Effect of extra dimensions

I am consuming json messages via tranquility into druid. At times, certain extra unwanted dimensions may come in those messages. Will druid consider these extra dimensions only in that particular segment or will it store these extra dimensions “globally (i.e. forever)” ? If druid stores these extra dimensions forever, it may cause more memory for indexing purpose in the future segments too.

Druid segments are self-contained and self-describing, i.e. there is no “global” dimension list that would cause extra indexing but each segment has its own list of dimensions.

If your events contain dimensions that you are not interested in, you may want to consider explicitly listing your wanted dimensions in the dimensionsSpec or alternately populating the dimensionExclusions list with your unwanted dimensions (if either of these work with your use case).