Druid supports Long as a basic type for dimensions. But ingesting integer values as dimensions also works which means Druid is casting Integers to Long type somewhere while reading the segment. Could somebody help me understand how this casting works or point to the class which may be responsible for this? Thanks!

yea, the values are cast during ingestion, if you specify ‘long’ in the dimension schema currently it goes through https://github.com/apache/druid/blob/master/processing/src/main/java/org/apache/druid/segment/LongDimensionIndexer.java#L56