ESQL: Implement coalesce for exponential histograms (#137897)

Author: JonasKunz

x-pack/plugin/esql/build.gradle

@@ -468,6 +468,7 @@ tasks.named('stringTemplates').configure {
   var doubleProperties = prop("Double", "Double", "double", "DOUBLE", "Double.BYTES", "DoubleArray")
   var bytesRefProperties = prop("BytesRef", "BytesRef", "BytesRef", "BYTES_REF", "org.apache.lucene.util.RamUsageEstimator.NUM_BYTES_OBJECT_REF", "")
   var booleanProperties = prop("Boolean", "Boolean", "boolean", "BOOLEAN", "Byte.BYTES", "BitArray")
+  var expHistoProperties = prop("ExponentialHistogram", "ExponentialHistogram", "ExponentialHistogram", "EXPONENTIAL_HISTOGRAM", "", "")
 
   File inInputFile = file("src/main/java/org/elasticsearch/xpack/esql/expression/predicate/operator/comparison/X-InEvaluator.java.st")
   template {
@@ -532,6 +533,11 @@ tasks.named('stringTemplates').configure {
     it.inputFile = coalesceInputFile
     it.outputFile = "org/elasticsearch/xpack/esql/expression/function/scalar/nulls/CoalesceBytesRefEvaluator.java"
   }
+  template {
+    it.properties = expHistoProperties
+    it.inputFile = coalesceInputFile
+    it.outputFile = "org/elasticsearch/xpack/esql/expression/function/scalar/nulls/CoalesceExponentialHistogramEvaluator.java"
+  }
 
   File roundToInput = file("src/main/java/org/elasticsearch/xpack/esql/expression/function/scalar/math/X-RoundTo.java.st")
   template {

x-pack/plugin/esql/compute/src/main/java/org/elasticsearch/compute/data/ExponentialHistogramBlock.java

@@ -9,6 +9,7 @@
 
 import org.apache.lucene.util.BytesRef;
 import org.elasticsearch.exponentialhistogram.ExponentialHistogram;
+import org.elasticsearch.index.mapper.BlockLoader;
 
 /**
  * A block that holds {@link ExponentialHistogram} values.
@@ -93,6 +94,20 @@ static boolean equals(ExponentialHistogramBlock blockA, ExponentialHistogramBloc
         };
     }
 
+    /**
+     * Builder for {@link ExponentialHistogramBlock}
+     */
+    sealed interface Builder extends Block.Builder, BlockLoader.ExponentialHistogramBuilder permits ExponentialHistogramBlockBuilder {
+
+        /**
+         * Copy the values in {@code block} from the given positon into this builder.
+         */
+        Builder copyFrom(ExponentialHistogramBlock block, int position);
+
+        @Override
+        ExponentialHistogramBlock build();
+    }
+
     /**
      * Abstraction to use for writing individual values via {@link #serializeExponentialHistogram(int, SerializedOutput, BytesRef)}.
      */

x-pack/plugin/esql/compute/src/main/java/org/elasticsearch/compute/data/ExponentialHistogramBlockBuilder.java

@@ -17,7 +17,7 @@
 
 import java.io.IOException;
 
-public class ExponentialHistogramBlockBuilder implements Block.Builder, BlockLoader.ExponentialHistogramBuilder {
+public final class ExponentialHistogramBlockBuilder implements ExponentialHistogramBlock.Builder {
 
     private final DoubleBlock.Builder minimaBuilder;
     private final DoubleBlock.Builder maximaBuilder;
@@ -221,6 +221,12 @@ public ExponentialHistogramBlockBuilder copyFrom(Block block, int beginInclusive
         return this;
     }
 
+    @Override
+    public ExponentialHistogramBlock.Builder copyFrom(ExponentialHistogramBlock block, int position) {
+        copyFrom(block, position, position + 1);
+        return this;
+    }
+
     @Override
     public ExponentialHistogramBlockBuilder mvOrdering(Block.MvOrdering mvOrdering) {
         assert mvOrdering == Block.MvOrdering.UNORDERED
@@ -238,4 +244,5 @@ public long estimatedBytes() {
     public void close() {
         Releasables.close(minimaBuilder, maximaBuilder, sumsBuilder, valueCountsBuilder, zeroThresholdsBuilder, encodedHistogramsBuilder);
     }
+
 }