Boolean Filters

If the documents being indexed represent people, you might have a gender field (e.g. M for Male, F for Female, X for Unknown). Gender doesn't have a standard prefix, so you might allocated "XGENDER". And then lowercase the field contents to avoid needing to always add a colon. So documents will be indexed by one of XGENDERm, XGENDERf, or XGENDERx.

If you're indexing using scriptindex, and have a field in the input file which can be "gender=M", etc, then your index script would have a rule such as:

gender : lower boolean=XGENDER

You can then restrict a search in Omega by passing a B parameter with one of these as the value, e.g. B=XGENDERf

In your HTML search form, you can allow the user to select this using a set of radio buttons:

Gender:<br>
<input type="radio" name="B" value=""> any<br>
<input type="radio" name="B" value="XGENDERf"> female<br>
<input type="radio" name="B" value="XGENDERm"> male<br>

If you want to have multiple sets of radio buttons for selecting different boolean filters, you can make use of Omega's preprocessing of CGI parameter names by calling them "B 1", "B 2", etc (names are truncated at the first space - see cgiparams.html for full details).

You can also use a select tag:

Gender:
<select name="B">
<option value="">any</option>
<option value="XGENDERf">female</option>
<option value="XGENDERm">male</option>
<option value="XGENDERx">unknown</option>
</select>

You can also allow the user to restrict a search with a boolean filter specified in text query (e.g. sex:f -> XGENDERf) by adding this to the start of your OmegaScript template:

$setmap{boolprefix,sex,XGENDER}

Multiple aliases are allowed:

$setmap{boolprefix,sex,XGENDER,gender,XGENDER}

This decoupling of internal and external names is also useful if you want to offer search frontends in more than one language, as it allows the prefixes the user sees to be translated.

Probabilistic Fields

Say you want to index the title of the document such that the user can search within the title by specifying title:report (for example) in their query.

Title has standard prefix S, so you'd generate terms as normal, but then add an "S" prefix. If you're using scriptindex, then you do this by adding "index=S" to the scriptindex rule like so:

title : field=title index=S

You then need to tell Xapian::QueryParser that "title:" maps to an "S" prefix. If you're using Omega, then you do so by adding this to your OmegaScript template (at the start is best):

$setmap{prefix,title,S}

Or if you're writing your own search frontend, like this:

Xapian::QueryParser qp;
qp.add_prefix("subject", "S");
// And similar lines for other probabilistic prefixes...
// And any other QueryParser configuration (e.g. stemmer, stopper).
Xapian::Query query = qp.parse_query(user_query_string);

You can add multiple aliases for a prefix (e.g. title and subject for S), and the decoupling of "UI prefix" and "term prefix" means you can easily translate the "UI prefixes" if you have frontends in different languages.

Note that if you want words from the subject to be found without a prefix, you either need to generate unprefixed terms as well as the prefixed ones, or map the empty prefix to both "" and "S" like so:

Xapian::QueryParser qp;
// Search both subject and body if no field is specified:
qp.add_prefix("", "");
qp.add_prefix("", "S");
// Search just the subject if 'subject:' is specified:
qp.add_prefix("subject", "S");
Xapian::Query query = qp.parse_query(user_query_string);