File: sorting.html

package info (click to toggle)
xapian-core 1.4.18-3%2Bdeb11u1
  • links: PTS, VCS
  • area: main
  • in suites: bullseye
  • size: 22,416 kB
  • sloc: cpp: 91,787; ansic: 9,970; sh: 4,794; perl: 850; makefile: 503; tcl: 319; javascript: 249; python: 40
file content (498 lines) | stat: -rw-r--r-- 15,862 bytes parent folder | download
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
<?xml version="1.0" encoding="utf-8" ?>
<!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" xml:lang="en" lang="en">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<meta name="generator" content="Docutils 0.16: http://docutils.sourceforge.net/" />
<title>Sorting of Search Results</title>
<style type="text/css">

/*
:Author: David Goodger (goodger@python.org)
:Id: $Id: html4css1.css 7952 2016-07-26 18:15:59Z milde $
:Copyright: This stylesheet has been placed in the public domain.

Default cascading style sheet for the HTML output of Docutils.

See http://docutils.sf.net/docs/howto/html-stylesheets.html for how to
customize this style sheet.
*/

/* used to remove borders from tables and images */
.borderless, table.borderless td, table.borderless th {
  border: 0 }

table.borderless td, table.borderless th {
  /* Override padding for "table.docutils td" with "! important".
     The right padding separates the table cells. */
  padding: 0 0.5em 0 0 ! important }

.first {
  /* Override more specific margin styles with "! important". */
  margin-top: 0 ! important }

.last, .with-subtitle {
  margin-bottom: 0 ! important }

.hidden {
  display: none }

.subscript {
  vertical-align: sub;
  font-size: smaller }

.superscript {
  vertical-align: super;
  font-size: smaller }

a.toc-backref {
  text-decoration: none ;
  color: black }

blockquote.epigraph {
  margin: 2em 5em ; }

dl.docutils dd {
  margin-bottom: 0.5em }

object[type="image/svg+xml"], object[type="application/x-shockwave-flash"] {
  overflow: hidden;
}

/* Uncomment (and remove this text!) to get bold-faced definition list terms
dl.docutils dt {
  font-weight: bold }
*/

div.abstract {
  margin: 2em 5em }

div.abstract p.topic-title {
  font-weight: bold ;
  text-align: center }

div.admonition, div.attention, div.caution, div.danger, div.error,
div.hint, div.important, div.note, div.tip, div.warning {
  margin: 2em ;
  border: medium outset ;
  padding: 1em }

div.admonition p.admonition-title, div.hint p.admonition-title,
div.important p.admonition-title, div.note p.admonition-title,
div.tip p.admonition-title {
  font-weight: bold ;
  font-family: sans-serif }

div.attention p.admonition-title, div.caution p.admonition-title,
div.danger p.admonition-title, div.error p.admonition-title,
div.warning p.admonition-title, .code .error {
  color: red ;
  font-weight: bold ;
  font-family: sans-serif }

/* Uncomment (and remove this text!) to get reduced vertical space in
   compound paragraphs.
div.compound .compound-first, div.compound .compound-middle {
  margin-bottom: 0.5em }

div.compound .compound-last, div.compound .compound-middle {
  margin-top: 0.5em }
*/

div.dedication {
  margin: 2em 5em ;
  text-align: center ;
  font-style: italic }

div.dedication p.topic-title {
  font-weight: bold ;
  font-style: normal }

div.figure {
  margin-left: 2em ;
  margin-right: 2em }

div.footer, div.header {
  clear: both;
  font-size: smaller }

div.line-block {
  display: block ;
  margin-top: 1em ;
  margin-bottom: 1em }

div.line-block div.line-block {
  margin-top: 0 ;
  margin-bottom: 0 ;
  margin-left: 1.5em }

div.sidebar {
  margin: 0 0 0.5em 1em ;
  border: medium outset ;
  padding: 1em ;
  background-color: #ffffee ;
  width: 40% ;
  float: right ;
  clear: right }

div.sidebar p.rubric {
  font-family: sans-serif ;
  font-size: medium }

div.system-messages {
  margin: 5em }

div.system-messages h1 {
  color: red }

div.system-message {
  border: medium outset ;
  padding: 1em }

div.system-message p.system-message-title {
  color: red ;
  font-weight: bold }

div.topic {
  margin: 2em }

h1.section-subtitle, h2.section-subtitle, h3.section-subtitle,
h4.section-subtitle, h5.section-subtitle, h6.section-subtitle {
  margin-top: 0.4em }

h1.title {
  text-align: center }

h2.subtitle {
  text-align: center }

hr.docutils {
  width: 75% }

img.align-left, .figure.align-left, object.align-left, table.align-left {
  clear: left ;
  float: left ;
  margin-right: 1em }

img.align-right, .figure.align-right, object.align-right, table.align-right {
  clear: right ;
  float: right ;
  margin-left: 1em }

img.align-center, .figure.align-center, object.align-center {
  display: block;
  margin-left: auto;
  margin-right: auto;
}

table.align-center {
  margin-left: auto;
  margin-right: auto;
}

.align-left {
  text-align: left }

.align-center {
  clear: both ;
  text-align: center }

.align-right {
  text-align: right }

/* reset inner alignment in figures */
div.align-right {
  text-align: inherit }

/* div.align-center * { */
/*   text-align: left } */

.align-top    {
  vertical-align: top }

.align-middle {
  vertical-align: middle }

.align-bottom {
  vertical-align: bottom }

ol.simple, ul.simple {
  margin-bottom: 1em }

ol.arabic {
  list-style: decimal }

ol.loweralpha {
  list-style: lower-alpha }

ol.upperalpha {
  list-style: upper-alpha }

ol.lowerroman {
  list-style: lower-roman }

ol.upperroman {
  list-style: upper-roman }

p.attribution {
  text-align: right ;
  margin-left: 50% }

p.caption {
  font-style: italic }

p.credits {
  font-style: italic ;
  font-size: smaller }

p.label {
  white-space: nowrap }

p.rubric {
  font-weight: bold ;
  font-size: larger ;
  color: maroon ;
  text-align: center }

p.sidebar-title {
  font-family: sans-serif ;
  font-weight: bold ;
  font-size: larger }

p.sidebar-subtitle {
  font-family: sans-serif ;
  font-weight: bold }

p.topic-title {
  font-weight: bold }

pre.address {
  margin-bottom: 0 ;
  margin-top: 0 ;
  font: inherit }

pre.literal-block, pre.doctest-block, pre.math, pre.code {
  margin-left: 2em ;
  margin-right: 2em }

pre.code .ln { color: grey; } /* line numbers */
pre.code, code { background-color: #eeeeee }
pre.code .comment, code .comment { color: #5C6576 }
pre.code .keyword, code .keyword { color: #3B0D06; font-weight: bold }
pre.code .literal.string, code .literal.string { color: #0C5404 }
pre.code .name.builtin, code .name.builtin { color: #352B84 }
pre.code .deleted, code .deleted { background-color: #DEB0A1}
pre.code .inserted, code .inserted { background-color: #A3D289}

span.classifier {
  font-family: sans-serif ;
  font-style: oblique }

span.classifier-delimiter {
  font-family: sans-serif ;
  font-weight: bold }

span.interpreted {
  font-family: sans-serif }

span.option {
  white-space: nowrap }

span.pre {
  white-space: pre }

span.problematic {
  color: red }

span.section-subtitle {
  /* font-size relative to parent (h1..h6 element) */
  font-size: 80% }

table.citation {
  border-left: solid 1px gray;
  margin-left: 1px }

table.docinfo {
  margin: 2em 4em }

table.docutils {
  margin-top: 0.5em ;
  margin-bottom: 0.5em }

table.footnote {
  border-left: solid 1px black;
  margin-left: 1px }

table.docutils td, table.docutils th,
table.docinfo td, table.docinfo th {
  padding-left: 0.5em ;
  padding-right: 0.5em ;
  vertical-align: top }

table.docutils th.field-name, table.docinfo th.docinfo-name {
  font-weight: bold ;
  text-align: left ;
  white-space: nowrap ;
  padding-left: 0 }

/* "booktabs" style (no vertical lines) */
table.docutils.booktabs {
  border: 0px;
  border-top: 2px solid;
  border-bottom: 2px solid;
  border-collapse: collapse;
}
table.docutils.booktabs * {
  border: 0px;
}
table.docutils.booktabs th {
  border-bottom: thin solid;
  text-align: left;
}

h1 tt.docutils, h2 tt.docutils, h3 tt.docutils,
h4 tt.docutils, h5 tt.docutils, h6 tt.docutils {
  font-size: 100% }

ul.auto-toc {
  list-style-type: none }

</style>
</head>
<body>
<div class="document" id="sorting-of-search-results">
<h1 class="title">Sorting of Search Results</h1>

<!-- Copyright (C) 2007,2009,2011,2014 Olly Betts -->
<div class="contents topic" id="table-of-contents">
<p class="topic-title">Table of contents</p>
<ul class="simple">
<li><a class="reference internal" href="#introduction" id="id1">Introduction</a></li>
<li><a class="reference internal" href="#sorting-by-relevance" id="id2">Sorting by Relevance</a></li>
<li><a class="reference internal" href="#sorting-by-other-properties" id="id3">Sorting by Other Properties</a><ul>
<li><a class="reference internal" href="#sorting-by-value" id="id4">Sorting by Value</a></li>
<li><a class="reference internal" href="#sorting-by-generated-key" id="id5">Sorting by Generated Key</a></li>
</ul>
</li>
</ul>
</div>
<div class="section" id="introduction">
<h1><a class="toc-backref" href="#id1">Introduction</a></h1>
<p>By default, Xapian orders search results by decreasing relevance score.
However, it also allows results to be ordered by other criteria, or
a mixture of other criteria and relevance score.</p>
<p>If two or more results compare equal by the sorting criteria, then their order
is decided by their document ids.  By default, the document ids sort in
ascending order (so a lower document id is &quot;better&quot;), but this can be set
to descending using <tt class="docutils literal">enquire.set_docid_order(enquire.DESCENDING);</tt>.  If you
have no preference, you can tell Xapian to use whatever order is most efficient
using <tt class="docutils literal">enquire.set_docid_order(enquire.DONT_CARE);</tt>.</p>
</div>
<div class="section" id="sorting-by-relevance">
<h1><a class="toc-backref" href="#id2">Sorting by Relevance</a></h1>
<p>The BM25 weighting formula which Xapian uses by default has a number of
parameters.  We have picked some default parameter values which do a good job
in general.  The optimal values of these parameters depend on the data being
indexed and the type of queries being run, so you may be able to improve the
effectiveness of your search system by adjusting these values, but it's a
fiddly process to tune them so people tend not to bother.</p>
<p>See the <a class="reference external" href="bm25.html">BM25 documentation</a> for more details of BM25.</p>
<p>The other included weighting schemes are <tt class="docutils literal">TradWeight</tt> and <tt class="docutils literal">BoolWeight</tt>.</p>
<p>TradWeight implements the original probabilistic weighting formula, which is
essentially a special case of BM25 (it's BM25 with k2 = 0, k3 = 0, b = 1, and
min_normlen = 0, except that the weights are scaled by a constant factor).</p>
<p>BoolWeight assigns a weight of 0 to all documents, so the ordering is
determined solely by other factors.</p>
<p>You can also implement your own weighting scheme, provided it can be expressed
in the form of a sum over the matching terms, plus an extra term which depends
on term-independent statistics (such as the normalised document length).</p>
<p>For example, here's an implementation of &quot;coordinate matching&quot; - each matching
term scores one point:</p>
<pre class="literal-block">
class CoordinateWeight : public Xapian::Weight {
  public:
    CoordinateWeight * clone() const { return new CoordinateWeight; }
    CoordinateWeight() { }
    ~CoordinateWeight() { }

    std::string name() const { return &quot;Coord&quot;; }
    std::string serialise() const { return &quot;&quot;; }
    CoordinateWeight * unserialise(const std::string &amp;) const {
        return new CoordinateWeight;
    }

    double get_sumpart(Xapian::termcount, Xapian::termcount, Xapian::termcount) const {
        return 1;
    }
    double get_maxpart() const { return 1; }

    double get_sumextra(Xapian::termcount, Xapian::termcount) const { return 0; }
    double get_maxextra() const { return 0; }

    bool get_sumpart_needs_doclength() const { return false; }
};
</pre>
<!-- FIXME: add a more complex example once user-defined weight classes can
see the statistics. -->
</div>
<div class="section" id="sorting-by-other-properties">
<h1><a class="toc-backref" href="#id3">Sorting by Other Properties</a></h1>
<p>If you want to offer a &quot;sort by date&quot; feature, and can arrange for documents to
be indexed in date order (or a close-enough approximation), then you can
implement a very efficient &quot;sort by date&quot; feature by using a boolean search
(i.e. call <tt class="docutils literal"><span class="pre">enquire.set_weighting_scheme(Xapian::BoolWeight());</span></tt>) with
<tt class="docutils literal"><span class="pre">enquire.set_docid_order(Xapian::Enquire::DESCENDING);</span></tt> (for newest first) or
<tt class="docutils literal"><span class="pre">enquire.set_docid_order(Xapian::Enquire::ASCENDING);</span></tt> (for oldest first).
There's no inherent reason why this technique can't be used for sorting by
something other than date, but it's usually much easier to arrange for new
documents to arrive in date order than in other orders.</p>
<div class="section" id="sorting-by-value">
<h2><a class="toc-backref" href="#id4">Sorting by Value</a></h2>
<p>You can order documents by comparing a specified document value.  Note that the
comparison used compares the byte values in the value (i.e. it's a string sort
ignoring locale), so <tt class="docutils literal">1</tt> &lt; <tt class="docutils literal">10</tt> &lt; <tt class="docutils literal">2</tt>.  If you want to encode the value
such that it sorts numerically, use <tt class="docutils literal"><span class="pre">Xapian::sortable_serialise()</span></tt> to encode
values at index time - this works equally will on integers and floating point
values:</p>
<pre class="literal-block">
Xapian::Document doc;
doc.add_value(0, Xapian::sortable_serialise(price));
</pre>
<p>There are three methods which are used to specify how the value is used to
sort, depending if/how you want relevance used in the ordering:</p>
<blockquote>
<ul class="simple">
<li><tt class="docutils literal"><span class="pre">Enquire::set_sort_by_value()</span></tt> specifies the relevance doesn't affect the
ordering at all.</li>
<li><tt class="docutils literal"><span class="pre">Enquire::set_sort_by_value_then_relevance()</span></tt> specifies that relevance is
used for ordering any groups of documents for which the value is the same.</li>
<li><tt class="docutils literal"><span class="pre">Enquire::set_sort_by_relevance_then_value()</span></tt> specifies that documents are
ordered by relevance, and the value is only used to order groups of documents
with identical relevance values (note: the weight has to be the same, not
just the rounded percentage score).  This method isn't very useful with the
default BM25 weighting, which rarely assigns identical scores to
different documents.</li>
</ul>
</blockquote>
</div>
<div class="section" id="sorting-by-generated-key">
<h2><a class="toc-backref" href="#id5">Sorting by Generated Key</a></h2>
<p>To allow more elaborate sorting schemes, Xapian allows you to provide a functor
object subclassed from <tt class="docutils literal"><span class="pre">Xapian::KeyMaker</span></tt> which generates a sort key for each
matching document which is under consideration.  This is called at most once
for each document, and then the generated sort keys are ordered by comparing
byte values (i.e. with a string sort ignoring locale).</p>
<p>There's a standard subclass <tt class="docutils literal"><span class="pre">Xapian::MultiValueKeyMaker</span></tt> which allows sorting
on more than one document value (so the first document value specified
determines the order except among groups which have the same value, when
the second document value specified is used, and so on).</p>
<p><tt class="docutils literal"><span class="pre">Xapian::KeyMaker</span></tt> can also be subclassed to offer features such as &quot;sort by
geographical distance&quot;.  A subclass could take a coordinate pair - e.g.
(latitude, longitude) - for the user's location and sort results using
coordinates stored in a document value so that the nearest results ranked
highest.</p>
</div>
</div>
</div>
</body>
</html>