optimize: only return 3 percentiles from backend

online/src/main/scala/ai/chronon/online/stats/PivotUtils.scala

@@ -45,7 +45,7 @@ object PivotUtils {
       while (seriesIndex < seriesLength) {
         val list = lists(seriesIndex)
         val value: T = if (list == null) {
-          null.asInstanceOf[T]
+          Constants.magicNullDouble.asInstanceOf[T]
         } else {
           val v = list.get(pctIndex)
           if (v == null || (v.isInstanceOf[Double] && v.asInstanceOf[Double].isNaN)) {
@@ -104,6 +104,19 @@ object PivotUtils {
 
     def iterator[T](func: TileSummary => T): Iterator[T] = summaries.iterator.map(func)
 
+    def filterPercentiles[T <: AnyRef](lists: Array[JList[T]]): Array[JList[T]] = {
+      if (lists == null || lists.isEmpty) return null
+      lists.map { list =>
+        if (list == null) null
+        else {
+          val filtered = new JArrayList[T]()
+          // Assuming 21 percentiles (0,5,10,...,95,100), indices 1,10,19 correspond to 5%,50%,95%
+          Seq(1, 10, 19).foreach(i => filtered.add(list.get(i)))
+          filtered
+        }
+      }
+    }
+
     def longIterator(isSetFunc: TileSummary => Boolean, extract: TileSummary => Long): Iterator[JLong] = {
       summaries.iterator.map { summary =>
         if (isSetFunc(summary)) {
@@ -115,7 +128,7 @@ object PivotUtils {
     }
 
     new TileSummarySeries()
-      .setPercentiles(iterator(_.getPercentiles).toArray |> transpose)
+      .setPercentiles(iterator(_.getPercentiles).toArray |> filterPercentiles |> transpose)
       .setHistogram(iterator(_.getHistogram).toArray |> merge)
       .setCount(longIterator(_.isSetCount, _.getCount) |> collect)
       .setNullCount(longIterator(_.isSetNullCount, _.getNullCount) |> collect)