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More consistent empty-set filtering behavior on multi-value columns. #2753
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add(..) on MutableBitmap is expensive and leads to a performance regression here because this will, in many cases, get called for num_rows * num_dimensions
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I got this code from IndexMergerV9 so it is probably an issue there too.
Although now I wonder if this check is really necessary. nullRowsList is just used to add empty rows to the inverted index for null. Those rows already containing explicit nulls should already be in the inverted index – so this might be duplicating work?
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I removed this and a bunch of filter tests failed so it does seem like it's doing something.
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I think in IndexMergerV9, nullRowsList was analogous to this part of IndexIO, in convertV8toV9():
https://github.com/druid-io/druid/blob/master/processing/src/main/java/io/druid/segment/IndexIO.java#L621
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Looked into this a bit, I think this "null bitmap" code in IndexMerger/IndexMergerV9 is handling two things:
jon-wei@59e1090. You can see that if you disable all of the "null bitmap" logic and run IndexMergerTest.testPersistWithDifferentDims()
If I disable the code pointed to by this comment thread in IndexMerger, I think some of the recently introduced filter tests on multival dimensions fails; enabling the "null bitmap" adjustment code in IndexIO.convertV8toV9() for multivalued dimensions as well as single valued dimensions seems to have the same effect as this change in question (but I haven't tested this extensively, only ran SelectorFilterTest)
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So I think this code may be redundant for the single value dimension case