yashmayya commented on code in PR #13835:
URL: https://github.com/apache/pinot/pull/13835#discussion_r1720962052


##########
pinot-segment-spi/src/main/java/org/apache/pinot/segment/spi/AggregationFunctionType.java:
##########
@@ -344,6 +346,55 @@ public static AggregationFunctionType 
getAggregationFunctionType(String function
     }
   }
 
+  public static int 
compareFunctionParametersForStarTree(AggregationFunctionType functionType,
+      Map<String, Object> configuration1, Map<String, Object> configuration2) {
+    switch (functionType) {
+      case DISTINCTCOUNTHLL:
+      case DISTINCTCOUNTRAWHLL: {
+        // Compare the log2m value accounting for defaults
+        if ((MapUtils.isEmpty(configuration1) || 
!configuration1.containsKey(Constants.HLL_LOG2M_KEY))
+            && (MapUtils.isEmpty(configuration2) || 
!configuration2.containsKey(Constants.HLL_LOG2M_KEY))) {
+          return 0;
+        }
+        if ((MapUtils.isEmpty(configuration1) || 
!configuration1.containsKey(Constants.HLL_LOG2M_KEY))) {
+          return 
Integer.compare(CommonConstants.Helix.DEFAULT_HYPERLOGLOG_LOG2M, 
Integer.parseInt(String.valueOf(
+              configuration2.get(Constants.HLL_LOG2M_KEY))));
+        }
+        if ((MapUtils.isEmpty(configuration2)) || 
!configuration2.containsKey(Constants.HLL_LOG2M_KEY)) {
+          return 
Integer.compare(Integer.parseInt(String.valueOf(configuration1.get(Constants.HLL_LOG2M_KEY))),
+              CommonConstants.Helix.DEFAULT_HYPERLOGLOG_LOG2M);
+        }
+        return 
Integer.compare(Integer.parseInt(String.valueOf(configuration1.get(Constants.HLL_LOG2M_KEY))),
+            
Integer.parseInt(String.valueOf(configuration2.get(Constants.HLL_LOG2M_KEY))));
+      }
+
+      case DISTINCTCOUNTHLLPLUS:
+      case DISTINCTCOUNTRAWHLLPLUS: {
+        // Only p value needs to be compared; HyperLogLogPlus with any sp 
value can be merged as long as p value is the
+        // same
+        if ((MapUtils.isEmpty(configuration1) || 
!configuration1.containsKey(Constants.HLLPLUS_ULL_P_KEY))
+            && (MapUtils.isEmpty(configuration2) || 
!configuration2.containsKey(Constants.HLLPLUS_ULL_P_KEY))) {
+          return 0;
+        }
+        if ((MapUtils.isEmpty(configuration1) || 
!configuration1.containsKey(Constants.HLLPLUS_ULL_P_KEY))) {
+          return 
Integer.compare(CommonConstants.Helix.DEFAULT_HYPERLOGLOG_PLUS_P,
+              
Integer.parseInt(String.valueOf(configuration2.get(Constants.HLLPLUS_ULL_P_KEY))));
+        }
+        if ((MapUtils.isEmpty(configuration2) || 
!configuration2.containsKey(Constants.HLLPLUS_ULL_P_KEY))) {
+          return 
Integer.compare(Integer.parseInt(String.valueOf(configuration1.get(Constants.HLLPLUS_ULL_P_KEY))),
+              CommonConstants.Helix.DEFAULT_HYPERLOGLOG_PLUS_P);
+        }
+        return 
Integer.compare(Integer.parseInt(String.valueOf(configuration1.get(Constants.HLLPLUS_ULL_P_KEY))),
+            
Integer.parseInt(String.valueOf(configuration2.get(Constants.HLLPLUS_ULL_P_KEY))));
+      }
+
+      // By default, assume that the rest of the aggregation functions do not 
have relevant configurations to be checked
+      // for a star-tree index
+      default:
+        return 0;

Review Comment:
   While this is definitely true for some aggregation functions like `COUNT` / 
`MIN` / `MAX` / `SUM` / `AVG` / `MINMAXRANGE` / `DISTINCTCOUNTBITMAP` / 
`PERCENTILEEST`, it's a little more nuanced for certain other aggregation 
functions like `DISTINCTCOUNTULL` and `SUMPRECISION` when there is a mismatch 
in the query time aggregation parameter value (different `p` value for 
`DISTINCTCOUNTULL`, different precision value for `SUMPRECISION`) and the 
star-tree index aggregation parameter value. In these cases, there will never 
be a query error since intermediate results with any parameter value can be 
merged but the result could have lower / higher precision than desired by the 
user. On second thought, I'm wondering if it might just be better to do a 
stricter parameter match for these aggregation functions too.



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