Introduction to libre « fulltext » technology

[ Rencontres Mondiales du Logiciel Libre 2010 - Thursday, July 8 ]

Introduction to libre « fulltext »
technology
Author : Ir Robert Viseur

Who am I?

• My name : Robert Viseur
• Civil Engineer, Master in Management of Innovation.
• Specific expertise in the economics of free software and
practices of co-creation.
• Manager of logiciellibre.com (directory of free software
companies).
• Assistant in the Department of Economics and Management
of Innovation (mi.fpms.ac.be) of the Faculty of Engineering,
University of Mons (www.umons.ac.be).
• Technology advisor at CETIC (www.cetic.be).
• Belgian ICT Research and Technology Transfer Centre.
• Initiator of Cellavi project (Center of Expertise for Open Source use
in Industrial Applications).

What do we talk about?

• Limits of conventional DBMS for text searching.
• Technologies for fulltext in DBMS (MySQL fulltext,
PostgreSQL tsearch, Sphynx Search for MySQL).
• Free indexers (Lucene family, Xapian).
• We will not speak about NoSQL databases
(Cassandra, CouchDB, HBase,...).

Why is this useful?

• Look for articles in a CMS (Content Management
System), for posts in a forum or for items in an
online shop.
• Research in news, podcasts or RSS / Atom feeds.
• Research in the content of books or PDF papers.
• ...

What are the important steps?

• Four important steps:
• the extraction,
• the indexing,
• the research,
• the presentation of results.

Step 1 : the extraction (1/2)

• Conversion of the file to index in plain text.
• Simple cases :
• structured text files
• Examples:
– XML (with PHP::SimpleXML),
– RSS (with PHP::SimplePie),
– HTML (with PHP::strip_tags or HTML analyzer).
• documented complex formats
• Examples: ODF = XML compressed file (ZIP).

Step 1 : the extraction (2/2)

• Complex cases: undocumented binary formats.
• Example: Office formats (97, 2000, XP ,...)
• Use of Open Source projects:
• Apache Jakarta POI (MS Office), Apache Tika (various
documents), xls2csv (Microsoft Excel), catdoc (Microsoft
Word), pdfinfo (PDF ),...
• Extraction often imperfect (~ 20% error with POI).
• Use IFilters (MS Windows).
• Extensions proposed by the publishers themselves to extract
the contents of files (Microsoft Office, Autocad, etc.).
• Scanned documents: OCR Open Source solutions.
• See Tesseract, OCRAD and GOCR (still emerging).

Steps 2 & 3: indexing and search

• What everybody knows: SELECT ... WHERE ...
LIKE ...
• What is less known: the regular expressions.
• What you may be tempted to do: « do it
yourself ».
• What it is instead recommended to do: use
standard technologies.

Step 4: presentation

• Export results to XML (API: OpenSearch standard
format)
• XSL transformations to RSS, Atom,...
• Spellchecking (eg with Aspell or with algorithms
like Soundex or Levenshtein distance)
• Classification of results (eg with Reverend).

« Do it yourself » approaches

What happens if there are no
fulltext solutions ? (1/2)
• This is particularly the case in SQLite.
• The most famous: LIKE
• Example: SELECT news.title, news.url FROM news
WHERE news.title LIKE '%linux%'
• Possible improvements: decomposition of research
in tokens, regular expression filtering, ...
• Disadvantage: problem of relevance, not suitable
for large volumes of data.

What happens if there are no
fulltext solutions ? (2/2)
• The attached functions or regular expressions in SQL:
• Example (PHP, SQLite):
• sqlite_create_function ($db, 'sqlite_fulltext', 'sqlite_fulltext', 2);
• $sql = "SELECT * FROM torrents WHERE sqlite_fulltext (Search,".
Sqlite_escape_string ($ q )."')== 1 ORDER BY Title ";
• In "sqlite_fulltext", using: preg_match ("/b($word)b/i", $text).
• Disadvantage: not suitable for large volumes of data.
• Regular expressions also supported by MySQL, PostgreSQL
and Firebird (since 2.5).

Is it possible to do by yourself?
(1/2)
• Yes, but difficult development.
• Create the dictionary:
• Filtering of text by removing non-alphanumeric characters.
• Decomposition of text filtered "terms" (tokens).
• Removal of black words (the, the, the, my, your, their,
our, your, their,...).
• Establishment of a correspondence table (identifier of the
document, term).
– Each term is associated with a list of documents containing
that term.
• Write the good SQL requests...

Is it possible to do by yourself?
(2/2)
• Possible improvements:
• Lemmatization terms.
• Each term is replaced by its canonical form.
– Example: studying, studying, student, students, ... => "study".
• Several open source implementations of the Porter algorithm
(eg Snowball).
• Associations of the terms or lemmas with a phonetic
form (soundex, Metaphone, etc.).
• Warning: stemming and phonetic forms depend on
the language.
• Automatic language detection (eg in PHP :
Text_LanguageDetect in PEAR).

Fulltext standard solutions for DBMS

What exists?

• SQLite: not included as standard (?) but SQLite FTS3
extension (untested).
• Firebird (untested): no standard module but extensions
(including Sphinx Search).
• MySQL: MySQL FULLTEXT standard module and
extensions (including Sphinx Search).
• PostgreSQL: PostgreSQL standard module tsearch
(standard since v8.3).
• Other: Senna (untested).
• Triton = MySQL + Senna | Ludia = PostgreSQL + Senna.
• Cfr. Kazuhiko Shiozaki (Solutions Linux 2008).

MySQL fulltext (1/2)

• MySQL provides an automatic fulltext mode.
• Creation:
• CREATE TABLE news ( id INT UNSIGNED AUTO_INCREMENT
NOT NULL PRIMARY KEY, title VARCHAR(256), body TEXT,
FULLTEXT (title, body) )
• Selection:
• SELECT id, title, body, MATCH (title,body) AGAINST ('linux')
AS score FROM news WHERE MATCH (title,body) AGAINST
('linux') ORDER BY score

MySQL Fulltext (2/2)

• Strengths:
• Supported on most shared web hosting services;
• Support the creation of the dictionary and the analysis of the
request;
• Availability of search operators,
• Evaluation of a score of relevance,
• Mechanism for query expansion, ...
• Weaknesses:
• No control over the analysis of the text (tokenisation but not
stemming)
• Minimum size of tokens (terms) set by default to 4
characters (not editable on a shared web hosting service).

MySQL with Sphinx Search

• Sphinx Search extension must be compiled for
MySQL.
• Support for PostgreSQL.
• Used by craiglist.org, mininova.org, ...
• Strengths: support very large volumes of data (>
100 GB of text), storage always provided by
MySQL, portable.
• Weaknesses: stemming limited to English and
Russian.

PostgreSQL tsearch (1/2)

• PostgreSQL offers an automatic fulltext mode.
• Creation:
• ALTER TABLE tpages ADD COLUMN vecteur tsvector;
UPDATE tpages SET vecteur=to_tsvector(contenu);
• Selection:
• SELECT * FROM docs WHERE vecteur @@
to_tsquery('tsearch2');
• Advanced Features:
• Score of relevance with ts_rank.
• Creation of "snippet" with ts_headline.

PostgreSQL tsearch (2/2)

• Strengths: query parsing, stemming function of
language.
• Weakness: not supported on shared web hosting
services.

What exists?

• Lucene (and its multiple ports),
• Other: Xapian,...

Xapian

• Fork of Open Muscat (BrightStation PLC); C ++,
GPL.
• Strengths: many bindings, import filters
(extraction), stemming (many languages
supported), synonymy (extensions of requests),
correction of requests, support for indexing SQL
databases (MySQL, PostgreSQL, SQLite, Oracle,
DB2, MS SQL, LDAP and ODBC).
• Weaknesses: less popular.

Lucene (1/3)

• Supported by the Apache Foundation.
• Wide ecosystem:
• Used in Alfresco, Jahia ...
• Multiple integrations (eg CouchDB-lucene).
• Many third-party tools: Luke (read index), Solr
(search server; without crawler), Nutch (search
engine with crawler), Carrot² (search interface
compatible with OpenSearch and Solr),...
• Lucene index format becomes a kind of standard.

Lucene (2/3)

• Many ports (Perl, Python, .Net,...).
• Lucene.Net (. Net) PyLucene (Python), CLucene
(C++) Plucene (Perl), Zend Search (PHP).
• Warning: functional coverage, release of supported
index!
• Three types of port:
• by literal translation (API compatible),
• translation adapted for the target language (best
performance),
• by binding (for Python).

Lucene (3/3)

• Ability to change the text analyzers.
• Access to the dictionary of terms.
• Multiple search operators (AND, OR, NOT, +, -, ?,
*, ...).
• Exact or fuzzy search, management of
synonyms, ...
• Ability to search by fields (eg title: linux).
• Ability to sort by field.

What was tested?

• Taking into account:
• the speed of index creation,
• the speed of insertion
• the speed of removal,
• the speed of search.
• No systematic consideration of relevance.
• Two sets of data:
• 20,000 textual data from 1kB to 900kB,
• 200,000 textual data from 2kB to 5kB.

MySQL, PostgreSQL, Sphinx Search
(2008) (1/2)
• MySQL:
• smaller index,
• slower search compared to PostgreSQL or Sphinx
Search,
• deletion is slow
• Insertion is very slow with bigger data.
• PostgreSQL:
• index creation is slow,
• insertion is very slow with bigger data.

MySQL, PostgreSQL, Sphinx Search
(2008) (2/2)
• Sphinx Search (with MySQL) :
• manual (re)indexation (but very fast),
• fast searches,
• relatively insensitive to data size.

Xapian, Lucene, PyLucene,
Lucene.Net (2008)
• Xapian:
• Slow when creating or updating the index, large
index (compared to Lucene);
• Installation more difficult.
• Lucene:
• Performance fairly homogeneous (Lucene, PyLucene
and Lucene.Net);
• PyLucene significantly slower in creating and
updating of the index (why?).

Zend Search (2010)

• PHP technology built into the Zend framework.
• Easily hostable.
• Very useful for small volumes of data.
• Fragility of index (corruption under heavy
solicitation in insertion).
• Example: www.retronimo.com (search RSS).

Which technology to choose? (1/2)

• Database or index?
• Indexer whether purely textual data.
• Database if:
• Structured data,
• Need of relational model,
• Need of SQL language.

Which technology to choose? (2/2)

• Databases:
• MySQL: well suited for basic solutions (relevance average,
good performance on small data sizes), easily hostable,
integrated platform LAMP / MAMP / WAMP.
• PostgreSQL: well suited for professional solutions (but
avoid with bigger data).
• Sphinx Search: suitable for large volumes of data of any
size.
• Indexers:
• Lucene confirms its reputation as a reference.
• Zend Search only useful for small volumes of data.

Thanks!

Thank you for your attention.

Questions?

Tools (1/2)

• SQLite (www.sqlite.org).
• MySQL (www.mysql.com).
• WampServer (www.wampserver.com).
• Sphinx Search (www.sphinxsearch.com).
• PostgreSQL (www.postgresql.org).
• Tritonn (qwik.jp/tritonn/).
• Lucene (lucene.apache.org).
• Zend framework (framework.zend.com).
• Xapian (xapian.org).

Tools (2/2)

• SolR (lucene.apache.org/solr/).
• Carrot² (project.carrot2.org).
• Luke (www.getopt.org/luke/).
• Nutch (nutch.apache.org).
• Tesseract (tesseract-ocr.googlecode.com).
• Apache POI (poi.apache.org).
• Snowball (snowball.tartarus.org).
• Reverend
(divmod.org/trac/wiki/DivmodReverend).

Resources and useful links (1/3)

• Justine Demeyer (stagiaire), Robert Viseur (maître de stage) et
Tom Mens (directeur de stage) (2008). Comparaison de
technologies d'indexation fulltext. UMons / CETIC, 2008.
• Robert Viseur (2008). "Solutions Linux: session sur l'indexation
fulltext dans les SGBD". URL:
http://www.robertviseur.be/news-20080222.php .
• Robert Viseur (2008). "Atelier de présentation du mode
FULLTEXT de PostgreSQL 8.3 aux RMLL 2008". URL:
• Robert Viseur (2009). "Première comparaison de Tesseract,
OCRAD, GOCR et... PhpOCR". URL:


• Erik Hatcher et Otis Gospodnetić (2004). "Lucene in
Action". Manning Publications Co.
• "Annexe F. Expressions régulières MySQL". URL:
http://dev.mysql.com/doc/refman/5.0/fr/regexp.html .
• "9.7. Pattern Matching". URL: http://www.regular-
expressions.info/postgresql.html .
• Philippe Makowski (2009). "Firebird 2.5, les principales
nouveautés". Code way 3. 16-20 novembre 2009. URL:
http://www.firebirdsql.org/download/rabbits/pmakowski
/firebird-25.pdf .
• "Does Firebird support full-text search?". URL:
http://www.firebirdfaq.org/faq328/ .


• Björn Reimer & Dirk Baumeister (2006). "Full text search in Firebird without a full
text search engine". Firebird Conference Prague 2006. URL:
http://www.ibphoenix.com/downloads/FirebirdConf2006/TECH-TPZ303-R/TECH-
TPZ303-R.zip .
• "SQLite FTS3 Extension". URL: http://www.sqlite.org/fts3.html .
• "Full-Text Search on SQLite". URL: http://michaeltrier.com/2008/7/13/full-text-
search-on-sqlite .
• "Tritonn - MySQL with Senna". Sumisho Computer Systems Corporation Brazil, Inc.
URL: http://qwik.jp/tritonn/about_en.files/tritonn-eng.pdf .
• Kazuhiko Shiozaki (2008). "Moteurs plein texte sous MySQL et PostgreSQL pour la
gestion de connaissances". Solutions Linux, 2008. URL:

Contact

• Ir. Robert Viseur.
• Email : robert.viseur@cetic.be
• Phone : 0032 (0) 479 66 08 76

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