The index plan¶
In order to index the CSV, we want to take two fields from each row, title and description, and turn them into suitable terms. For straightforward textual search we don’t need document values.
Because we’re dealing with free text, and because we know the whole dataset is in English, we can use stemming so that for instance searching for “sundial” and “sundials” will both match the same documents. This way people don’t need to worry too much about exactly which words to use in their query.
Finally, we want a way of separating the two fields. In Xapian this is done using term prefixes, basically by putting short strings at the beginning of terms to indicate which field the term indexes. As well as prefixed terms, we also want to generate unprefixed terms, so that as well as searching within fields you can also search for text in any field.
There are some conventional prefixes used, which is helpful if you ever need to interoperate with omega (a web-based search engine) or other compatible systems. From this, we’ll use ‘S’ to prefix title (it stands for ‘subject’), and for description we’ll use ‘XD’. A full list of conventional prefixes is given at the top of the omega documentation on termprefixes.
When you’re indexing multiple fields like this, the term positions used for each field when indexed unprefixed need to be kept apart. Say you have a title of “The Saints”, and description “Don’t like rabbits? Keep reading.” If you index those fields without a gap, the phrase search “Saints don’t like rabbits” will match, where it really shouldn’t. Usually a gap of 100 between each field is enough.
To write to a database, we use the WritableDatabase class, which allows us to create, update or overwrite a database.
To create terms, we use Xapian’s TermGenerator, a built-in class to make turning free text into terms easier. It will split into words, apply stemming, and then add term prefixes as needed. It can also take care of term positions, including the gap between different fields.