Don't union with empty bitmap in the end in Filters.matchPredicate()

[leventov]

Problem
I've inspected the graph of bitmap manipulations which are done before processing the query on Druid historical nodes and seen something like this:

union {
  intersection {
    union {dimValue=14, empty=1}=1,
    union {dimValue=3, empty=1}=1
  }=1,
  union {dimValue=7, empty=1}=1
}

(BTW if you want such intronspection capabilities to be commited to Druid sooner please review this: #3693)

It means that we often make unions of something + empty bitmap.

How it is fixed
Optimized Iterator<ImmutableBitmap> implementation inside Filters.matchPredicate() so that it doesn't emit empty bitmap in the end of the iteration, and make it to follow Iterator contract, that is throw NoSuchElementException from next() if there are no more bitmaps.

BTW disadvantage of Druid's "anonymous class style" is that it's impossible to add tests to such anonymous interface implementations.

[gianm]

@leventov have you measured the improvement here?

[gianm]

👍 LGTM

[leventov]

@gianm no, I didn't. I don't think improvement is significant, because empty bitmaps in compressed representations (concise and roaring) doesn't take much space and cheap to union with. However for roaring it saves one allocation/merge cycle because roaring doesn't support union (and intersection) of more than 2 bitmaps, for making union of many bitmaps it merges them one-by-one.

[leventov]

@gianm thank you very much for quick reviews!

[jon-wei]

👍