somandal commented on code in PR #9740:
URL: https://github.com/apache/pinot/pull/9740#discussion_r1018550943


##########
pinot-segment-local/src/main/java/org/apache/pinot/segment/local/segment/index/loader/ForwardIndexHandler.java:
##########
@@ -112,20 +122,50 @@ public void updateIndices(SegmentDirectory.Writer 
segmentWriter, IndexCreatorPro
       Operation operation = entry.getValue();
 
       switch (operation) {
-        case CHANGE_RAW_INDEX_COMPRESSION_TYPE: {
-          rewriteRawForwardIndex(column, segmentWriter, indexCreatorProvider);
+        case DISABLE_FORWARD_INDEX_FOR_DICT_COLUMN: {
+          // Deletion of the forward index will be handled outside the index 
handler to ensure that other index
+          // handlers that need the forward index to construct their own 
indexes will have it available.
+          // The existing forward index must be in dictionary format for this 
to be a no-op.
+          _forwardIndexDisabledColumnsToCleanup.add(column);
+          break;
+        }
+        case DISABLE_FORWARD_INDEX_FOR_RAW_COLUMN: {
+          // The forward index has been disabled for a column which has a 
noDictionary based forward index. A dictionary
+          // and inverted index need to be created before we can delete the 
forward index. We create a dictionary here,
+          // but let the InvertedIndexHandler handle the creation of the 
inverted index. We create a temporary
+          // forward index here which is dictionary based and allow the post 
deletion step handle the actual deletion
+          // of the forward index.
+          createDictBasedForwardIndex(column, segmentWriter, 
indexCreatorProvider);
+          Preconditions.checkState(segmentWriter.hasIndexFor(column, 
ColumnIndexType.FORWARD_INDEX),
+              String.format("Temporary forward index was not created for 
column: %s", column));
+          _forwardIndexDisabledColumnsToCleanup.add(column);

Review Comment:
   So there is actually no worry here about TableConfig inconsistency. The 
TableConfig will have to be updated to disable the forward index. We added a 
check in the TableConfigUtils which checks for the following:
   
   ```
     /**
      * Validates the compatibility of the indexes if the column has the 
forward index disabled. Throws exceptions due to
      * compatibility mismatch. The checks performed are:
      *     - Validate dictionary is enabled.
      *     - Validate inverted index is enabled.
      *     - Validate that either no range index exists for column or the 
range index version is at least 2 and isn't a
      *       multi-value column (since mulit-value defaults to index v1).
      */
     private static void validateForwardIndexDisabledIndexCompatibility()
   ```
   
   I will update this to add back my Precondition checks just as a sanity check



##########
pinot-segment-local/src/main/java/org/apache/pinot/segment/local/segment/index/loader/ForwardIndexHandler.java:
##########
@@ -112,20 +122,50 @@ public void updateIndices(SegmentDirectory.Writer 
segmentWriter, IndexCreatorPro
       Operation operation = entry.getValue();
 
       switch (operation) {
-        case CHANGE_RAW_INDEX_COMPRESSION_TYPE: {
-          rewriteRawForwardIndex(column, segmentWriter, indexCreatorProvider);
+        case DISABLE_FORWARD_INDEX_FOR_DICT_COLUMN: {
+          // Deletion of the forward index will be handled outside the index 
handler to ensure that other index
+          // handlers that need the forward index to construct their own 
indexes will have it available.
+          // The existing forward index must be in dictionary format for this 
to be a no-op.
+          _forwardIndexDisabledColumnsToCleanup.add(column);
+          break;
+        }
+        case DISABLE_FORWARD_INDEX_FOR_RAW_COLUMN: {
+          // The forward index has been disabled for a column which has a 
noDictionary based forward index. A dictionary
+          // and inverted index need to be created before we can delete the 
forward index. We create a dictionary here,
+          // but let the InvertedIndexHandler handle the creation of the 
inverted index. We create a temporary
+          // forward index here which is dictionary based and allow the post 
deletion step handle the actual deletion
+          // of the forward index.
+          createDictBasedForwardIndex(column, segmentWriter, 
indexCreatorProvider);
+          Preconditions.checkState(segmentWriter.hasIndexFor(column, 
ColumnIndexType.FORWARD_INDEX),
+              String.format("Temporary forward index was not created for 
column: %s", column));
+          _forwardIndexDisabledColumnsToCleanup.add(column);

Review Comment:
   So there is actually no worry here about TableConfig inconsistency. The 
TableConfig will have to be updated to disable the forward index. We added a 
check in the TableConfigUtils which checks for the following:
   
   ```
     /**
      * Validates the compatibility of the indexes if the column has the 
forward index disabled. Throws exceptions due to
      * compatibility mismatch. The checks performed are:
      *     - Validate dictionary is enabled.
      *     - Validate inverted index is enabled.
      *     - Validate that either no range index exists for column or the 
range index version is at least 2 and isn't a
      *       multi-value column (since multi-value defaults to index v1).
      */
     private static void validateForwardIndexDisabledIndexCompatibility()
   ```
   
   I will update this to add back my Precondition checks just as a sanity check



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