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
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
|
<?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>BTree Configuration</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="dbconfig.html" title="Chapter 11. Database Configuration" />
<link rel="prev" href="cachesize.html" title="Selecting the Cache Size" />
</head>
<body>
<div xmlns="" class="navheader">
<div class="libver">
<p>Library Version 12.1.6.1</p>
</div>
<table width="100%" summary="Navigation header">
<tr>
<th colspan="3" align="center">BTree Configuration</th>
</tr>
<tr>
<td width="20%" align="left"><a accesskey="p" href="cachesize.html">Prev</a> </td>
<th width="60%" align="center">Chapter 11. Database Configuration</th>
<td width="20%" align="right"> </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="btree"></a>BTree Configuration</h2>
</div>
</div>
</div>
<div class="toc">
<dl>
<dt>
<span class="sect2">
<a href="btree.html#duplicateRecords">Allowing Duplicate Records</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="btree.html#comparators">Setting Comparison Functions</a>
</span>
</dt>
</dl>
</div>
<p>
In going through the previous chapters in this book, you may notice that
we touch on some topics that are specific to BTree, but we do not cover
those topics in any real detail. In this section, we will discuss
configuration issues that are unique to BTree.
</p>
<p>
Specifically, in this section we describe:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
Allowing duplicate records.
</p>
</li>
<li>
<p>
Setting comparator callbacks.
</p>
</li>
</ul>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="duplicateRecords"></a>Allowing Duplicate Records</h3>
</div>
</div>
</div>
<p>
BTree databases can contain duplicate records. One record is
considered to be a duplicate of another when both records use keys
that compare as equal to one another.
</p>
<p>
By default, keys are compared using a lexicographical comparison,
with shorter keys collating higher than longer keys.
You can override this default using the
<code class="methodname">DatabaseConfig.setBtreeComparator()</code>
method. See the next section for details.
</p>
<p>
By default, DB databases do not allow duplicate records. As a
result, any attempt to write a record that uses a key equal to a
previously existing record results in the previously existing record
being overwritten by the new record.
</p>
<p>
Allowing duplicate records is useful if you have a database that
contains records keyed by a commonly occurring piece of information.
It is frequently necessary to allow duplicate records for secondary
databases.
</p>
<p>
For example, suppose your primary database contained records related
to automobiles. You might in this case want to be able to find all
the automobiles in the database that are of a particular color, so
you would index on the color of the automobile. However, for any
given color there will probably be multiple automobiles. Since the
index is the secondary key, this means that multiple secondary
database records will share the same key, and so the secondary
database must support duplicate records.
</p>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="sorteddups"></a>Sorted Duplicates</h4>
</div>
</div>
</div>
<p>
Duplicate records can be stored in sorted or unsorted order.
You can cause DB to automatically sort your duplicate
records by
<span>
setting <code class="methodname">DatabaseConfig.setSortedDuplicates()</code>
to <code class="literal">true</code>. Note that this property must be
set prior to database creation time and it cannot be changed
afterwards.
</span>
</p>
<p>
If sorted duplicates are supported, then the
<span>
<code class="classname">java.util.Comparator</code> implementation
identified to
<code class="methodname">DatabaseConfig.setDuplicateComparator()</code>
</span>
is used to determine the location of the duplicate record in its
duplicate set. If no such function is provided, then the default
lexicographical comparison is used.
</p>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="nosorteddups"></a>Unsorted Duplicates</h4>
</div>
</div>
</div>
<p>
For performance reasons, BTrees should always contain sorted
records. (BTrees containing unsorted entries must potentially
spend a great deal more time locating an entry than does a BTree
that contains sorted entries). That said, DB provides support
for suppressing automatic sorting of duplicate records because it may be that
your application is inserting records that are already in a
sorted order.
</p>
<p>
That is, if the database is configured to support unsorted
duplicates, then the assumption is that your application
will manually perform the sorting. In this event,
expect to pay a significant performance penalty. Any time you
place records into the database in a sort order not know to
DB, you will pay a performance penalty
</p>
<p>
That said, this is how DB behaves when inserting records
into a database that supports non-sorted duplicates:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
If your application simply adds a duplicate record using
<span><code class="methodname">Database.put()</code>,</span>
then the record is inserted at the end of its sorted duplicate set.
</p>
</li>
<li>
<p>
If a cursor is used to put the duplicate record to the database,
then the new record is placed in the duplicate set according to the
actual method used to perform the put. The relevant methods
are:
</p>
<div class="itemizedlist">
<ul type="circle">
<li>
<p>
<code class="methodname">Cursor.putAfter()</code>
</p>
<p>
The data
is placed into the database
as a duplicate record. The key used for this operation is
the key used for the record to which the cursor currently
refers. Any key provided on the call
is therefore ignored.
</p>
<p>
The duplicate record is inserted into the database
immediately after the cursor's current position in the
database.
</p>
</li>
<li>
<p>
<code class="methodname">Cursor.putBefore()</code>
</p>
<p>
Behaves the same as
<code class="methodname">Cursor.putAfter()</code>
except that the new record is inserted immediately before
the cursor's current location in the database.
</p>
</li>
<li>
<p>
<code class="methodname">Cursor.putKeyFirst()</code>
</p>
<p>
If the key
already exists in the
database, and the database is configured to use duplicates
without sorting, then the new record is inserted as the first entry
in the appropriate duplicates list.
</p>
</li>
<li>
<p>
<code class="methodname">Cursor.putKeyLast()</code>
</p>
<p>
Behaves identically to
<code class="methodname">Cursor.putKeyFirst()</code>
except that the new duplicate record is inserted as the last
record in the duplicates list.
</p>
</li>
</ul>
</div>
</li>
</ul>
</div>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="specifyingDups"></a>Configuring a Database to Support Duplicates</h4>
</div>
</div>
</div>
<p>
Duplicates support can only be configured
at database creation time. You do this by specifying the appropriate
<span>
<code class="classname">DatabaseConfig</code> method
</span>
before the database is opened for the first time.
</p>
<p>
The
<span>methods</span>
that you can use are:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
<code class="methodname">DatabaseConfig.setUnsortedDuplicates()</code>
</p>
<p>
The database supports non-sorted duplicate records.
</p>
</li>
<li>
<p>
<code class="methodname">DatabaseConfig.setSortedDuplicates()</code>
</p>
<p>
The database supports sorted duplicate records. Note
that this flag also sets the
flag for
you.
</p>
</li>
</ul>
</div>
<p>
The following code fragment illustrates how to configure a database
to support sorted duplicate records:
</p>
<a id="java_btree_dupsort"></a>
<pre class="programlisting">package db.GettingStarted;
import java.io.FileNotFoundException;
import com.sleepycat.db.Database;
import com.sleepycat.db.DatabaseConfig;
import com.sleepycat.db.DatabaseException;
import com.sleepycat.db.DatabaseType;
...
Database myDb = null;
try {
// Typical configuration settings
DatabaseConfig myDbConfig = new DatabaseConfig();
myDbConfig.setType(DatabaseType.BTREE);
myDbConfig.setAllowCreate(true);
// Configure for sorted duplicates
myDbConfig.setSortedDuplicates(true);
// Open the database
myDb = new Database("mydb.db", null, myDbConfig);
} catch(DatabaseException dbe) {
System.err.println("MyDbs: " + dbe.toString());
System.exit(-1);
} catch(FileNotFoundException fnfe) {
System.err.println("MyDbs: " + fnfe.toString());
System.exit(-1);
} </pre>
</div>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="comparators"></a>Setting Comparison Functions</h3>
</div>
</div>
</div>
<p>
By default, DB uses a lexicographical comparison function where
shorter records collate before longer records. For the majority of
cases, this comparison works well and you do not need to manage
it in any way.
</p>
<p>
However, in some situations your application's performance can
benefit from setting a custom comparison routine. You can do this
either for database keys, or for the data if your
database supports sorted duplicate records.
</p>
<p>
Some of the reasons why you may want to provide a custom sorting
function are:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
Your database is keyed using strings and you want to provide
some sort of language-sensitive ordering to that data. Doing
so can help increase the locality of reference that allows
your database to perform at its best.
</p>
</li>
<li>
<p>
You are using a little-endian system (such as x86) and you
are using integers as your database's keys. Berkeley DB
stores keys as byte strings and little-endian integers
do not sort well when viewed as byte strings. There are
several solutions to this problem, one being to provide a
custom comparison function. See
<a class="ulink" href="http://docs.oracle.com/cd/E17076_02/html/programmer_reference/am_misc_faq.html" target="_top">http://docs.oracle.com/cd/E17076_02/html/programmer_reference/am_misc_faq.html</a>
for more information.
</p>
</li>
<li>
<p>
You you do not want the entire key to participate in the
comparison, for whatever reason. In
this case, you may want to provide a custom comparison
function so that only the relevant bytes are examined.
</p>
</li>
</ul>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="creatingComparisonFunctions"></a>
<span>Creating Java Comparators</span>
</h4>
</div>
</div>
</div>
<p>
You set a BTree's key
<span>
comparator
</span>
using
<span><code class="methodname">DatabaseConfig.setBtreeComparator()</code>.</span>
You can also set a BTree's duplicate data comparison function using
<span><code class="methodname">DatabaseConfig.setDuplicateComparator()</code>.</span>
</p>
<p>
<span>
If
</span>
the database already exists when it is opened, the
<span>
comparator
</span>
provided to these methods must be the same as
that historically used to create the database or corruption can
occur.
</p>
<p>
You override the default comparison function by providing a Java
<code class="classname">Comparator</code> class to the database.
The Java <code class="classname">Comparator</code> interface requires you to implement the
<code class="methodname">Comparator.compare()</code> method
(see <a class="ulink" href="http://docs.oracle.com/javase/7/docs/api/java/util/Comparator.html" target="_top">http://docs.oracle.com/javase/7/docs/api/java/util/Comparator.html</a> for details).
</p>
<p>
DB hands your <code class="methodname">Comparator.compare()</code> method
the <code class="literal">byte</code> arrays that you stored in the database. If
you know how your data is organized in the <code class="literal">byte</code>
array, then you can write a comparison routine that directly examines
the contents of the arrays. Otherwise, you have to reconstruct your
original objects, and then perform the comparison.
</p>
<p>
For example, suppose you want to perform unicode lexical comparisons
instead of UTF-8 byte-by-byte comparisons. Then you could provide a
comparator that uses <code class="methodname">String.compareTo()</code>,
which performs a Unicode comparison of two strings (note that for
single-byte roman characters, Unicode comparison and UTF-8
byte-by-byte comparisons are identical – this is something you
would only want to do if you were using multibyte unicode characters
with DB). In this case, your comparator would look like the
following:
</p>
<a id="java_btree1"></a>
<pre class="programlisting">package db.GettingStarted;
import java.util.Comparator;
public class MyDataComparator implements Comparator {
public MyDataComparator() {}
public int compare(Object d1, Object d2) {
byte[] b1 = (byte[])d1;
byte[] b2 = (byte[])d2;
String s1 = new String(b1);
String s2 = new String(b2);
return s1.compareTo(s2);
}
} </pre>
<p>
To use this comparator:
</p>
<a id="java_btree2"></a>
<pre class="programlisting">package db.GettingStarted;
import java.io.FileNotFoundException;
import java.util.Comparator;
import com.sleepycat.db.Database;
import com.sleepycat.db.DatabaseConfig;
import com.sleepycat.db.DatabaseException;
...
Database myDatabase = null;
try {
// Get the database configuration object
DatabaseConfig myDbConfig = new DatabaseConfig();
myDbConfig.setAllowCreate(true);
// Set the duplicate comparator class
MyDataComparator mdc = new MyDataComparator();
myDbConfig.setDuplicateComparator(mdc);
// Open the database that you will use to store your data
myDbConfig.setSortedDuplicates(true);
myDatabase = new Database("myDb", null, myDbConfig);
} catch (DatabaseException dbe) {
// Exception handling goes here
} catch (FileNotFoundException fnfe) {
// Exception handling goes here
}</pre>
</div>
</div>
</div>
<div class="navfooter">
<hr />
<table width="100%" summary="Navigation footer">
<tr>
<td width="40%" align="left"><a accesskey="p" href="cachesize.html">Prev</a> </td>
<td width="20%" align="center">
<a accesskey="u" href="dbconfig.html">Up</a>
</td>
<td width="40%" align="right"> </td>
</tr>
<tr>
<td width="40%" align="left" valign="top">Selecting the Cache Size </td>
<td width="20%" align="center">
<a accesskey="h" href="index.html">Home</a>
</td>
<td width="40%" align="right" valign="top"> </td>
</tr>
</table>
</div>
</body>
</html>
|
