00_Intro.asciidoc

Partial matching

A keen observer will notice that all the queries so far in this book have operated on whole terms. To match something, the smallest ``unit'' had to be a single term — you can only find terms that exist in the inverted index.

But what happens if you want to match parts of a term but not the whole thing? Partial matching allows the user to specify a portion of the term they are looking for and find any words that contain that fragment.

The requirement to match on part of a term is less common in the full text search engine world than you might think. If you have come from an SQL background, it is likely that you have, at some stage of your career, implemented a ``poor man’s full text search'' using SQL constructs like:

    WHERE text LIKE "*quick*"
      AND text LIKE "*brown*"
      AND text LIKE "*fox*" (1)

1. "fox" would match fox'' and foxes''.

Of course, with Elasticsearch we have the analysis process and the inverted index which remove the need for such brute-force techniques. To handle the case of matching both fox'' and foxes'' we could simply use a stemmer to index words in their root form. There is no need to match partial terms.

That said, there are occasions when partial matching can be very useful. Common use cases include:

• matching postcodes, product serial numbers or other not_analyzed values which start with a particular prefix or match some wildcard pattern or even a regular expression.

• search-as-you-type — displaying the most likely results before the user has finished typing their search terms

• matching in languages like German or Dutch which contain long compound words, like Weltgesundheitsorganisation (World Health Organisation)

We will start by examining prefix matching on exact value not_analyzed fields.