Fix CalcitePPLAggregationIT on the analytics-engine route (parquet testSimpleCount0 + APPROX_COUNT_DISTINCT name) (#5525)

* Use a parquet-backed index in CalcitePPLAggregationIT.testSimpleCount0

A bare auto-created index isn't composite/parquet-backed, so on the analytics-engine
route it doesn't route to the analytics engine. Switch to TEST_INDEX_BANK (loaded via
loadIndex, which injects parquet settings when the flag is set, 7 docs) so the test is
meaningful on both routes. Diagnosis by Sandesh Kumar.

Signed-off-by: Kai Huang <ahkcs@amazon.com>

* Emit APPROX_COUNT_DISTINCT as the distinct_count_approx runtime name

distinct_count_approx() failed to bind on the analytics-engine (DataFusion) route
because the SqlAggFunction was named DISTINCT_COUNT_APPROX; the backend resolves
aggregates by the Calcite/Substrait-standard name APPROX_COUNT_DISTINCT. The Java
field name and PPL function name are unchanged. The OpenSearch V3 path is unaffected
(it overrides this via the external HLL registration). Analytics-route binding is
completed by opensearch-project/OpenSearch#22013. Per Sandesh Kumar.

Signed-off-by: Kai Huang <ahkcs@amazon.com>

---------

Signed-off-by: Kai Huang <ahkcs@amazon.com>

Author: ahkcs

core/src/main/java/org/opensearch/sql/expression/function/PPLBuiltinOperators.java

@@ -521,7 +521,8 @@ public class PPLBuiltinOperators extends ReflectiveSqlOperatorTable {
   public static final SqlAggFunction DISTINCT_COUNT_APPROX =
       createUserDefinedAggFunction(
           DistinctCountApproxLogicalAggFunction.class,
-          "DISTINCT_COUNT_APPROX",
+          // Substrait-standard name the analytics-engine backend resolves by (V3 overrides it).
+          "APPROX_COUNT_DISTINCT",
           ReturnTypes.BIGINT_FORCE_NULLABLE,
           null);
 

integ-test/src/test/java/org/opensearch/sql/calcite/remote/CalcitePPLAggregationIT.java

@@ -25,7 +25,6 @@
 import java.util.List;
 import org.json.JSONObject;
 import org.junit.jupiter.api.Test;
-import org.opensearch.client.Request;
 import org.opensearch.sql.common.utils.StringUtils;
 import org.opensearch.sql.exception.SemanticCheckException;
 import org.opensearch.sql.ppl.PPLIntegTestCase;
@@ -52,16 +51,11 @@ public void init() throws Exception {
 
   @Test
   public void testSimpleCount0() throws IOException {
-    Request request1 = new Request("PUT", "/test/_doc/1?refresh=true");
-    request1.setJsonEntity("{\"name\": \"hello\", \"age\": 20}");
-    client().performRequest(request1);
-    Request request2 = new Request("PUT", "/test/_doc/2?refresh=true");
-    request2.setJsonEntity("{\"name\": \"world\", \"age\": 30}");
-    client().performRequest(request2);
-
-    JSONObject actual = executeQuery("source=test | stats count() as c");
+    // A bare auto-created index isn't parquet-backed; use the parquet-aware bank index (7 docs).
+    JSONObject actual =
+        executeQuery(String.format("source=%s | stats count() as c", TEST_INDEX_BANK));
     verifySchema(actual, schema("c", "bigint"));
-    verifyDataRows(actual, rows(2));
+    verifyDataRows(actual, rows(7));
   }
 
   @Test