How to find similar results and sort by similarity?

Question

I have found out that the Levenshtein distance may be good when you are searching a full string against another full string, but when you are looking for keywords within a string, this method does not return (sometimes) the wanted results. Moreover, the SOUNDEX function is not suitable for languages other than english, so it is quite limited. You could get away with LIKE, but it’s really for basic searches. You may want to look into other search methods for what you want to achieve. For example:

You may use Lucene as search base for your projects. It’s implemented in most major programming languages and it’d quite fast and versatile. This method is probably the best, as it not only search for substrings, but also letter transposition, prefixes and suffixes (all combined). However, you need to keep a separate index (using CRON to update it from a independent script once in a while works though).

Or, if you want a MySQL solution, the fulltext functionality is pretty good, and certainly faster than a stored procedure. If your tables are not MyISAM, you can create a temporary table, then perform your fulltext search :

CREATE TABLE IF NOT EXISTS `tests`.`data_table` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `title` varchar(2000) CHARACTER SET latin1 NOT NULL,
  `description` text CHARACTER SET latin1 NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB  DEFAULT CHARSET=utf8 COLLATE=utf8_bin AUTO_INCREMENT=1 ;

Use a data generator to generate some random data if you don’t want to bother creating it yourself…

** NOTE ** : the column type should be latin1_bin to perform a case sensitive search instead of case insensitive with latin1. For unicode strings, I would recommend utf8_bin for case sensitive and utf8_general_ci for case insensitive searches.

DROP TABLE IF EXISTS `tests`.`data_table_temp`;
CREATE TEMPORARY TABLE `tests`.`data_table_temp`
   SELECT * FROM `tests`.`data_table`;

ALTER TABLE `tests`.`data_table_temp`  ENGINE = MYISAM;

ALTER TABLE `tests`.`data_table_temp` ADD FULLTEXT `FTK_title_description` (
  `title` ,
  `description`
);

SELECT *,
       MATCH (`title`,`description`)
       AGAINST ('+so* +nullam lorem' IN BOOLEAN MODE) as `score`
  FROM `tests`.`data_table_temp`
 WHERE MATCH (`title`,`description`)
       AGAINST ('+so* +nullam lorem' IN BOOLEAN MODE)
 ORDER BY `score` DESC;

DROP TABLE `tests`.`data_table_temp`;

Read more about it from the MySQL API reference page

The downside to this is that it will not look for letter transposition or “similar, sounds like” words.

** UPDATE **

Using Lucene for your search, you will simply need to create a cron job (all web hosts have this “feature”) where this job will simply execute a PHP script (i.g. “cd /path/to/script; php searchindexer.php”) that will update the indexes. The reason being that indexing thousands of “documents” (rows, data, etc.) may take several seconds, even minutes, but this is to ensure that all searches are performed as fast as possible. Therefore, you may want to create a delay job to be run by the server. It may be overnight, or in the next hour, this is up to you. The PHP script should look something like this:

$indexer = Zend_Search_Lucene::create('/path/to/lucene/data');

Zend_Search_Lucene_Analysis_Analyzer::setDefault(
  // change this option for your need
  new Zend_Search_Lucene_Analysis_Analyzer_Common_Utf8Num_CaseInsensitive()
);

$rowSet = getDataRowSet();  // perform your SQL query to fetch whatever you need to index
foreach ($rowSet as $row) {
   $doc = new Zend_Search_Lucene_Document();
   $doc->addField(Zend_Search_Lucene_Field::text('field1', $row->field1, 'utf-8'))
       ->addField(Zend_Search_Lucene_Field::text('field2', $row->field2, 'utf-8'))
       ->addField(Zend_Search_Lucene_Field::unIndexed('someValue', $someVariable))
       ->addField(Zend_Search_Lucene_Field::unIndexed('someObj', serialize($obj), 'utf-8'))
  ;
  $indexer->addDocument($doc);
}

// ... you can get as many $rowSet as you want and create as many documents
// as you wish... each document doesn't necessarily need the same fields...
// Lucene is pretty flexible on this

$indexer->optimize();  // do this every time you add more data to you indexer...
$indexer->commit();    // finalize the process

Then, this is basically how you search (basic search) :

$index = Zend_Search_Lucene::open('/path/to/lucene/data');

// same search options
Zend_Search_Lucene_Analysis_Analyzer::setDefault(
   new Zend_Search_Lucene_Analysis_Analyzer_Common_Utf8Num_CaseInsensitive()
);

Zend_Search_Lucene_Search_QueryParser::setDefaultEncoding('utf-8');

$query = 'php +field1:foo';  // search for the word 'php' in any field,
                                 // +search for 'foo' in field 'field1'

$hits = $index->find($query);

$numHits = count($hits);
foreach ($hits as $hit) {
   $score = $hit->score;  // the hit weight
   $field1 = $hit->field1;
   // etc.
}

Here are great sites about Lucene in Java, PHP, and .Net.

In conclusion each search methods have their own pros and cons :

You mentioned Sphinx search and it looks very good, as long as you can make the deamon run on your web host.
Zend Lucene requires a cron job to re-index the database. While it is quite transparent to the user, this means that any new data (or deleted data!) is not always in sync with the data in your database and therefore won’t show up right away on user search.
MySQL FULLTEXT search is good and fast, but will not give you all the power and flexibility of the first two.

Please feel free to comment if I have forgotten/missed anything.

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