netudima commented on code in PR #3730:
URL: https://github.com/apache/cassandra/pull/3730#discussion_r1918796431


##########
src/java/org/apache/cassandra/db/ReadCommand.java:
##########
@@ -867,6 +868,130 @@ protected LongPredicate getPurgeEvaluator()
         return Transformation.apply(iterator, new 
WithoutPurgeableTombstones());
     }
 
+
+    /**
+     * Wraps the provided iterator so that metrics on count of purgeable 
tombstones are tracked and traced.
+     * It tracks only tombstones with localDeletionTime < now - 
gc_grace_period.
+     * Other (non-purgeable) tombstones will be tracked by regular Cassandra 
logic later.
+     */
+    private UnfilteredPartitionIterator 
withPurgeableTombstonesMetricRecording(UnfilteredPartitionIterator iter,
+                                                                               
ColumnFamilyStore cfs)
+    {
+        class PurgeableTombstonesMetricRecording extends 
Transformation<UnfilteredRowIterator>
+        {
+            private int purgeableTombstones = 0;
+
+            @Override
+            public UnfilteredRowIterator 
applyToPartition(UnfilteredRowIterator iter)
+            {
+                return Transformation.apply(iter, this);

Review Comment:
   I have looked again through the added code and now I am a bit concerned 
about the iteration over cells to check for cell tombstones - it may create a 
performance overhead + I am not sure if information about droppable cell 
tombstones is really valuable: all the times when I saw issues with droppable 
tombstones - it was about row tombstones. I am thinking now about an option to 
count only partition level and row level tombstones in this added logic and 
skip cell tombstones part..
   I am going to measure the overhead to see how big the impact can be..



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