Full Text Index

Using match="..."

In addition to defining an index on a given qualified name, you can also specify a "path" with wildcards. This feature might be subject to change, so please be careful when using it.

Assume you want to define an index on all the possible elements below <SPEECH>. You can do this by creating one index for every element:

<text qname="LINE"/>
<text qname="SPEAKER"/>

As a shortcut, you can use a match attribute with a wildcard:

<text match="//SPEECH/*"/>

This will create a separate index on each child element of SPEECH it encounters. Please note that the argument to match is a simple path pattern, not a full XPath expression. For the time being, it only allows:

• / and // to denote child or descendant steps,

• * wildcard selector to match an arbitrary element,

• matching a single attribute's value, e.g. foo[@bar = 'xyz']

As explained above, you have to figure out which parts of your document will likely be interesting as context for a full text query. The full text index works best if the context isn't too narrow. For example, if you have a document structure with section <div>s, headings and paragraphs, you would probably want to create an index on the <div>s and maybe on the headings, so the user can differentiate between the two.

In some cases, you could decide to put the index on the paragraph level. Then you don't need the index on the section, since you can always get from the paragraph back to the section.

If you query a larger context, you can use the KWIC module to show the user text surrounding each match. Or you can ask eXist-db to highlight each match with an <exist:match> tag, which you can later use to locate the matches within the text.

Whitespace Treatment and Ignored Content

<size><width>12</width><height>8</height></size>

<p>This is <b>un</b>clear.</p>

<inline qname="b"/>

<text qname="p">
    <inline qname="em"/>
</text>

<p>This is a paragraph
<note>containing an inline note</note>.</p>

<ignore qname="note"/>

<lucene>
    <text qname="p"/>
    <text qname="note"/>
    <ignore qname="note"/>
</lucene>

//p[ft:query(., "note")]

//p[ft:query(note, "note")]

Boost

A boost value can be assigned to an index to give it a higher score. The score for each match will be multiplied by the boost factor (default is: 1.0). For example, you may want to rank matches in titles higher than other matches.

Here's how to configure the documentation search indexes in eXist-db:

<lucene>
    <analyzer class="org.apache.lucene.analysis.standard.StandardAnalyzer"/>
    <text qname="section">
        <ignore qname="title"/>
        <ignore qname="programlisting"/>
        <ignore qname="screen"/>
        <ignore qname="synopsis"/>
    </text>
    <text qname="para"/>
    <text qname="title" boost="2.0"/>
    <ignore qname="title"/>
</lucene>

The <title> index gets a boost of 2.0 to make sure that its matches get a higher score. Since the <title> element occurs within <section>, we add an ignore rule to the index definition on the section and create a separate index on title. We also ignore titles occurring inside paragraphs. Without this, title would be matched two times.

Because the title is now indexed separately, we need to query it explicitly. For example, to search the section and the title at the same time, one could issue the following query:

for $sect in /book//section[ft:query(., "ngram")] | /book//section[ft:query(title, "ngram")]
order by ft:score($sect) descending 
return $sect

<text qname="@val">
    <match-sibling-attr boost="25" qname="att" value="writtenForm"/>
</text>

<text qname="feat">
    <has-attr boost="0" qname="xml:lang"/>
</text>

Analyzers

One of the strengths of Lucene is that it allows the developer to determine nearly every aspect of text analysis. This is done through analyzer classes, which combine a tokenizer with a chain of filters to post-process the tokenized text. eXist-db's Lucene module already allows different analyzers to be used for different indexes.

<lucene>
    <analyzer class="org.apache.lucene.analysis.standard.StandardAnalyzer"/>
    <analyzer id="ws" class="org.apache.lucene.analysis.core.WhitespaceAnalyzer"/>
    <text match="//SPEECH//*"/>
    <text qname="TITLE" analyzer="ws"/>
</lucene>

In the example above, we define that Lucene's StandardAnalyzer should be used by default (the <analyzer> element without id attribute). We provide an additional analyzer and assign it the id ws, by which the analyzer can be referenced in the actual index definitions.

The whitespace analyzer is the most basic one. As the name implies, it tokenizes the text at white space characters, but treats all other characters - including punctuation - as part of the token. The tokens are not converted to lower case and there's no stopword filter applied.

<analyzer id="stdstops" class="org.apache.lucene.analysis.standard.StandardAnalyzer">
    <param name="stopwords" type="java.io.FileReader" value="/tmp/stop.txt"/>
</analyzer>

<analyzer id="stdstops" class="org.apache.lucene.analysis.standard.StandardAnalyzer">
    <param name="stopwords" type="org.apache.lucene.analysis.util.CharArraySet">
        <value>the</value>
        <value>this</value>
        <value>and</value>
        <value>that</value>
    </param>
</analyzer>

<analyzer id="sbstops" class="org.apache.lucene.analysis.snowball.SnowballAnalyzer">
    <param name="name" value="English"/>
    <param name="stopwords" type="org.apache.lucene.analysis.util.CharArraySet">
        <value>the</value>
        <value>this</value>
        <value>and</value>
        <value>that</value>
    </param>
</analyzer>

Facets and Fields

Starting with eXist 5.0, an index configuration may define additional facets and fields. Both can hold arbitrary content, which will be attached to the indexed parent node and can be used to further refine a query, sort results or display additional information to the user:

facet

a facet defines a concept or information item by which the indexed items can be grouped. Typical facets would be categories taken from some pre-defined taxonomy, languages, dates, places or names occurring in a text corpus. The goal is to enable users to "drill down" into a potentially large result set by selecting from a list of facets displayed to them. For example, if you shop for a laptop, you are often presented with a list of facets with which you may restrict your result by CPU type, memory or screen size etc. As you select facets, the result set will become smaller and smaller.

Facets are always pre-defined at indexing time, so the drill down is very fast. They are meant for refining other queries, the assumption always is that the user selects one or more facets from a list of facet values associated with the current query results.

field

a field contains additional, searchable content attached to an indexed parent node. In many cases fields will contain constructed content which is not directly found in the indexed XML or requires costly computation. For example, determining publication dates or author names for a set of articles may require some pre-processing which may be too expensive at query time. A field allows you to pre-compute those information items at indexing time.

Fields can be queried in the same expression as the parent node, resulting in fast response times. Their content can optionally be stored to speed up display or sorting. Fields may also use a different analyzer than the parent node, which allows e.g. multiple languages to be handled separately.

<?xml version="1.0" encoding="UTF-8"?>
<collection xmlns="http://exist-db.org/collection-config/1.0">
    <index xmlns:db="http://docbook.org/ns/docbook" xmlns:xs="http://www.w3.org/2001/XMLSchema">
        <lucene>
            <text qname="db:article">
                <facet dimension="keyword" expression="db:info/db:keywordset/db:keyword"/>
            </text>
        </lucene>
    </index>
</collection>

<?xml version="1.0" encoding="UTF-8"?>
<collection xmlns="http://exist-db.org/collection-config/1.0">
    <index xmlns:db="http://docbook.org/ns/docbook" xmlns:xs="http://www.w3.org/2001/XMLSchema">
        <lucene>
            <text qname="db:article">
                <facet dimension="keyword" expression="db:info/db:keywordset/db:keyword"/>
                <facet dimension="date" expression="tokenize(db:info/db:pubdate, '-')" hierarchical="yes"/>
            </text>
        </lucene>
    </index>
</collection>

<collection xmlns="http://exist-db.org/collection-config/1.0">
    <index xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns:db="http://docbook.org/ns/docbook">
        <lucene>
            <module uri="http://exist-db.org/lucene/test/" prefix="idx" at="module.xql"/>
            <text qname="db:article">
                <facet dimension="keyword" expression="db:info/db:keywordset/db:keyword"/>
                <facet dimension="date" expression="tokenize(db:info/db:pubdate, '-')" hierarchical="yes"/>
                <facet dimension="subject" expression="idx:subject-hierarchy(db:info/db:subjectset/db:subject/db:subjectterm)" hierarchical="yes"/>
            </text>
        </lucene>
    </index>
</collection>

declare function idx:subject-hierarchy($key as xs:string*) {
    array:for-each (array {$key}, function($k) {
        doc('/db/subjects/subjects.xml')//subject[@name=$k]/ancestor-or-self::subject/@name
    })
};

<subject>
    <subject name="science">
        <subject name="math"/>
        <subject name="physics"/>
    </subject>
    <subject name="humanities">
        <subject name="art"/>
        <subject name="sociology"/>
        <subject name="history"/>
    </subject>
</subject>

<?xml version="1.0" encoding="UTF-8"?>
<collection xmlns="http://exist-db.org/collection-config/1.0">
    <index xmlns:db="http://docbook.org/ns/docbook" xmlns:xs="http://www.w3.org/2001/XMLSchema">
        <lucene>
            <text qname="db:article">
                <facet dimension="keyword" expression="db:info/db:keywordset/db:keyword"/>
                <field name="title" expression="db:info/db:title"/>
                <field name="author" expression="for $au in db:info/db:author string-join(($au/db:personname/db:firstname, $au/db:personname/db:surname), ' ')"/>
            </text>
        </lucene>
    </index>
</collection>

<collection xmlns="http://exist-db.org/collection-config/1.0">
    <index xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns:db="http://docbook.org/ns/docbook">
        <lucene>
            <module uri="http://exist-db.org/lucene/test/" prefix="idx" at="module.xql"/>
            <text qname="db:article">
                <facet dimension="keyword" expression="db:info/db:keywordset/db:keyword"/>
                <field name="title" expression="db:info/db:title"/>
                <field name="author" expression="idx:author(db:info/db:author)"/>
            </text>
        </lucene>
    </index>
</collection>

<collection xmlns="http://exist-db.org/collection-config/1.0">
    <index xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns:db="http://docbook.org/ns/docbook">
        <lucene>
            <analyzer class="org.apache.lucene.analysis.de.GermanAnalyzer" id="german"/>
            <analyzer class="org.apache.lucene.analysis.en.EnglishAnalyzer" id="english"/>
            <text qname="db:article" index="no">
                <field name="english" if="@xml:lang='en'" analyzer="english"/>
                <field name="german" if="@xml:lang='de'" analyzer="german"/>
            </text>
        </lucene>
    </index>
</collection>

Querying Fields

Fields associated with the indexed parent node (see above) can be queried with ft:query by prefixing parts of the query expression with the field name followed by a colon (':') as described in the documentation for Lucene's default query syntax. For example, the following expression searches for a docbook article containing the terms "xml" in the text and "xquery language" in the title:

//db:article[ft:query(., "title:(xquery AND language) AND xml")]

Note how subexpressions can be grouped with parentheses to clearly state to which field they apply.

Retrieving Field Content

You can retrieve the content of a field for display or sorting purposes using the ft:field function. However, fields are always bound to the result of a full text query, so you cannot retrieve them without calling ft:query first. Also, keeping track of fields can be expensive, so ft:query needs to be explicitly passed a parameter specifying which fields should be made available via the query result. The third, optional, parameter to ft:query supports a fields option listing the names of the fields to be provided. The values of the associated field can then be retrieved for each item in the query result using ft:field.

One of the most common uses for retrieving field contents will be for sorting the results of a query. The order by in the example below sorts results by title first and then by author.

for $article in collection("/db/articles")//db:article[ft:query(., "xquery", map { "fields": ("title", "author") })]
order by ft:field($article, "title"), ft:field($article, "author")[1]
return
    $article

Note that even though fields are only available with the results of the ft:query, it is still possible to use them for sorting and displaying the whole available data set. For example, to view all articles in the collection you could pass in an empty sequence in place of the query string like this:

//db:article[ft:query(., (), map { "fields": ("title", "author") })]

Typed fields: If you declared a different type than xs:string on a field, you should remember to use the 3-parameter variant of ft:field and pass in the name of the desired target type as 3rd parameter. Reason: lucene basically stores all non-text data types as numbers and eXist has no way to figure out the original type of the field. So if you defined a field with type xs:date, make sure to retrieve it with ft:field($node, "date", "xs:date"), otherwise all you get is a number.

Displaying Facet Counts

Facet counts for the query result can be retrieved if facets are associated with an indexed parent element. Facet counts for a particular dimension are available as a map containing an entry for each facet value occurring in one or more items of the query result. The map links the facet value given as a map key with a positive count corresponding to the number of times the value occurs in the result set. Facet values with zero count are never included.

For example, we may use the following query to display the facet counts for the "keyword" dimension in our set of docbook articles:

let $result := collection("/db/articles")//db:article[ft:query(., "xml")]
let $facets := ft:facets($result, "keyword", ())
return
    <table>
    {
        map:for-each($facets, function($label, $count) {
            <tr><td>{$label}</td><td>{$count}</td></tr>
        })
    }
    </table>

Function ft:facets expects a sequence of nodes belonging to a result set obtained from one or more calls to ft:query. If the sequence was combined from multiple expressions calling ft:query, the facet counts will be merged. Second parameter of ft:facets specifies the dimension for which facet counts should be retrieved. The third parameter should be either empty sequence or a positive integer denoting the maximum number of facets to show. In the case it is smaller than the total number of facets, only those with the highest counts are returned. Passing an empty sequence means that all facet value counts should be shown. Please note that facets with a zero occurrence count (i.e. facets not appearing anywhere in the result) are never returned.

For hierarchical facets only the top-most facet value in the hierarchy will be returned by default. For example, if you indexed a date facet with separate year, month and day component, a call to ft:facets($node, "date", ()) will return facet counts for years only. To also get counts for months, you have to call ft:facets with a fourth parameter, passing in the year for which sub-facet counts should be retrieved. To get days, you also need to specify month and so on. ft:facets($node, "date", (), ("2018", "06")) will thus return facet counts for all days in June 2018.

Refining a Query with Facets

The main purpose of facets is to quickly narrow down a query result, limiting it to only items which match a certain facet value. To drill down by a given facet dimension and value, pass a key "facets" in the options map given in the third parameter of ft:query:

let $options := map { 
    "facets": map { 
        "keyword": ("indexing", "facets")
    }
}
return
    collection("/db/articles")//db:article[ft:query(., "xml", $options)]

If you specify multiple dimensions, these will be linked together with a logical and, limiting the result to elements matching both dimensions.

Treatment of multiple values for one facet dimension depends on the type of facet. For non-hierarchical facets, as in example above, if you specify more than one value these will be linked together with a logical or, returning elements matching any of the alternative facet values for that dimension.

In case of hierarchical facets, a sequence of items is interpreted as a hierarchical value/subvalue facet path, therefore expression like ("2018", "06", "25") for the date dimension mentioned earlier will return nodes from the 25th of June 2018. Nested sequences are not allowed in XQuery, so the only way to pass in multiple hierarchical facet paths is by wrapping the whole structure in an array. Each array element then is logically linked with its other members with or expression. To query for elements from June or May 2018 we therefore need to specify date dimension values as [("2018", "06"), ("2018", "05")]

let $options := map { 
    "facets": map { 
        "keyword": ("indexing", "facets"),
        "date": [("2018", "06"), ("2018", "05")]
    }
}
return
    collection("/db/articles")//db:article[ft:query(., "xml", $options)]

Describing Queries in XML

Lucene's default query syntax does not provide access to all available features. However, eXist-db's ft:query function also accepts a description of the query in XML, as an alternative to passing a query string. The XML description closely mirrors Lucene's query API. It is transformed into an internal tree of query objects, which is directly passed to Lucene for execution. This has several advantages, for example you can specify if the order of terms should be relevant for a phrase query:

let $query :=
    <query>
        <near ordered="no">miserable nation</near>
    </query>
return
    //SPEECH[ft:query(., $query)]

The following elements may occur within a query description:

<term>

Defines a single term to be searched in the index. If the root query element contains a sequence of term elements, wrap them in <bool/> and they will be combined as in a boolean "or" query. For example:

let $query :=
    <query>
        <bool><term>nation</term><term>miserable</term></bool>
    </query>
return
//SPEECH[ft:query(., $query)]

This finds all <SPEECH> elements containing either nation or miserable or both.

<wildcard>

A string with a * wildcard in it. This will be matched against the terms of a document. Can be used instead of a <term> element. For example:

let $query :=
    <query>
        <bool><term>nation</term><wildcard>miser*</wildcard></bool>
    </query>
return
//SPEECH[ft:query(., $query)]

<regex>

A regular expression which will be matched against the terms of a document. Can be used instead of a <term> element. For example:

let $query :=
    <query>
        <bool><term>nation</term><regex>miser.*</regex></bool>
    </query>
return
//SPEECH[ft:query(., $query)]

<bool>

Constructs a boolean query from its children. Each child element may have an occurrence indicator, which could be either must, should or not:

must

this part of the query must be matched

should

this part of the query should be matched, but doesn't need to

not

this part of the query must not be matched

For instance:

let $query :=
    <query>
        <bool><term occur="must">boil</term><term occur="should">bubble</term></bool>
    </query>
return //SPEECH[ft:query(LINE, $query)]

To optimize performance you can specify a minimum number of matches to prevent needless disjunctive searches, using the min attribute. If no occurence indicator is provided the query will default to should, as this is the only indicator that supports min:

let $query := <query><bool min="3"><term>witch</term></bool></query>

return
  //SPEECH[ft:query(LINE, $query)]        

<phrase>

Searches for a group of terms occurring in the correct order. The element may either contain explicit <term> elements or text content. Text will be automatically tokenized into a sequence of terms. For example:

let $query :=
    <query>
        <phrase>cauldron boil</phrase>
    </query>
return //SPEECH[ft:query(., $query)]

This has the same effect as:

let $query :=
    <query>
        <phrase><term>cauldron</term><term>boil</term></phrase>
    </query>
return //SPEECH[ft:query(., $query)]

The attribute slop can be used for a proximity search: Lucene will try to find terms which are within the specified distance:

let $query :=
    <query>
        <phrase slop="10"><term>frog</term><term>dog</term></phrase>
    </query>
return //SPEECH[ft:query(., $query)]

<near>

<near> is a powerful alternative to <phrase> and one of the features not available through the standard Lucene query parser.

If the element has text content only, it will be tokenized into terms and the expression behaves like <phrase>. Otherwise it may contain any combination of <term>, <first> and nested <near> elements. This makes it possible to search for two sequences of terms which are within a specific distance. For example:

let $query :=
    <query>
        <near slop="20"><term>snake</term><near slop="1">tongue dog</near></near>
    </query>
return //SPEECH[ft:query(., $query)]

Element <first> matches a span against the start of the text in the context node. It takes an optional attribute end to specify the maximum distance from the start of the text. For example:

let $query :=
    <query>
        <near slop="50"><first end="2"><near>second witch</near></first><near
slop="1">tongue dog</near></near>
    </query>
    return //SPEECH[ft:query(., $query)]

As shown above, the content of <first> can again be text, a <term> or <near>.

Contrary to <phrase>, <near> can be told to ignore the order of its components. Use parameter ordered="yes|no" to change near's behaviour. For example:

let $query :=
    <query>
        <near slop="100" ordered="no"><term>bubble</term><term>fillet</term></near>
    </query>
return //SPEECH[ft:query(., $query)]

All elements in a query may have an optional boost parameter (float). The score of the nodes matching the corresponding query part will be multiplied by this factor.

Additional parameters

The ft:query function allows a third parameter for passing additional settings to the query engine. This parameter must be an XML fragment which lists the configuration properties to be set as child elements:

let $options :=
    <options>
        <default-operator>and</default-operator>
        <phrase-slop>1</phrase-slop>
        <leading-wildcard>no</leading-wildcard>
        <filter-rewrite>yes</filter-rewrite>
    </options>
return
    //SPEECH[ft:query(., $query, $options)]

The meaning of those properties is as follows

filter-rewrite

Controls how terms are expanded for wildcard or regular expression searches. If set to yes, Lucene will use a filter to pre-process matching terms. If set to no, all matching terms will be added to a single boolean query which is then executed. This may generate a "too many clauses" exception when applied to large data sets. Setting filter-rewrite to yes avoids those issues.

default-operator

The default operator with which multiple terms will be combined. Allowed values: or, and.

phrase-slop

Sets the default slop for phrases. If 0, then exact phrase matches are required. Default value is 0.

leading-wildcard

When set to yes, * or ? are allowed as the first character of a PrefixQuery and WildcardQuery. Note that this can produce very slow queries on big indexes.