@gianm Parallelizing union queries with the existing design doesn't work even using AsyncQueryRunner as the merging for all the queries except GroupByV1 are performed lazily. I think what would help fix this and union queries overall would be to modify the `Query` interface to accept a list of datasources instead of a single datasource. In this case, the `BrokerServerView` would have to merge the timelines for all the datasources in the list before the segments to be queried are identified. This would essentially condense the queries for each datasource into a single query. What do you think of this proposal?
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