Reported via Discuss forum: https://discuss.elastic.co/t/bucket-selector-aggregation-on-date-histogram--key/80986

The bucket_script and bucket_selector aggregations use the BucketHelpers.resolveBucketValue() method to get the bucket_path values from the buckets. That method requires that the return value is a Double so currently it is required that all the bucket_paths are numeric values. This presents a problem when trying to use the _key of a bucket since the key might be a DateTime (in the case of a date_histogram or date_range aggs) or a String (in the case of a terms agg).

One thing to note here is that all current pipeline aggs except the bucket_script and bucket_selector require the value from the bucket_path to be a double, so whatever the solution to this bug we should maintain a route that guarantees a double to be returned.

I have a few thoughts on how to solve this but I don't know if which (if any) of them are a good idea yet:

1. Change BucketHelpers.resolveBucketValue() to have a generic return type and somehow check that type is compatible before returning (not sure if this is possible since generics are not available at runtime)
2. Add a public Object resolveBucketObject() method in BucketHelpers that just returns the value it gets from the bucket as long as its not an instance of InternalAggregation (So that its an actual value rather than an aggregation
3. Let the bucket_script and bucket_selector aggs get the bucket path values directly using Bucket.getProperty()