fix auto type column indexer realtime logical type to report long if all nulls since it is most restrictive type

processing/src/main/java/org/apache/druid/segment/AutoTypeColumnIndexer.java

@@ -405,8 +405,10 @@ public ColumnType getLogicalType()
       return ColumnType.NESTED_DATA;
     }
     if (isConstant && constantValue == null) {
-      // we didn't see anything, so we can be anything, so why not a string?
-      return ColumnType.STRING;
+      // we didn't see anything, so we can be anything, so be long as it is the most restrictive type in SQL compatible
+      // mode. Default value mode however doesn't allow null numeric values to exist, so we're going to be a STRING
+      // since they can have nulls
+      return NullHandling.sqlCompatible() ? ColumnType.LONG : ColumnType.STRING;
     }
     if (fieldIndexers.size() == 1 && fieldIndexers.containsKey(NestedPathFinder.JSON_PATH_ROOT)) {
       FieldIndexer rootField = fieldIndexers.get(NestedPathFinder.JSON_PATH_ROOT);
@@ -553,6 +555,9 @@ private ColumnValueSelector<?> getRootLiteralValueSelector(
     if (fieldIndexers.size() > 1 || hasNestedData) {
       return null;
     }
+    if (isConstant && constantValue == null) {
+      return NilColumnValueSelector.instance();
+    }
     final FieldIndexer root = fieldIndexers.get(NestedPathFinder.JSON_PATH_ROOT);
     if (root == null) {
       return null;

processing/src/main/java/org/apache/druid/segment/AutoTypeColumnMerger.java

@@ -22,6 +22,7 @@
 import com.google.common.base.Preconditions;
 import com.google.common.collect.Iterators;
 import com.google.common.collect.PeekingIterator;
+import org.apache.druid.common.config.NullHandling;
 import org.apache.druid.error.DruidException;
 import org.apache.druid.java.util.common.io.Closer;
 import org.apache.druid.java.util.common.logger.Logger;
@@ -93,7 +94,6 @@ public AutoTypeColumnMerger(
       Closer closer
   )
   {
-
     this.name = name;
     this.castToType = castToType;
     this.indexSpec = indexSpec;
@@ -159,17 +159,28 @@ public void writeMergedValueDictionary(List<IndexableAdapter> adapters) throws I
         explicitType = null;
       }
 
-      // for backwards compat; remove this constant handling in druid 28 along with
-      // indexSpec.optimizeJsonConstantColumns in favor of always writing constant columns
-      // we also handle the numMergeIndex == 0 here, which also indicates that the column is a null constant
+      // null constant case. we also handle the numMergeIndex == 0 here, which also indicates that the column is a null
+      // constant
       if (explicitType == null && !forceNested && ((isConstant && constantValue == null) || numMergeIndex == 0)) {
-        logicalType = ColumnType.STRING;
-        serializer = new ScalarStringColumnSerializer(
-            name,
-            indexSpec,
-            segmentWriteOutMedium,
-            closer
-        );
+        // if sql compatible mode, use LONG since it is the most restrictive type. Default value mode however must
+        // write out a STRING since null values do not exist for numeric types
+        if (NullHandling.sqlCompatible()) {
+          logicalType = ColumnType.LONG;
+          serializer = new ScalarLongColumnSerializer(
+              name,
+              indexSpec,
+              segmentWriteOutMedium,
+              closer
+          );
+        } else {
+          logicalType = ColumnType.STRING;
+          serializer = new ScalarStringColumnSerializer(
+              name,
+              indexSpec,
+              segmentWriteOutMedium,
+              closer
+          );
+        }
       } else if (explicitType != null || (!forceNested && rootOnly && rootTypes.getSingleType() != null)) {
         logicalType = explicitType != null ? explicitType : rootTypes.getSingleType();
         // empty arrays can be missed since they don't have a type, so handle them here

processing/src/test/java/org/apache/druid/segment/AutoTypeColumnIndexerTest.java

@@ -571,7 +571,7 @@ public void testConstantNull()
     Assert.assertTrue(indexer.hasNulls);
     Assert.assertFalse(indexer.hasNestedData);
     Assert.assertTrue(indexer.isConstant());
-    Assert.assertEquals(ColumnType.STRING, indexer.getLogicalType());
+    Assert.assertEquals(ColumnType.LONG, indexer.getLogicalType());
   }
 
   @Test

sql/src/test/java/org/apache/druid/sql/calcite/CalciteNestedDataQueryTest.java

@@ -6567,7 +6567,7 @@ public void testScanAllTypesAuto()
                     .add("arrayBool", ColumnType.LONG_ARRAY)
                     .add("arrayNestedLong", ColumnType.NESTED_DATA)
                     .add("arrayObject", ColumnType.NESTED_DATA)
-                    .add("null", ColumnType.STRING)
+                    .add("null", NullHandling.sqlCompatible() ? ColumnType.LONG : ColumnType.STRING)
                     .add("cstr", ColumnType.STRING)
                     .add("clong", ColumnType.LONG)
                     .add("cdouble", ColumnType.DOUBLE)