; 783carray[1] = job_curs; 784carray[2] = NULL; 785 786if ((ret = 787 pers_db->join(pers_db, carray, &join_curs, 0)) != 0) 788 goto err; 789while ((ret = 790 join_curs->c_get(join_curs, &key, &data, 0)) == 0) { 791 /* Process record returned in key/data. */ 792} 793 794/* 795 * If we exited the loop because we ran out of records, 796 * then it has completed successfully. 797 */ 798if (ret == DB_NOTFOUND) 799 ret = 0; 800 801err: 802if (join_curs != NULL && 803 (tret = join_curs->c_close(join_curs)) != 0 && ret == 0) 804 ret = tret; 805if (name_curs != NULL && 806 (tret = name_curs->c_close(name_curs)) != 0 && ret == 0) 807 ret = tret; 808if (job_curs != NULL && 809 (tret = job_curs->c_close(job_curs)) != 0 && ret == 0) 810 ret = tret; 811 812return (ret); 813</pre> 814 <p>The name cursor is positioned at the beginning of the duplicate list 815for <span class="bold"><strong>smith</strong></span> and the job cursor is placed at the beginning of 816the duplicate list for <span class="bold"><strong>manager</strong></span>. The join cursor is returned 817from the join method. This code then loops over the join cursor getting 818the personnel records of each one until there are no more.</p> 819 </div> 820 </div> 821 <div class="sect2" lang="en" xml:lang="en"> 822 <div class="titlepage"> 823 <div> 824 <div> 825 <h3 class="title"><a id="am_count"></a>Data item count</h3> 826 </div> 827 </div> 828 </div> 829 <p>Once a cursor has been initialized to refer to a particular key in the 830database, it can be used to determine the number of data items that are 831stored for any particular key. The <a href="../api_reference/C/dbccount.html" class="olink">DBC->count()</a> method returns 832this number of data items. The returned value is always one, unless 833the database supports duplicate data items, in which case it may be any 834number of items.</p> 835 </div> 836 <div class="sect2" lang="en" xml:lang="en"> 837 <div class="titlepage"> 838 <div> 839 <div> 840 <h3 class="title"><a id="am_curclose"></a>Cursor close</h3> 841 </div> 842 </div> 843 </div> 844 <p>The <a href="../api_reference/C/dbcclose.html" class="olink">DBC->close()</a> method closes the <a href="../api_reference/C/dbc.html" class="olink">DBC</a> cursor, after which the 845cursor may no longer be used. Although cursors are implicitly closed 846when the database they point to are closed, it is good programming 847practice to explicitly close cursors. In addition, in transactional 848systems, cursors may not exist outside of a transaction and so must be 849explicitly closed.</p> 850 </div> 851 </div> 852 <div class="navfooter"> 853 <hr /> 854 <table width="100%" summary="Navigation footer"> 855 <tr> 856 <td width="40%" align="left"><a accesskey="p" href="am_foreign.html">Prev</a>��</td> 857 <td width="20%" align="center"> 858 <a accesskey="u" href="am.html">Up</a> 859 </td> 860 <td width="40%" align="right">��<a accesskey="n" href="am_misc.html">Next</a></td> 861 </tr> 862 <tr> 863 <td width="40%" align="left" valign="top">Foreign key indexes��</td> 864 <td width="20%" align="center"> 865 <a accesskey="h" href="index.html">Home</a> 866 </td> 867 <td width="40%" align="right" valign="top">��Chapter��4.�� 868 Access Method Wrapup 869 </td> 870 </tr> 871 </table> 872 </div> 873 </body> 874</html> 875