will-lauer commented on issue #6519: Lookups should be able to return list of values URL: https://github.com/apache/incubator-druid/issues/6519#issuecomment-433161183 Instead of `LookupExtractor@apply()` returning a `String` (which it does now) or a `Map<String, Object>`, I think the correct contract is for a new method that returns a `List<String>` with a default implementation that simply invokes `return Collections.singletonList(apply())`, creating a list from the standard single value return, thus allowing any existing lookups to continue to work without any changes. One of the tricky issues here is that `LookupExtractor.unapply()` makes less sense for mutlivalued values. We would need to understand what that API would do for this case.
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