10_Slop.asciidoc

Mixing it up

Requiring exact phrase matches may be too strict a constraint. Perhaps we do want documents which contain quick brown fox'' to be considered a match for the query quick fox'', even though the positions aren’t exactly equivalent.

We can introduce a degree of flexibility into phrase matching by using the slop parameter:

GET /my_index/my_type/_search
{
    "query": {
        "match_phrase": {
            "title": "quick fox",
            "slop":  1
        }
    }
}

The slop parameter tells the match_phrase query how far apart terms are allowed to be while still considering the document a match. By ``how far apart'' we mean: how many times do you need to move a term in order to make the query and document match?

We’ll start with a simple example. To make the query "quick fox" match a document containing "quick brown fox" we need a slop of just 1:

            Pos 1         Pos 2         Pos 3
-----------------------------------------------
Doc:        quick         brown         fox
-----------------------------------------------
Query:      quick         fox
Slop 1:     quick                 ↳     fox

While all words need to be present in phrase matching, even when using slop, the words don’t necessarily need to be in the same sequence in order to match. With a high enough slop value, words can be arranged in any order.

To make the query "fox quick" match our document, we need a slop of 3:

            Pos 1         Pos 2         Pos 3
-----------------------------------------------
Doc:        quick         brown         fox
-----------------------------------------------
Query:      fox           quick
Slop 1:     fox|quick  ↵  (1)
Slop 2:     quick      ↳  fox
Slop 3:     quick                 ↳     fox

1. Note that fox and quick occupy the same position in this step. Switching word order from fox quick'' to quick fox'' thus requires two steps, or a slop of 2.