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How it works...
The Percolator field stores an Elasticsearch query inside it.
Because all the percolators are cached and always active for performances, all the fields required in the query must be defined in the mapping of the document.
Due to the fact the all the queries in all percolators documents will be executed against every document, for the best performance, the query inside the percolators must be optimized to provide fast execution of them inside the percolator query.