richardstartin edited a comment on issue #9920:
URL: https://github.com/apache/druid/issues/9920#issuecomment-633717928


   The code change in #6764 aimed to improve the performance of bitmap 
construction, and it applies run compression as the bitmap is built, but only 
to containers which are run-compressible. If this results in slower 
intersections in some cases, this can be fixed in the RoaringBitmap library; 
some changes were recently made to significantly improve run vs bitmap 
`andCardinality` and `intersects` and these changes are applicable to run vs 
bitmap `and`. 
   
   Note that #6764 will not have changed any behaviour when the roaring bitmaps 
have been memory mapped from disk and where run compression has already been 
applied (immediately before serialisation), which might explain if no 
regression has otherwise come to light since the change...


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