rohityadav1993 commented on code in PR #11983:
URL: https://github.com/apache/pinot/pull/11983#discussion_r1552327323


##########
pinot-segment-local/src/main/java/org/apache/pinot/segment/local/upsert/PartialUpsertHandler.java:
##########
@@ -32,76 +31,79 @@
 
 /**
  * Handler for partial-upsert.
+ *
+ * This class is responsible for merging the new record with the previous 
record.
+ * It uses the configured merge strategies to merge the columns. If no merge 
strategy is configured for a column,
+ * it uses the default merge strategy.
+ *
+ * It is also possible to define a custom logic for merging rows by 
implementing {@link PartialUpsertMerger}.
+ * If a merger for row is defined then it takes precedence and ignores column 
mergers.
  */
 public class PartialUpsertHandler {
-  // _column2Mergers maintains the mapping of merge strategies per columns.
-  private final Map<String, PartialUpsertMerger> _column2Mergers = new 
HashMap<>();
-  private final PartialUpsertMerger _defaultPartialUpsertMerger;
   private final List<String> _comparisonColumns;
   private final List<String> _primaryKeyColumns;
+  private final PartialUpsertMerger _partialUpsertMerger;
 
-  public PartialUpsertHandler(Schema schema, Map<String, 
UpsertConfig.Strategy> partialUpsertStrategies,
-      UpsertConfig.Strategy defaultPartialUpsertStrategy, List<String> 
comparisonColumns) {
-    _defaultPartialUpsertMerger = 
PartialUpsertMergerFactory.getMerger(defaultPartialUpsertStrategy);
+  public PartialUpsertHandler(Schema schema, List<String> comparisonColumns, 
UpsertConfig upsertConfig) {
     _comparisonColumns = comparisonColumns;
     _primaryKeyColumns = schema.getPrimaryKeyColumns();
 
-    for (Map.Entry<String, UpsertConfig.Strategy> entry : 
partialUpsertStrategies.entrySet()) {
-      _column2Mergers.put(entry.getKey(), 
PartialUpsertMergerFactory.getMerger(entry.getValue()));
-    }
+    _partialUpsertMerger =
+        PartialUpsertMergerFactory.getPartialUpsertMerger(_primaryKeyColumns, 
comparisonColumns, upsertConfig);
   }
 
-  /**
-   * Merges records and returns the merged record.
-   * We used a map to indicate all configured fields for partial upsert. For 
these fields
-   * (1) If the prev value is null, return the new value
-   * (2) If the prev record is not null, the new value is null, return the 
prev value.
-   * (3) If neither values are not null, then merge the value and return.
-   * For un-configured fields, they are using default override behavior, 
regardless null values.
-   *
-   * For example, overwrite merger will only override the prev value if the 
new value is not null.
-   * Null values will override existing values if not configured. They can be 
ignored by using ignoreMerger.
-   *
-   * @param prevRecord wrapper for previous record, which lazily reads column 
values of previous row and caches for
-   *                   re-reads.
-   * @param newRecord the new consumed record.
-   */
-  public void merge(LazyRow prevRecord, GenericRow newRecord) {
-    for (String column : prevRecord.getColumnNames()) {
-      if (!_primaryKeyColumns.contains(column)) {
-        PartialUpsertMerger merger = _column2Mergers.getOrDefault(column, 
_defaultPartialUpsertMerger);
-        // Non-overwrite mergers
-        // (1) If the value of the previous is null value, skip merging and 
use the new value
-        // (2) Else If the value of new value is null, use the previous value 
(even for comparison columns).
-        // (3) Else If the column is not a comparison column, we applied the 
merged value to it.
-        if (!(merger instanceof OverwriteMerger)) {
-          Object prevValue = prevRecord.getValue(column);
-          if (prevValue != null) {
-            if (newRecord.isNullValue(column)) {
-              // Note that we intentionally want to overwrite any previous 
_comparisonColumn value in the case of
-              // using
-              // multiple comparison columns. We never apply a merge function 
to it, rather we just take any/all
-              // non-null comparison column values from the previous record, 
and the sole non-null comparison column
-              // value from the new record.
-              newRecord.putValue(column, prevValue);
-              newRecord.removeNullValueField(column);
-            } else if (!_comparisonColumns.contains(column)) {
-              newRecord.putValue(column, merger.merge(prevValue, 
newRecord.getValue(column)));
-            }
-          }
-        } else {
-          // Overwrite mergers.
-          // (1) If the merge strategy is Overwrite merger and newValue is not 
null, skip and use the new value
-          // (2) Otherwise, if previous is not null, init columnReader and use 
the previous value.
-          if (newRecord.isNullValue(column)) {
-            Object prevValue = prevRecord.getValue(column);
-            if (prevValue != null) {
-              newRecord.putValue(column, prevValue);
-              newRecord.removeNullValueField(column);
-            }
-          }
+  public void merge(LazyRow prevRecord, GenericRow newRecord, Map<String, 
Object> reuseMergerResult) {
+    reuseMergerResult.clear();
+
+    // merger current row with previously indexed row
+    _partialUpsertMerger.merge(prevRecord, newRecord, reuseMergerResult);
+
+    if (_partialUpsertMerger instanceof PartialUpsertColumnarMerger) {

Review Comment:
   I wanted to keep the logic for `PartialUpsertColumnarMerger`(behaviour so 
far) to be unmodified and was doing null handling wrongly for custom merger. I 
have removed the differentiation and using `putDefaultNullValue()` for null 
handling based on another [review 
comment](https://github.com/apache/pinot/pull/11983#discussion_r1550951066). We 
will never have a null merger result from `PartialUpsertColumnarMerger`.



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