better handle dim extraction regex cases

processing/src/main/java/io/druid/query/extraction/JavascriptDimExtractionFn.java

@@ -96,6 +96,6 @@ public class JavascriptDimExtractionFn implements DimExtractionFn
   @Override
   public boolean preservesOrdering()
   {
-    return true;
+    return false;
   }
 }

processing/src/main/java/io/druid/query/extraction/PartialDimExtractionFn.java

@@ -70,7 +70,7 @@ public class PartialDimExtractionFn implements DimExtractionFn
   @Override
   public boolean preservesOrdering()
   {
-    return false;
+    return true;
   }
 
   @Override

processing/src/main/java/io/druid/query/extraction/RegexDimExtractionFn.java

@@ -70,7 +70,7 @@ public class RegexDimExtractionFn implements DimExtractionFn
   @Override
   public boolean preservesOrdering()
   {
-    return true;
+    return false;
   }
 
   @Override

processing/src/main/java/io/druid/query/extraction/SearchQuerySpecDimExtractionFn.java

@@ -66,7 +66,7 @@ public class SearchQuerySpecDimExtractionFn implements DimExtractionFn
   @Override
   public boolean preservesOrdering()
   {
-    return false;
+    return true;
   }
 
   @Override

processing/src/main/java/io/druid/query/extraction/TimeDimExtractionFn.java

@@ -90,7 +90,7 @@ public class TimeDimExtractionFn implements DimExtractionFn
   @Override
   public boolean preservesOrdering()
   {
-    return true;
+    return false;
   }
 
   @Override

processing/src/main/java/io/druid/query/topn/DimExtractionTopNAlgorithm.java

@@ -65,9 +65,11 @@ public class DimExtractionTopNAlgorithm extends BaseTopNAlgorithm<Aggregator[][]
         query,
         params.getCardinality()
     );
-    if (query.getDimensionSpec().preservesOrdering()) {
+
+    if (!query.getDimensionSpec().preservesOrdering()) {
       return provider.build();
     }
+
     return query.getTopNMetricSpec().configureOptimizer(provider).build();
   }
 

processing/src/main/java/io/druid/query/topn/InvertedTopNMetricSpec.java

@@ -21,6 +21,7 @@ package io.druid.query.topn;
 
 import com.fasterxml.jackson.annotation.JsonCreator;
 import com.fasterxml.jackson.annotation.JsonProperty;
+import com.google.common.collect.Ordering;
 import com.metamx.common.guava.Comparators;
 import io.druid.query.aggregation.AggregatorFactory;
 import io.druid.query.aggregation.PostAggregator;
@@ -63,11 +64,27 @@ public class InvertedTopNMetricSpec implements TopNMetricSpec
 
   @Override
   public Comparator getComparator(
-      List<AggregatorFactory> aggregatorSpecs,
-      List<PostAggregator> postAggregatorSpecs
+      final List<AggregatorFactory> aggregatorSpecs,
+      final List<PostAggregator> postAggregatorSpecs
   )
   {
-    return Comparators.inverse(delegate.getComparator(aggregatorSpecs, postAggregatorSpecs));
+    return Comparators.inverse(
+        new Comparator()
+        {
+          @Override
+          public int compare(Object o1, Object o2)
+          {
+            // nulls last
+            if (o1 == null) {
+              return 1;
+            }
+            if (o2 == null) {
+              return -1;
+            }
+            return delegate.getComparator(aggregatorSpecs, postAggregatorSpecs).compare(o1, o2);
+          }
+        }
+    );
   }
 
   @Override

processing/src/main/java/io/druid/query/topn/LexicographicTopNMetricSpec.java

@@ -43,6 +43,13 @@ public class LexicographicTopNMetricSpec implements TopNMetricSpec
     @Override
     public int compare(String s, String s2)
     {
+      // null first
+      if (s == null) {
+        return -1;
+      }
+      if (s2 == null) {
+        return 1;
+      }
       return UnsignedBytes.lexicographicalComparator().compare(s.getBytes(Charsets.UTF_8), s2.getBytes(Charsets.UTF_8));
     }
   };
@@ -54,7 +61,7 @@ public class LexicographicTopNMetricSpec implements TopNMetricSpec
       @JsonProperty("previousStop") String previousStop
   )
   {
-    this.previousStop = (previousStop == null) ? "" : previousStop;
+    this.previousStop = previousStop;
   }
 
   @Override
@@ -98,7 +105,7 @@ public class LexicographicTopNMetricSpec implements TopNMetricSpec
   @Override
   public byte[] getCacheKey()
   {
-    byte[] previousStopBytes = previousStop.getBytes(Charsets.UTF_8);
+    byte[] previousStopBytes = previousStop == null ? new byte[]{} : previousStop.getBytes(Charsets.UTF_8);
 
     return ByteBuffer.allocate(1 + previousStopBytes.length)
                      .put(CACHE_TYPE_ID)

processing/src/test/java/io/druid/query/topn/TopNQueryRunnerTest.java

@@ -1263,6 +1263,58 @@ public class TopNQueryRunnerTest
     TestHelper.assertExpectedResults(expectedResults, runner.run(query));
   }
 
+  @Test
+  public void testInvertedTopNLexicographicDimExtraction2()
+  {
+    TopNQuery query = new TopNQueryBuilder()
+        .dataSource(QueryRunnerTestHelper.dataSource)
+        .granularity(QueryRunnerTestHelper.allGran)
+        .dimension(
+            new ExtractionDimensionSpec(
+                providerDimension, providerDimension, new RegexDimExtractionFn("..(.)")
+            )
+        )
+        .metric(new InvertedTopNMetricSpec(new LexicographicTopNMetricSpec(null)))
+        .threshold(4)
+        .intervals(QueryRunnerTestHelper.firstToThird)
+        .aggregators(QueryRunnerTestHelper.commonAggregators)
+        .postAggregators(Arrays.<PostAggregator>asList(QueryRunnerTestHelper.addRowsIndexConstant))
+        .build();
+
+    List<Result<TopNResultValue>> expectedResults = Arrays.asList(
+        new Result<TopNResultValue>(
+            new DateTime("2011-04-01T00:00:00.000Z"),
+            new TopNResultValue(
+                Arrays.<Map<String, Object>>asList(
+                    ImmutableMap.<String, Object>of(
+                        providerDimension, "t",
+                        "rows", 4L,
+                        "index", 5351.814697265625D,
+                        "addRowsIndexConstant", 5356.814697265625D,
+                        "uniques", QueryRunnerTestHelper.UNIQUES_2
+                    ),
+                    ImmutableMap.<String, Object>of(
+                        providerDimension, "o",
+                        "rows", 18L,
+                        "index", 2231.8768157958984D,
+                        "addRowsIndexConstant", 2250.8768157958984D,
+                        "uniques", QueryRunnerTestHelper.UNIQUES_9
+                    ),
+                    ImmutableMap.<String, Object>of(
+                        providerDimension, "f",
+                        "rows", 4L,
+                        "index", 4875.669677734375D,
+                        "addRowsIndexConstant", 4880.669677734375D,
+                        "uniques", QueryRunnerTestHelper.UNIQUES_2
+                    )
+                )
+            )
+        )
+    );
+
+    TestHelper.assertExpectedResults(expectedResults, runner.run(query));
+  }
+
   @Test
   public void testTopNLexicographicDimExtractionWithPreviousStop()
   {
@@ -1354,6 +1406,51 @@ public class TopNQueryRunnerTest
     TestHelper.assertExpectedResults(expectedResults, runner.run(query));
   }
 
+  @Test
+  public void testInvertedTopNLexicographicDimExtractionWithPreviousStop2()
+  {
+    TopNQuery query = new TopNQueryBuilder()
+        .dataSource(QueryRunnerTestHelper.dataSource)
+        .granularity(QueryRunnerTestHelper.allGran)
+        .dimension(
+            new ExtractionDimensionSpec(
+                providerDimension, providerDimension, new RegexDimExtractionFn("..(.)")
+            )
+        )
+        .metric(new InvertedTopNMetricSpec(new LexicographicTopNMetricSpec("p")))
+        .threshold(4)
+        .intervals(QueryRunnerTestHelper.firstToThird)
+        .aggregators(QueryRunnerTestHelper.commonAggregators)
+        .postAggregators(Arrays.<PostAggregator>asList(QueryRunnerTestHelper.addRowsIndexConstant))
+        .build();
+
+    List<Result<TopNResultValue>> expectedResults = Arrays.asList(
+        new Result<TopNResultValue>(
+            new DateTime("2011-04-01T00:00:00.000Z"),
+            new TopNResultValue(
+                Arrays.<Map<String, Object>>asList(
+                    ImmutableMap.<String, Object>of(
+                        providerDimension, "o",
+                        "rows", 18L,
+                        "index", 2231.8768157958984D,
+                        "addRowsIndexConstant", 2250.8768157958984D,
+                        "uniques", QueryRunnerTestHelper.UNIQUES_9
+                    ),
+                    ImmutableMap.<String, Object>of(
+                        providerDimension, "f",
+                        "rows", 4L,
+                        "index", 4875.669677734375D,
+                        "addRowsIndexConstant", 4880.669677734375D,
+                        "uniques", QueryRunnerTestHelper.UNIQUES_2
+                    )
+                )
+            )
+        )
+    );
+
+    TestHelper.assertExpectedResults(expectedResults, runner.run(query));
+  }
+
   @Test
   public void testInvertedTopNQuery()
   {