Imagine that you are running a website that allows users to listen to their favorite songs. To make it easier for them to manage their music library, users can tag songs with whatever tags make sense to them. You will end up with a lot of tracks tagged with rock, hiphop, and electronica, but also with some tracks tagged with my_16th_birthday_favorite_anthem.

Now imagine that you want to show users the most popular three tags for each song. It is highly likely that tags like rock will show up in the top three, but my_16th_birthday_favorite_anthem is very unlikely to make the grade. However, in order to calculate the most popular tags, you have been forced to load all of these one-off terms into memory.

Thanks to fielddata filtering, we can take control of this situation. We know that we’re interested in only the most popular terms, so we can simply avoid loading any terms that fall into the less interesting long tail:

PUT /music/_mapping/song
{
  "properties": {
    "tag": {
      "type": "string",
      "fielddata": { 
        "filter": {
          "frequency": { 
            "min":              0.01, 
            "min_segment_size": 500  
          }
        }
      }
    }
  }
}

With this mapping in place, only terms that appear in at least 1% of the documents in that segment will be loaded into memory. You can also specify a max term frequency, which could be used to exclude terms that are too common, such as stopwords.

Term frequencies, in this case, are calculated per segment. This is a limitation of the implementation: fielddata is loaded per segment, and at that point the only term frequencies that are visible are the frequencies for that segment. However, this limitation has interesting properties: it allows newly popular terms to rise to the top quickly.

Let’s say that a new genre of song becomes popular one day. You would like to include the tag for this new genre in the most popular list, but if you were relying on term frequencies calculated across the whole index, you would have to wait for the new tag to become as popular as rock and electronica. Because of the way frequency filtering is implemented, the newly added tag will quickly show up as a high-frequency tag within new segments, so will quickly float to the top.

The min_segment_size parameter tells Elasticsearch to ignore segments below a certain size. If a segment holds only a few documents, the term frequencies are too coarse to have any meaning. Small segments will soon be merged into bigger segments, which will then be big enough to take into account.

Fielddata filtering can have a massive impact on memory usage. The trade-off is fairly obvious: you are essentially ignoring data. But for many applications, the trade-off is reasonable since the data is not being used anyway. The memory savings is often more important than including a large and relatively useless long tail of terms.