Highlighting Results

To present search results it is ideal to show a part of each document and how it is related to the query. Usually, search engines show fragments of the document with marked search terms. GaussDB(DWS) provides function ts_headline that implements this functionality.

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ts_headline([ config regconfig, ] document text, query tsquery [, options text ]) returns text

ts_headline accepts a document along with a query, and returns an excerpt from the document in which terms from the query are highlighted. The configuration to be used to parse the document can be specified by config. If config is omitted, the default_text_search_config configuration is used.

If an options string is specified it must consist of a comma-separated list of one or more option=value pairs. The available options are:

• StartSel, StopSel: The strings with which to delimit query words appearing in the document, to distinguish them from other excerpted words. You must double-quote these strings if they contain spaces or commas.
• MaxWords, MinWords: These numbers determine the longest and shortest headlines to output.

• ShortWord: Words of this length or less will be dropped at the start and end of a headline. The default value of three eliminates common English articles.

• HighlightAll: Boolean flag. If true the whole document will be used as the headline, ignoring the preceding three parameters.

• MaxFragments: Maximum number of text excerpts or fragments to display. The default value of zero selects a non-fragment-oriented headline generation method. A value greater than zero selects fragment-based headline generation. This method finds text fragments with as many query words as possible and stretches those fragments around the query words. As a result query words are close to the middle of each fragment and have words on each side. Each fragment will be of at most MaxWords and words of length ShortWord or less are dropped at the start and end of each fragment. If not all query words are found in the document, then a single fragment of the first MinWords in the document will be displayed.

• FragmentDelimiter: When more than one fragment is displayed, the fragments will be separated by this string.

Any unspecified options receive these defaults:

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StartSel=<b>, StopSel=</b>,
MaxWords=35, MinWords=15, ShortWord=3, HighlightAll=FALSE,
MaxFragments=0, FragmentDelimiter=" ... "

For example:

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SELECT ts_headline('english',
'The most common type of search
is to find all documents containing given query terms
and return them in order of their similarity to the
query.',
to_tsquery('english', 'query & similarity'));
                        ts_headline                         
------------------------------------------------------------
 containing given <b>query</b> terms
 and return them in order of their <b>similarity</b> to the
 <b>query</b>.
(1 row)

SELECT ts_headline('english',
'The most common type of search
is to find all documents containing given query terms
and return them in order of their similarity to the
query.',
to_tsquery('english', 'query & similarity'),
'StartSel = <, StopSel = >');
                      ts_headline                      
-------------------------------------------------------
 containing given <query> terms
 and return them in order of their <similarity> to the
 <query>.
(1 row)

ts_headline uses the original document, not a tsvector summary, so it can be slow and should be used with care.