enable pushdown distinct count approx

Signed-off-by: xinyual <xinyual@amazon.com>

Author: xinyual

opensearch/src/main/java/org/opensearch/sql/opensearch/executor/OpenSearchExecutionEngine.java

@@ -55,6 +55,8 @@
 import org.opensearch.sql.planner.physical.PhysicalPlan;
 import org.opensearch.sql.storage.TableScanOperator;
 
+import static org.opensearch.sql.expression.function.BuiltinFunctionName.DISTINCT_COUNT_APPROX;
+
 /** OpenSearch execution engine implementation. */
 public class OpenSearchExecutionEngine implements ExecutionEngine {
   private static final Logger logger = LogManager.getLogger(OpenSearchExecutionEngine.class);
@@ -282,10 +284,10 @@ private void registerOpenSearchFunctions() {
     SqlUserDefinedAggFunction approxDistinctCountFunction =
         UserDefinedFunctionUtils.createUserDefinedAggFunction(
             DistinctCountApproxAggFunction.class,
-            "APPROX_DISTINCT_COUNT",
+            DISTINCT_COUNT_APPROX.toString(),
             ReturnTypes.BIGINT_FORCE_NULLABLE,
             null);
     PPLFuncImpTable.INSTANCE.registerExternalAggOperator(
-        BuiltinFunctionName.DISTINCT_COUNT_APPROX, approxDistinctCountFunction);
+        DISTINCT_COUNT_APPROX, approxDistinctCountFunction);
   }
 }

opensearch/src/main/java/org/opensearch/sql/opensearch/request/AggregateAnalyzer.java

@@ -495,6 +495,11 @@ yield switch (functionName) {
             }
             yield Pair.of(aggBuilder, new SinglePercentileParser(aggFieldName));
           }
+          case DISTINCT_COUNT_APPROX ->  Pair.of(
+                  helper.build(
+                          !args.isEmpty() ? args.getFirst() : null,
+                          AggregationBuilders.cardinality(aggFieldName)),
+                  new SingleValueParser(aggFieldName));
           default -> throw new AggregateAnalyzer.AggregateAnalyzerException(
               String.format("Unsupported push-down aggregator %s", aggCall.getAggregation()));
         };