SegmentSelector (Pinot Broker 0.12.1 API)

All Known Implementing Classes:

OfflineSegmentSelector, RealtimeSegmentSelector
```
public interface SegmentSelector
```
The segment selector selects the segments for the query. The segments selected should cover the whole dataset (table) without overlap.
Segment selector examples:
- For real-time table, when HLC and LLC segments coexist (during LLC migration), select only HLC segments or LLC segments
- For HLC real-time table, select segments in one group
- For table with segment merge/rollup enabled, select the merged segments over the original segments with the same data

Method Summary

Modifier and Type	Method	Description
`void`	`init(org.apache.helix.model.IdealState idealState, org.apache.helix.model.ExternalView externalView, Set<String> onlineSegments)`	Initializes the segment selector with the ideal state, external view and online segments (segments with ONLINE/CONSUMING instances in the ideal state and pre-selected by the `SegmentPreSelector`).
`void`	`onAssignmentChange(org.apache.helix.model.IdealState idealState, org.apache.helix.model.ExternalView externalView, Set<String> onlineSegments)`	Processes the segment assignment (ideal state or external view) change based on the given online segments (segments with ONLINE/CONSUMING instances in the ideal state and pre-selected by the `SegmentPreSelector`).
`Set<String>`	`select(org.apache.pinot.common.request.BrokerRequest brokerRequest)`	Selects the segments queried by the given broker request.

- Method Detail
  - init
```
void init(org.apache.helix.model.IdealState idealState,
          org.apache.helix.model.ExternalView externalView,
          Set<String> onlineSegments)
```
    Initializes the segment selector with the ideal state, external view and online segments (segments with ONLINE/CONSUMING instances in the ideal state and pre-selected by the SegmentPreSelector). Should be called only once before calling other methods.
  - onAssignmentChange
```
void onAssignmentChange(org.apache.helix.model.IdealState idealState,
                        org.apache.helix.model.ExternalView externalView,
                        Set<String> onlineSegments)
```
    Processes the segment assignment (ideal state or external view) change based on the given online segments (segments with ONLINE/CONSUMING instances in the ideal state and pre-selected by the SegmentPreSelector).
  - select
```
Set<String> select(org.apache.pinot.common.request.BrokerRequest brokerRequest)
```
    Selects the segments queried by the given broker request. The segments selected should cover the whole dataset (table) without overlap.