haibow commented on a change in pull request #4954: Support schema evolution 
for consuming segments
URL: https://github.com/apache/incubator-pinot/pull/4954#discussion_r385554338
 
 

 ##########
 File path: 
pinot-core/src/main/java/org/apache/pinot/core/segment/virtualcolumn/ColumnProviderFactory.java
 ##########
 @@ -20,18 +20,29 @@
 
 import 
org.apache.pinot.common.utils.CommonConstants.Segment.BuiltInVirtualColumn;
 import org.apache.pinot.common.utils.NetUtil;
+import org.apache.pinot.core.segment.index.column.ColumnContext;
+import org.apache.pinot.core.segment.index.column.ColumnProvider;
+import 
org.apache.pinot.core.segment.index.column.DefaultNullValueColumnProvider;
 import org.apache.pinot.spi.data.DimensionFieldSpec;
 import org.apache.pinot.spi.data.FieldSpec;
 import org.apache.pinot.spi.data.Schema;
 
 
 /**
- * Factory for virtual column providers.
+ * Factory for column providers.
  */
-public class VirtualColumnProviderFactory {
-  public static VirtualColumnProvider buildProvider(String 
virtualColumnProvider) {
+public class ColumnProviderFactory {
+  public static ColumnProvider buildProvider(ColumnContext columnContext) {
+    String virtualColumnProvider = 
columnContext.getFieldSpec().getVirtualColumnProvider();
     try {
-      return (VirtualColumnProvider) 
Class.forName(virtualColumnProvider).newInstance();
+      // Use the preset virtualColumnProvider if available
+      if (virtualColumnProvider != null && !virtualColumnProvider
+          .equals(DefaultNullValueColumnProvider.class.getName())) {
+        return (ColumnProvider) 
Class.forName(virtualColumnProvider).newInstance();
+      }
+      // Create the columnProvider that returns default null values based on 
the columnContext
+      return 
DefaultNullValueColumnProvider.class.getDeclaredConstructor(ColumnContext.class)
+          .newInstance(columnContext);
 
 Review comment:
   Didn't know this. updated

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