Unfortunately, I haven't seen any tutorial on the topic. You can start with
this thread
- https://groups.google.com/d/topic/elasticsearch/NlLoXMfx4fw/discussion
and then take a look at how other facets are implemented. 
For prototyping, I am typically using the facet script pluginhttps://github.com/imotov/elasticsearch-facet-scriptthat allows me to quickly create custom facets using any scripting language
that elasticsearch supports.
On Wednesday, November 21, 2012 10:35:58 AM UTC-5, Valentin wrote:
Are there any tutorials where facet plugins are explained? I would like to
write one myself and wondered where to start looking.
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