1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
|
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<title>Database Joins</title>
<link rel="stylesheet" href="gettingStarted.css" type="text/css" />
<meta name="generator" content="DocBook XSL Stylesheets V1.73.2" />
<link rel="start" href="index.html" title="Getting Started with Berkeley DB" />
<link rel="up" href="indexes.html" title="Chapter 10. Secondary Databases" />
<link rel="prev" href="secondaryCursor.html" title="Using Secondary Cursors" />
<link rel="next" href="javaindexusage.html" title="Secondary Database Example" />
</head>
<body>
<div xmlns="" class="navheader">
<div class="libver">
<p>Library Version 11.2.5.3</p>
</div>
<table width="100%" summary="Navigation header">
<tr>
<th colspan="3" align="center">Database Joins</th>
</tr>
<tr>
<td width="20%" align="left"><a accesskey="p" href="secondaryCursor.html">Prev</a> </td>
<th width="60%" align="center">Chapter 10. Secondary Databases</th>
<td width="20%" align="right"> <a accesskey="n" href="javaindexusage.html">Next</a></td>
</tr>
</table>
<hr />
</div>
<div class="sect1" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h2 class="title" style="clear: both"><a id="joins"></a>Database Joins</h2>
</div>
</div>
</div>
<div class="toc">
<dl>
<dt>
<span class="sect2">
<a href="joins.html#joinUsage">Using Join Cursors</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="joins.html#joinconfig">JoinCursor Properties</a>
</span>
</dt>
</dl>
</div>
<p>
If you have two or more secondary databases associated with a primary
database, then you can retrieve primary records based on the intersection of
multiple secondary entries. You do this using a
<span><code class="classname">JoinCursor</code>.</span>
</p>
<p>
Throughout this document we have presented a
<span>class</span>
that stores
<span>inventory</span>
information on grocery
That
<span>class</span>
is fairly simple with a limited
number of data members, few of which would be interesting from a query
perspective. But suppose, instead, that we were storing
information on something with many more characteristics that can be queried, such
as an automobile. In that case, you may be storing information such as
color, number of doors, fuel mileage, automobile type, number of
passengers, make, model, and year, to name just a few.
</p>
<p>
In this case, you would still likely be using some unique value to key your
primary entries (in the United States, the automobile's VIN would be
ideal for this purpose). You would then create a
<span>class</span>
that identifies
all the characteristics of the automobiles in your inventory.
<span>
You would
also have to create some mechanism by which you would move instances of
this class in and out of Java <code class="literal">byte</code> arrays. We
described the concepts and mechanisms by which you can perform these
activities in <a class="xref" href="DBEntry.html" title="Chapter 8. Database Records">Database Records</a>.
</span>
</p>
<p>
To query this data, you might then create multiple secondary databases,
one for each of the characteristics that you want to query. For
example, you might create a secondary for color, another for number of
doors, another for number of passengers, and so forth. Of course, you
will need a unique
<span>key creator</span>
for each such secondary database. You do
all of this using the concepts and techniques described throughout this
chapter.
</p>
<p>
Once you have created this primary database and all interesting
secondaries, what you have is the ability to retrieve automobile records
based on a single characteristic. You can, for example, find all the
automobiles that are red. Or you can find all the automobiles that have
four doors. Or all the automobiles that are minivans.
</p>
<p>
The next most natural step, then, is to form compound queries, or joins.
For example, you might want to find all the automobiles that are red,
and that were built by Toyota, and that are minivans. You can do this
using a
<span><code class="classname">JoinCursor</code> class instance.</span>
</p>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="joinUsage"></a>Using Join Cursors</h3>
</div>
</div>
</div>
<p>
To use a join cursor:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
Open two or more
<span>secondary cursors. These </span>
cursors
<span>for</span>
secondary databases that are associated with
the same primary database.
</p>
</li>
<li>
<p>
Position each such cursor to the secondary key
value in which you are interested. For example, to build on
the previous description, the cursor for the color
database is positioned to the <code class="literal">red</code> records
while the cursor for the model database is positioned to the
<code class="literal">minivan</code> records, and the cursor for the
make database is positioned to <code class="literal">Toyota</code>.
</p>
</li>
<li>
<p>
Create an array of <span>secondary</span> cursors, and
place in it each of the cursors that are participating in your join query.
</p>
</li>
<li>
<p>
Obtain a join cursor. You do this using the
<code class="methodname">Database.join()</code>
method. You must pass this method the array of secondary cursors that you
opened and positioned in the previous steps.
</p>
</li>
<li>
<p>
Iterate over the set of matching records
<span>using <code class="methodname">JoinCursor.getNext()</code></span>
until
<span><code class="classname">OperationStatus</code> is not <code class="literal">SUCCESS</code>.</span>
</p>
</li>
<li>
<p>
Close your <span>join</span> cursor.
</p>
</li>
<li>
<p>
If you are done with them, close all your <span>secondary</span> cursors.
</p>
</li>
</ul>
</div>
<p>
For example:
</p>
<a id="java_index9"></a>
<pre class="programlisting">package db.GettingStarted;
import com.sleepycat.db.Database;
import com.sleepycat.db.DatabaseEntry;
import com.sleepycat.db.DatabaseException;
import com.sleepycat.db.JoinCursor;
import com.sleepycat.db.LockMode;
import com.sleepycat.db.OperationStatus;
import com.sleepycat.db.SecondaryCursor;
import com.sleepycat.db.SecondaryDatabase;
...
// Database and secondary database opens omitted for brevity.
// Assume a primary database handle:
// automotiveDB
// Assume 3 secondary database handles:
// automotiveColorDB -- index based on automobile color
// automotiveTypeDB -- index based on automobile type
// automotiveMakeDB -- index based on the manufacturer
Database automotiveDB = null;
SecondaryDatabase automotiveColorDB = null;
SecondaryDatabase automotiveTypeDB = null;
SecondaryDatabase automotiveMakeDB = null;
// Query strings:
String theColor = "red";
String theType = "minivan";
String theMake = "Toyota";
// Secondary cursors used for the query:
SecondaryCursor colorSecCursor = null;
SecondaryCursor typeSecCursor = null;
SecondaryCursor makeSecCursor = null;
// The join cursor
JoinCursor joinCursor = null;
// These are needed for our queries
DatabaseEntry foundKey = new DatabaseEntry();
DatabaseEntry foundData = new DatabaseEntry();
// All cursor operations are enclosed in a try block to ensure that they
// get closed in the event of an exception.
try {
// Database entries used for the query:
DatabaseEntry color = new DatabaseEntry(theColor.getBytes("UTF-8"));
DatabaseEntry type = new DatabaseEntry(theType.getBytes("UTF-8"));
DatabaseEntry make = new DatabaseEntry(theMake.getBytes("UTF-8"));
colorSecCursor = automotiveColorDB.openSecondaryCursor(null, null);
typeSecCursor = automotiveTypeDB.openSecondaryCursor(null, null);
makeSecCursor = automotiveMakeDB.openSecondaryCursor(null, null);
// Position all our secondary cursors to our query values.
OperationStatus colorRet =
colorSecCursor.getSearchKey(color, foundData, LockMode.DEFAULT);
OperationStatus typeRet =
typeSecCursor.getSearchKey(type, foundData, LockMode.DEFAULT);
OperationStatus makeRet =
makeSecCursor.getSearchKey(make, foundData, LockMode.DEFAULT);
// If all our searches returned successfully, we can proceed
if (colorRet == OperationStatus.SUCCESS &&
typeRet == OperationStatus.SUCCESS &&
makeRet == OperationStatus.SUCCESS) {
// Get a secondary cursor array and populate it with our
// positioned cursors
SecondaryCursor[] cursorArray = {colorSecCursor,
typeSecCursor,
makeSecCursor};
// Create the join cursor
joinCursor = automotiveDB.join(cursorArray, null);
// Now iterate over the results, handling each in turn
while (joinCursor.getNext(foundKey, foundData, LockMode.DEFAULT) ==
OperationStatus.SUCCESS) {
// Do something with the key and data retrieved in
// foundKey and foundData
}
}
} catch (DatabaseException dbe) {
// Error reporting goes here
} catch (Exception e) {
// Error reporting goes here
} finally {
try {
// Make sure to close out all our cursors
if (colorSecCursor != null) {
colorSecCursor.close();
}
if (typeSecCursor != null) {
typeSecCursor.close();
}
if (makeSecCursor != null) {
makeSecCursor.close();
}
if (joinCursor != null) {
joinCursor.close();
}
} catch (DatabaseException dbe) {
// Error reporting goes here
}
} </pre>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="joinconfig"></a>JoinCursor Properties</h3>
</div>
</div>
</div>
<p>
You can set <code class="classname">JoinCursor</code> properties using the
<code class="classname">JoinConfig</code> class. Currently there is just one property that you can
set:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
<code class="methodname">JoinConfig.setNoSort()</code>
</p>
<p>
Specifies whether automatic sorting of input cursors is disabled. The cursors are sorted from the
one that refers to the least number of data items to the one that refers to the most.
</p>
<p>
If the data is 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. Turning off sorting permits applications to specify cursors in the proper order given this
scenario.
</p>
<p>
The default value is <code class="literal">false</code> (automatic cursor sorting is performed).
</p>
<p>
For example:
</p>
<a id="je_index10"></a>
<pre class="programlisting">// All database and environments omitted
JoinConfig config = new JoinConfig();
config.setNoSort(true);
JoinCursor joinCursor = myDb.join(cursorArray, config); </pre>
</li>
</ul>
</div>
</div>
</div>
<div class="navfooter">
<hr />
<table width="100%" summary="Navigation footer">
<tr>
<td width="40%" align="left"><a accesskey="p" href="secondaryCursor.html">Prev</a> </td>
<td width="20%" align="center">
<a accesskey="u" href="indexes.html">Up</a>
</td>
<td width="40%" align="right"> <a accesskey="n" href="javaindexusage.html">Next</a></td>
</tr>
<tr>
<td width="40%" align="left" valign="top">
<span>Using Secondary Cursors</span>
</td>
<td width="20%" align="center">
<a accesskey="h" href="index.html">Home</a>
</td>
<td width="40%" align="right" valign="top"> Secondary Database Example</td>
</tr>
</table>
</div>
</body>
</html>
|
