common_grams Token Filter

The common_grams token filter is designed to make phrase queries with stopwords more efficient. It is similar to the shingles token filter (see [shingles]), which creates bigrams out of every pair of adjacent words. It is most easily explained by example.

The common_grams token filter produces different output depending on whether query_mode is set to false (for indexing) or to true (for searching), so we have to create two separate analyzers:

PUT /my_index
{
  "settings": {
    "analysis": {
      "filter": {
        "index_filter": { (1)
          "type":         "common_grams",
          "common_words": "_english_" (2)
        },
        "search_filter": { (1)
          "type":         "common_grams",
          "common_words": "_english_", (2)
          "query_mode":   true
        }
      },
      "analyzer": {
        "index_grams": { (3)
          "tokenizer":  "standard",
          "filter":   [ "lowercase", "index_filter" ]
        },
        "search_grams": { (3)
          "tokenizer": "standard",
          "filter":  [ "lowercase", "search_filter" ]
        }
      }
    }
  }
}

  1. First we create two token filters based on the common_grams token filter: index_filter for index time (with query_mode set to the default false), and search_filter for query time (with query_mode set to true).

  2. The common_words parameter accepts the same options as the stopwords parameter (see [specifying-stopwords]). The filter also accepts a common_words_path parameter, which allows you to maintain the common words list in a file.

  3. Then we use each filter to create an analyzer for index time and another for query time.

With our custom analyzers in place, we can create a field that will use the index_grams analyzer at index time:

PUT /my_index/_mapping/my_type
{
  "properties": {
    "text": {
      "type":            "string",
      "analyzer":  "index_grams", (1)
      "search_analyzer": "standard" (1)
    }
  }
}

  1. The text field uses the index_grams analyzer at index time, but defaults to using the standard analyzer at search time, for reasons we will explain next.

At Index Time

If we were to analyze the phrase The quick and brown fox with shingles, it would produce these terms:

Pos 1: the_quick
Pos 2: quick_and
Pos 3: and_brown
Pos 4: brown_fox

Our new index_grams analyzer produces the following terms instead:

Pos 1: the, the_quick
Pos 2: quick, quick_and
Pos 3: and, and_brown
Pos 4: brown
Pos 5: fox

All terms are output as unigrams—`the`, quick, and so forth—​but if a word is a common word or is followed by a common word, then it also outputs a bigram in the same position as the unigram—`the_quick`, quick_and, and_brown.

Unigram Queries

Because the index contains unigrams, the field can be queried using the same techniques that we have used for any other field, for example:

GET /my_index/_search
{
  "query": {
    "match": {
      "text": {
        "query": "the quick and brown fox",
        "cutoff_frequency": 0.01
      }
    }
  }
}

The preceding query string is analyzed by the search_analyzer configured for the text field—​the standard analyzer in this example—​to produce the terms the, quick, and, brown, fox.

Because the index for the text field contains the same unigrams as produced by the standard analyzer, search functions as it would for any normal field.

Bigram Phrase Queries

However, when we come to do phrase queries, we can use the specialized search_grams analyzer to make the process much more efficient:

GET /my_index/_search
{
  "query": {
    "match_phrase": {
      "text": {
        "query":    "The quick and brown fox",
        "analyzer": "search_grams" (1)
      }
    }
  }
}

  1. For phrase queries, we override the default search_analyzer and use the search_grams analyzer instead.

The search_grams analyzer would produce the following terms:

Pos 1: the_quick
Pos 2: quick_and
Pos 3: and_brown
Pos 4: brown
Pos 5: fox

The analyzer has stripped out all of the common word unigrams, leaving the common word bigrams and the low-frequency unigrams. Bigrams like the_quick are much less common than the single term the. This has two advantages:

  • The positions data for the_quick is much smaller than for the, so it is faster to read from disk and has less of an impact on the filesystem cache.

  • The term the_quick is much less common than the, so it drastically decreases the number of documents that have to be examined.

Two-Word Phrases

There is one further optimization. By far the majority of phrase queries consist of only two words. If one of those words happens to be a common word, such as

GET /my_index/_search
{
  "query": {
    "match_phrase": {
      "text": {
        "query":    "The quick",
        "analyzer": "search_grams"
      }
    }
  }
}

then the search_grams analyzer outputs a single token: the_quick. This transforms what originally could have been an expensive phrase query for the and quick into a very efficient single-term lookup.

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