[barrkel]

I was reading the details on inverted index usage in Druid, but what is described seems to be bitmap indexes, not inverted indexes.

Inverted indexes map distinct values in a column to a list of document ids containing the value. Bitmap indexes map distinct values to an array of booleans the same length as the number of documents, with true for presence and false for absence. Both index types can be highly compressed, of course.

Can you clarify what Druid is using?

[gianm]

Logically, an array of booleans and a set of integers are equivalent. So in the Druid developer community we usually use the terms interchangeably. But to be precise, our indexes are all stored as bitmaps and compressed with bitmap compression libraries.