<?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 0.5: http://docutils.sourceforge.net/" /> <title>Xapian::PostingSource</title> <style type="text/css"> /* :Author: David Goodger (goodger@python.org) :Id: $Id: html4css1.css 5196 2007-06-03 20:25:28Z wiemann $ :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 } a.toc-backref { text-decoration: none ; color: black } blockquote.epigraph { margin: 2em 5em ; } dl.docutils dd { margin-bottom: 0.5em } /* 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 { 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 { clear: left } img.align-right { clear: right } 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-family: serif ; font-size: 100% } pre.literal-block, pre.doctest-block { margin-left: 2em ; margin-right: 2em } 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 } 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="xapian-postingsource"> <h1 class="title">Xapian::PostingSource</h1> <!-- Copyright (C) 2008,2009,2010,2011 Olly Betts --> <!-- Copyright (C) 2008,2009 Lemur Consulting Ltd --> <div class="contents topic" id="table-of-contents"> <p class="topic-title first">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="#anatomy" id="id2">Anatomy</a></li> <li><a class="reference internal" href="#examples" id="id3">Examples</a></li> <li><a class="reference internal" href="#multiple-databases-and-remote-databases" id="id4">Multiple databases, and remote databases</a></li> </ul> </div> <div class="section" id="introduction"> <h1><a class="toc-backref" href="#id1">Introduction</a></h1> <p>Xapian::PostingSource is an API class which you can subclass to feed data to Xapian's matcher. This feature can be made use of in a number of ways - for example:</p> <p>As a filter - a subclass could return a stream of document ids to filter a query against.</p> <p>As a weight boost - a subclass could return every document, but with a varying weight so that certain documents receive a weight boost. This could be used to prefer documents based on some external factor, such as age, price, proximity to a physical location, link analysis score, etc.</p> <p>As an alternative way of ranking documents - if the weighting scheme is set to Xapian::BoolWeight, then the ranking will be entirely by the weight returned by Xapian::PostingSource.</p> </div> <div class="section" id="anatomy"> <h1><a class="toc-backref" href="#id2">Anatomy</a></h1> <p>When first constructed, a PostingSource is not tied to a particular database. Before Xapian can get any postings (or statistics) from the source, it needs to be supplied with a database. This is performed by the init() method, which is passed a single parameter holding the database to use. This method will always be called before asking for any information about the postings in the list. If a posting source is used for multiple searches, the init() method will be called before each search; implementations must cope with init() being called multiple times, and should always use the database provided in the most recent call:</p> <pre class="literal-block"> virtual void init(const Xapian::Database & db) = 0; </pre> <p>Three methods return statistics independent of the iteration position. These are upper and lower bounds for the number of documents which can be returned, and an estimate of this number:</p> <pre class="literal-block"> virtual Xapian::doccount get_termfreq_min() const = 0; virtual Xapian::doccount get_termfreq_max() const = 0; virtual Xapian::doccount get_termfreq_est() const = 0; </pre> <p>These methods are pure-virtual in the base class, so you have to define them when deriving your subclass.</p> <p>It must always be true that:</p> <pre class="literal-block"> get_termfreq_min() <= get_termfreq_est() <= get_termfreq_max() </pre> <p>PostingSources must always return documents in increasing document ID order.</p> <p>After construction, a PostingSource points to a position <em>before</em> the first document id - so before a docid can be read, the position must be advanced.</p> <p>The <tt class="docutils literal"><span class="pre">get_weight()</span></tt> method returns the weight that you want to contribute to the current document. This weight must always be >= 0:</p> <pre class="literal-block"> virtual Xapian::weight get_weight() const; </pre> <p>The default implementation of <tt class="docutils literal"><span class="pre">get_weight()</span></tt> returns 0, for convenience when deriving "weight-less" subclasses.</p> <p>You also need to specify an upper bound on the value which <tt class="docutils literal"><span class="pre">get_weight()</span></tt> can return, which is used by the matcher to perform various optimisations. You should try hard to find a bound for efficiency, but if there really isn't one then you can set <tt class="docutils literal"><span class="pre">DBL_MAX</span></tt>:</p> <pre class="literal-block"> void get_maxweight(Xapian::weight max_weight); </pre> <p>This method specifies an upper bound on what <tt class="docutils literal"><span class="pre">get_weight()</span></tt> will return <em>from now on</em> (until the next call to <tt class="docutils literal"><span class="pre">init()</span></tt>). So if you know that the upper bound has decreased, you should call <tt class="docutils literal"><span class="pre">set_maxweight()</span></tt> with the new reduced bound.</p> <p>One thing to be aware of is that currently calling <tt class="docutils literal"><span class="pre">set_maxweight()</span></tt> during the match triggers an recursion through the postlist tree to recalculate the new overall maxweight, which takes a comparable amount of time to calculating the weight for a matching document. If your maxweight reduces for nearly every document, you may want to profile to see if it's beneficial to notify every single change. Experiments with a modified <tt class="docutils literal"><span class="pre">FixedWeightPostingSource</span></tt> which forces a pointless recalculation for every document suggest a worst case overhead in search times of about 37%, but reports of profiling results for real world examples are most welcome. In real cases, this overhead could easily be offset by the extra scope for matcher optimisations which a tighter maxweight bound allows.</p> <p>A simple approach to reducing the number of calculations is only to do it every N documents. If it's cheap to calculate the maxweight in your posting source, a more sophisticated strategy might be to decide an absolute maximum number of times to update the maxweight (say 100) and then to call it whenever:</p> <pre class="literal-block"> last_notified_maxweight - new_maxweight >= original_maxweight / 100.0 </pre> <p>This ensures that only reasonably significant drops result in a recalculation of the maxweight.</p> <p>Since <tt class="docutils literal"><span class="pre">get_weight()</span></tt> must always return >= 0, the upper bound must clearly also always be >= 0 too. If you don't call <tt class="docutils literal"><span class="pre">get_maxweight()</span></tt> then the bound defaults to 0, to match the default implementation of <tt class="docutils literal"><span class="pre">get_weight()</span></tt>.</p> <p>If you want to read the currently set upper bound, you can call:</p> <pre class="literal-block"> Xapian::weight get_maxweight() const; </pre> <p>This is just a getter method for a member variable in the <tt class="docutils literal"><span class="pre">Xapian::PostingSource</span></tt> class, and is inlined from the API headers, so there's no point storing this yourself in your subclass - it should be just as efficient to call <tt class="docutils literal"><span class="pre">get_maxweight()</span></tt> whenever you want to use it.</p> <p>The <tt class="docutils literal"><span class="pre">at_end()</span></tt> method checks if the current iteration position is past the last entry:</p> <pre class="literal-block"> virtual bool at_end() const = 0; </pre> <p>The <tt class="docutils literal"><span class="pre">get_docid()</span></tt> method returns the document id at the current iteration position:</p> <pre class="literal-block"> virtual Xapian::docid get_docid() const = 0; </pre> <p>There are three methods which advance the current position. All of these take a Xapian::Weight parameter <tt class="docutils literal"><span class="pre">min_wt</span></tt>, which indicates the minimum weight contribution which the matcher is interested in. The matcher still checks the weight of documents so it's OK to ignore this parameter completely, or to use it to discard only some documents. But it can be useful for optimising in some cases.</p> <p>The simplest of these three methods is <tt class="docutils literal"><span class="pre">next()</span></tt>, which simply advances the iteration position to the next document (possibly skipping documents with weight contribution < min_wt):</p> <pre class="literal-block"> virtual void next(Xapian::weight min_wt) = 0; </pre> <p>Then there's <tt class="docutils literal"><span class="pre">skip_to()</span></tt>. This advances the iteration position to the next document with document id >= that specified (possibly also skipping documents with weight contribution < min_wt):</p> <pre class="literal-block"> virtual void skip_to(Xapian::docid did, Xapian::weight min_wt); </pre> <p>A default implementation of <tt class="docutils literal"><span class="pre">skip_to()</span></tt> is provided which just calls <tt class="docutils literal"><span class="pre">next()</span></tt> repeatedly. This works but <tt class="docutils literal"><span class="pre">skip_to()</span></tt> can often be implemented much more efficiently.</p> <p>The final method of this group is <tt class="docutils literal"><span class="pre">check()</span></tt>. In some cases, it's fairly cheap to check if a given document matches, but the requirement that <tt class="docutils literal"><span class="pre">skip_to()</span></tt> must leave the iteration position on the next document is rather costly to implement (for example, it might require linear scanning of document ids). To avoid this where possible, the <tt class="docutils literal"><span class="pre">check()</span></tt> method allows the matcher to just check if a given document matches:</p> <pre class="literal-block"> virtual bool check(Xapian::docid did, Xapian::weight min_wt); </pre> <p>The return value is <tt class="docutils literal"><span class="pre">true</span></tt> if the method leaves the iteration position valid, and <tt class="docutils literal"><span class="pre">false</span></tt> if it doesn't. In the latter case, <tt class="docutils literal"><span class="pre">next()</span></tt> will advance to the first matching position after document id <tt class="docutils literal"><span class="pre">did</span></tt>, and <tt class="docutils literal"><span class="pre">skip_to()</span></tt> will act as it would if the iteration position was the first matching position after <tt class="docutils literal"><span class="pre">did</span></tt>.</p> <p>The default implementation of <tt class="docutils literal"><span class="pre">check()</span></tt> is just a thin wrapper around <tt class="docutils literal"><span class="pre">skip_to()</span></tt> which returns true - you should use this if <tt class="docutils literal"><span class="pre">skip_to()</span></tt> incurs only a small extra cost.</p> <p>There's also a method to return a string describing this object:</p> <pre class="literal-block"> virtual std::string get_description() const; </pre> <p>The default implementation returns a generic answer. This default is provided to avoid forcing you to provide an implementation if you don't really care what <tt class="docutils literal"><span class="pre">get_description()</span></tt> gives for your sub-class.</p> </div> <div class="section" id="examples"> <h1><a class="toc-backref" href="#id3">Examples</a></h1> <p>Here is an example of a Python PostingSource which contributes additional weight from some external source:</p> <pre class="literal-block"> class ExternalWeightPostingSource(xapian.PostingSource): """ A Xapian posting source returning weights from an external source. """ def __init__(self, db, wtsource): xapian.PostingSource.__init__(self) self.db = db self.wtsource = wtsource def init(self, db): self.alldocs = db.postlist('') def get_termfreq_min(self): return 0 def get_termfreq_est(self): return self.db.get_doccount() def get_termfreq_max(self): return self.db.get_doccount() def next(self, minweight): try: self.current = self.alldocs.next() except StopIteration: self.current = None def skip_to(self, docid, minweight): try: self.current = self.alldocs.skip_to(docid) except StopIteration: self.current = None def at_end(self): return self.current is None def get_docid(self): return self.current.docid def get_maxweight(self): return self.wtsource.get_maxweight() def get_weight(self): doc = self.db.get_document(self.current.docid) return self.wtsource.get_weight(doc) </pre> <p>ExternalWeightPostingSource doesn't restrict which documents match - it's intended to be combined with an existing query using <cite>OP_AND_MAYBE</cite> like so:</p> <pre class="literal-block"> extwtps = xapian.ExternalWeightPostingSource(db, wtsource) query = xapian.Query(query.OP_AND_MAYBE, query, xapian.Query(extwtps)) </pre> <p>The wtsource would be a class like this one:</p> <pre class="literal-block"> class WeightSource: def get_maxweight(self): return 12.34; def get_weight(self, doc): return some_func(doc.get_docid()) </pre> <!-- FIXME: Provide some more examples! --> <!-- FIXME "why you might want to do this" (e.g. scenario) too --> </div> <div class="section" id="multiple-databases-and-remote-databases"> <h1><a class="toc-backref" href="#id4">Multiple databases, and remote databases</a></h1> <p>In order to work with searches across multiple databases, or in remote databases, some additional methods need to be implemented in your Xapian::PostingSource subclass. The first of these is <tt class="docutils literal"><span class="pre">clone()</span></tt>, which is used for multi database searches. This method should just return a newly allocated instance of the same posting source class, initialised in the same way as the source that clone() was called on. The returned source will be deallocated by the caller (using "delete" - so you should allocate it with "new").</p> <p>If you don't care about supporting searches across multiple databases, you can simply return NULL from this method. In fact, the default implementation does this, so you can just leave the default implementation in place. If <tt class="docutils literal"><span class="pre">clone()</span></tt> returns NULL, an attempt to perform a search with multiple databases will raise an exception:</p> <pre class="literal-block"> virtual PostingSource * clone() const; </pre> <p>To work with searches across remote databases, you need to implement a few more methods. Firstly, you need to implement the <tt class="docutils literal"><span class="pre">name()</span></tt> method. This simply returns the name of your posting source (fully qualified with any namespace):</p> <pre class="literal-block"> virtual std::string name() const; </pre> <p>Next, you need to implement the serialise and unserialise methods. The <tt class="docutils literal"><span class="pre">serialise()</span></tt> method converts all the settings of the PostingSource to a string, and the <tt class="docutils literal"><span class="pre">unserialise()</span></tt> method converts one of these strings back into a PostingSource. Note that the serialised string doesn't need to include any information about the current iteration position of the PostingSource:</p> <pre class="literal-block"> virtual std::string serialise() const; virtual PostingSource * unserialise(const std::string &s) const; </pre> <p>Finally, you need to make a remote server which knows about your PostingSource. Currently, the only way to do this is to modify the source slightly, and compile your own xapian-tcpsrv. To do this, you need to edit <tt class="docutils literal"><span class="pre">xapian-core/bin/xapian-tcpsrv.cc</span></tt> and find the <tt class="docutils literal"><span class="pre">register_user_weighting_schemes()</span></tt> function. If <tt class="docutils literal"><span class="pre">MyPostingSource</span></tt> is your posting source, at the end of this function, add these lines:</p> <pre class="literal-block"> Xapian::Registry registry; registry.register_postingsource(MyPostingSource()); server.set_registry(registry); </pre> </div> </div> </body> </html>