Commits on Oct 27, 2020

1. Make sure non-collecting aggs include sub-aggs …

   Now that we're consistently using `cat_match` to filter which shards we
   run on we can get this confusing case:
   1. You have a search with, say, a range and a sub-agg.
   2. That search has a query that `can_match` can recognize will match no
      docs. On *any* shard.
   3. So we dutifully run it on a single shard so it can produce the
      "empty" aggs.
   4. The shard we pick happens to not have the target of the range mapped.
   5. This kicks in the special range aggregator that doesn't collect any
      documents.
   6. Before this commit, that range aggregator *also* never produced any
      sub-aggs.
   
   So, without this change, it was quite possible for a search that
   happened to match no documents to "throw away" the sub-aggs of a range
   and a few other aggs.
   
   We've had this problem for a long, long time but it is more confusing
   now because `can_match` is really kicking in and causing us to see cases
   where it looks like you are targeting a lot of shards but you really are
   only targeting a couple. It used to be that to get the "no sub-aggs"
   behavior you had to explicitly target only shards that didn't map the
   target field of the `range` agg. And, like, in that case it isn't too
   bad because you targeted a sort of degenerate shard. But now that
   `can_match` is doing its thing you can end up with the confusing steps
   above. It took me several hours to track down what what happening I know
   how the individual pieces of all of this works. It took four hours to
   figure out how they fit together in this case....
   
   Anyway! This replaces all the aggregator implementations that throw out
   the sub-aggregators with ones that keep them. I think this'll be less
   confusing in the future.
   
   Closes elastic#64142

   

   nik9000 committed Oct 27, 2020
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