Hey @mhoydis,
So when you rollup data into the rollup index, we have to change the structure of the data to roll it up. e.g. you might choose sum, max, min on one field, but we have to store that as 3 fields or you might choose avg on a field and we have to store the sum and the value count of that field to compute the avg for when you try to do average of averages.
Because of this as you can imagine if you tried to execute FooQuery against your original indices, it would not work as is against the new rollup index because all the data has been restructured. To make this work we actually transform the query during the request to match the new structure internally. That comes at a cost though of not being able to easily explore the data in discovery currently. We require the size to be set to 0, i.e. we just support queries on it. So you can use it for visualizations/dashboards. If you absolutely must be able to query the rollup index you can set the rollup search enabled cluster setting (opendistro.rollup.search.enabled) to false and we won’t intercept the request and transform it anymore. Then you should be able to look around on discover, but just know the limitations from above.