optionally dump list of segment-ids used to process the query

api/src/main/java/io/druid/query/SegmentDescriptor.java

@@ -21,6 +21,7 @@
 
 import com.fasterxml.jackson.annotation.JsonCreator;
 import com.fasterxml.jackson.annotation.JsonProperty;
+import io.druid.timeline.DataSegment;
 import org.joda.time.Interval;
 
 /**
@@ -86,6 +87,16 @@ public boolean equals(Object o)
     return true;
   }
 
+  public String getSegmentIdWithoutDataSource()
+  {
+    return DataSegment.makeDataSegmentIdentifierWithoutDataSource(
+        interval.getStart(),
+        interval.getEnd(),
+        version,
+        partitionNumber
+    );
+  }
+
   @Override
   public int hashCode()
   {

api/src/main/java/io/druid/timeline/DataSegment.java

@@ -48,7 +48,7 @@
  */
 public class DataSegment implements Comparable<DataSegment>
 {
-  public static String delimiter = "_";
+  public final static String DELIMITER = "_";
   private final Integer binaryVersion;
   private static final Interner<String> interner = Interners.newWeakInterner();
   private static final Function<String, String> internFun = new Function<String, String>()
@@ -70,18 +70,43 @@ public static String makeDataSegmentIdentifier(
   {
     StringBuilder sb = new StringBuilder();
 
-    sb.append(dataSource).append(delimiter)
-      .append(start).append(delimiter)
-      .append(end).append(delimiter)
+    sb.append(dataSource).append(DELIMITER);
+    updateBuilderWithSegmentIdWithoutDataSource(sb, start, end, version, shardSpec.getPartitionNum());
+
+    return sb.toString();
+  }
+
+  private static void updateBuilderWithSegmentIdWithoutDataSource(
+      StringBuilder sb,
+      DateTime start,
+      DateTime end,
+      String version,
+      int partitionNum
+  )
+  {
+    sb.append(start).append(DELIMITER)
+      .append(end).append(DELIMITER)
       .append(version);
 
-    if (shardSpec.getPartitionNum() != 0) {
-      sb.append(delimiter).append(shardSpec.getPartitionNum());
+    if (partitionNum != 0) {
+      sb.append(DELIMITER).append(partitionNum);
     }
+  }
+
+  public static String makeDataSegmentIdentifierWithoutDataSource(
+      DateTime start,
+      DateTime end,
+      String version,
+      int partitionNum
+  )
+  {
+    StringBuilder sb = new StringBuilder();
+
+    updateBuilderWithSegmentIdWithoutDataSource(sb, start, end, version, partitionNum);
 
     return sb.toString();
   }
-
+  
   private final String dataSource;
   private final Interval interval;
   private final String version;

api/src/main/java/io/druid/timeline/DataSegmentUtils.java

@@ -64,7 +64,7 @@ private static SegmentIdentifierParts parse(String dataSource, String identifier
       return null;
     }
     String remaining = identifier.substring(dataSource.length() + 1);
-    String[] splits = remaining.split(DataSegment.delimiter);
+    String[] splits = remaining.split(DataSegment.DELIMITER);
     if (splits.length < 3) {
       LOGGER.info("Invalid identifier %s", identifier);
       return null;
@@ -74,7 +74,7 @@ private static SegmentIdentifierParts parse(String dataSource, String identifier
     DateTime start = formatter.parseDateTime(splits[0]);
     DateTime end = formatter.parseDateTime(splits[1]);
     String version = splits[2];
-    String trail = splits.length > 3 ? join(splits, DataSegment.delimiter, 3, splits.length) : null;
+    String trail = splits.length > 3 ? join(splits, DataSegment.DELIMITER, 3, splits.length) : null;
 
     return new SegmentIdentifierParts(
         dataSource,
@@ -169,7 +169,7 @@ public String toString()
     {
       return join(
           new Object[]{dataSource, interval.getStart(), interval.getEnd(), version, trail},
-          DataSegment.delimiter, 0, version == null ? 3 : trail == null ? 4 : 5
+          DataSegment.DELIMITER, 0, version == null ? 3 : trail == null ? 4 : 5
       );
     }
   }

processing/src/main/java/io/druid/query/BaseQuery.java

@@ -67,6 +67,11 @@ public static <T> int getContextUncoveredIntervalsLimit(Query<T> query, int defa
     return parseInt(query, "uncoveredIntervalsLimit", defaultValue);
   }
 
+  public static <T> boolean getContextDumpSegmentList(Query<T> query, boolean defaultValue)
+  {
+    return parseBoolean(query, "dumpSegmentList", defaultValue);
+  }
+
   private static <T> int parseInt(Query<T> query, String key, int defaultValue)
   {
     Object val = query.getContextValue(key);

server/src/main/java/io/druid/client/CachingClusteredClient.java

@@ -248,6 +248,15 @@ public ShardSpec apply(PartitionChunk<ServerSelector> input)
       }
     }
 
+    // Optionally dump all used segmentIds in processing this query
+    if (BaseQuery.getContextDumpSegmentList(query, false)) {
+      List<String> segmentIds = new ArrayList<>(segments.size());
+      for (Pair<ServerSelector, SegmentDescriptor> segment : segments) {
+        segmentIds.add(segment.rhs.getSegmentIdWithoutDataSource());
+      }
+      responseContext.put("segments", segmentIds);
+    }
+
     final byte[] queryCacheKey;
 
     if ((populateCache || useCache) // implies strategy != null