Question

I’m not seeing how this would be done, but is it possible to have a facet that uses an interval to give the stats on every X number of occurrences? As an example, if net was a sequence of numbers ordered by date like:

1,2,3,4,5,6,7

and I set the interval to 2, I would like to get back a histogram like:

count: 2
value: 3,

count: 2,
value: 7,

count: 2,
value: 11,

...

Answer 1

Elasticsearch doesn’t support such operation out of the box. It’s possible to write such facet, but it’s not very practical since it would require writing quite complex custom facet processor and optionally controlling the way records are split into shards (so called routing).

In elasticsearch, any operation that relies on global order of elements is somewhat problematic from the architectural perspective. Elasticsearch splits records into shards, and most operations including searching and facet calculation occur on shards and then results of these shard-level operations are collected and merged into a global result. This is basically map/reduce architecture, and it is the key for horizontal scalability of elasticsearch. Optimal implementation of your facet would require changing routing in such a way that records are split into shards based on their order rather than hash code of id. Alternatively, it can be done by limiting shard-level phase to just extraction of the field values and performing the actual calculation of the facet in the merge phase. The latter approach seems to be more practical but at the same time it is not much different from simply extracting field values for all records and doing calculations on the client side, which is exactly what I would suggest doing here. Just extract all values using desired sort order and calculate all stats on the client. If the number of records in your index is large, you can use Scroll API to retrieve all records using multiple requests.