[BugFix] Fix the count() only aggregation pushdown issue

core/src/main/java/org/opensearch/sql/calcite/CalciteRelNodeVisitor.java

@@ -556,6 +556,11 @@ private Pair<List<RexNode>, List<AggCall>> aggregateWithTrimming(
     //        \- Project([c, b])
     //           \- Filter(a > 1)
     //              \- Scan t
+    // Example 3: source=t | stats count(): no project added for count()
+    // Before: Aggregate(count)
+    //           \- Scan t
+    // After: Aggregate(count)
+    //           \- Scan t
     Pair<List<RexNode>, List<AggCall>> resolved =
         resolveAttributesForAggregation(groupExprList, aggExprList, context);
     List<RexInputRef> trimmedRefs = new ArrayList<>();

integ-test/src/test/java/org/opensearch/sql/ppl/ExplainIT.java

@@ -97,6 +97,14 @@ public void testFilterAndAggPushDownExplain() throws IOException {
                 + "| stats avg(age) AS avg_age by state, city"));
   }
 
+  @Test
+  public void testCountAggPushDownExplain() throws IOException {
+    String expected = loadExpectedPlan("explain_count_agg_push.json");
+    assertJsonEqualsIgnoreId(
+        expected,
+        explainQueryToString("source=opensearch-sql_test_index_account | stats count() as cnt"));
+  }
+
   @Test
   public void testSortPushDownExplain() throws IOException {
     String expected = loadExpectedPlan("explain_sort_push.json");

integ-test/src/test/resources/expectedOutput/calcite/explain_count_agg_push.json

@@ -0,0 +1,6 @@
+{
+  "calcite": {
+    "logical": "LogicalAggregate(group=[{}], cnt=[COUNT()])\n  CalciteLogicalIndexScan(table=[[OpenSearch, opensearch-sql_test_index_account]])\n",
+    "physical": "CalciteEnumerableIndexScan(table=[[OpenSearch, opensearch-sql_test_index_account]], PushDownContext=[[AGGREGATION->rel#:LogicalAggregate.NONE.[](input=RelSubset#,group={},cnt=COUNT())], OpenSearchRequestBuilder(sourceBuilder={\"from\":0,\"size\":0,\"timeout\":\"1m\",\"aggregations\":{\"cnt\":{\"value_count\":{\"field\":\"_index\"}}}}, requestedTotalSize=2147483647, pageSize=null, startFrom=0)])\n"
+  }
+}

integ-test/src/test/resources/expectedOutput/calcite_no_pushdown/explain_count_agg_push.json

@@ -0,0 +1,6 @@
+{
+  "calcite": {
+    "logical": "LogicalAggregate(group=[{}], cnt=[COUNT()])\n  CalciteLogicalIndexScan(table=[[OpenSearch, opensearch-sql_test_index_account]])\n",
+    "physical": "EnumerableAggregate(group=[{}], cnt=[COUNT()])\n  CalciteEnumerableIndexScan(table=[[OpenSearch, opensearch-sql_test_index_account]])\n"
+  }
+}

integ-test/src/test/resources/expectedOutput/ppl/explain_count_agg_push.json

@@ -0,0 +1,15 @@
+{
+  "root": {
+    "name": "ProjectOperator",
+    "description": {
+      "fields": "[cnt]"
+    },
+    "children": [{
+      "name": "OpenSearchIndexScan",
+      "description": {
+        "request": "OpenSearchQueryRequest(indexName=opensearch-sql_test_index_account, sourceBuilder={\"from\":0,\"size\":0,\"timeout\":\"1m\",\"aggregations\":{\"cnt\":{\"value_count\":{\"field\":\"_index\"}}}}, needClean=true, searchDone=false, pitId=*, cursorKeepAlive=null, searchAfter=null, searchResponse=null)"
+      },
+      "children": []
+    }]
+  }
+}

opensearch/src/main/java/org/opensearch/sql/opensearch/planner/physical/OpenSearchAggregateIndexScanRule.java

@@ -9,6 +9,7 @@
 import org.apache.calcite.plan.RelRule;
 import org.apache.calcite.rel.logical.LogicalAggregate;
 import org.apache.calcite.rel.logical.LogicalProject;
+import org.apache.calcite.sql.SqlKind;
 import org.immutables.value.Value;
 import org.opensearch.sql.opensearch.storage.scan.CalciteLogicalIndexScan;
 
@@ -29,6 +30,11 @@ public void onMatch(RelOptRuleCall call) {
       final LogicalProject project = call.rel(1);
       final CalciteLogicalIndexScan scan = call.rel(2);
       apply(call, aggregate, project, scan);
+    } else if (call.rels.length == 2) {
+      // case of count() without group-by
+      final LogicalAggregate aggregate = call.rel(0);
+      final CalciteLogicalIndexScan scan = call.rel(1);
+      apply(call, aggregate, null, scan);
     } else {
       throw new AssertionError(
           String.format(
@@ -54,6 +60,7 @@ public interface Config extends RelRule.Config {
     Config DEFAULT =
         ImmutableOpenSearchAggregateIndexScanRule.Config.builder()
             .build()
+            .withDescription("Agg-Project-TableScan")
             .withOperandSupplier(
                 b0 ->
                     b0.operand(LogicalAggregate.class)
@@ -71,6 +78,28 @@ public interface Config extends RelRule.Config {
                                                             OpenSearchIndexScanRule
                                                                 ::noAggregatePushed))
                                                 .noInputs())));
+    Config COUNT_STAR =
+        ImmutableOpenSearchAggregateIndexScanRule.Config.builder()
+            .build()
+            .withDescription("Agg[count()]-TableScan")
+            .withOperandSupplier(
+                b0 ->
+                    b0.operand(LogicalAggregate.class)
+                        .predicate(
+                            agg ->
+                                agg.getGroupSet().isEmpty()
+                                    && agg.getAggCallList().stream()
+                                        .allMatch(
+                                            call ->
+                                                call.getAggregation().kind == SqlKind.COUNT
+                                                    && call.getArgList().isEmpty()))
+                        .oneInput(
+                            b1 ->
+                                b1.operand(CalciteLogicalIndexScan.class)
+                                    .predicate(
+                                        Predicate.not(OpenSearchIndexScanRule::isLimitPushed)
+                                            .and(OpenSearchIndexScanRule::noAggregatePushed))
+                                    .noInputs()));
 
     @Override
     default OpenSearchAggregateIndexScanRule toRule() {

opensearch/src/main/java/org/opensearch/sql/opensearch/planner/physical/OpenSearchIndexRules.java

@@ -16,6 +16,8 @@ public class OpenSearchIndexRules {
       OpenSearchFilterIndexScanRule.Config.DEFAULT.toRule();
   private static final OpenSearchAggregateIndexScanRule AGGREGATE_INDEX_SCAN =
       OpenSearchAggregateIndexScanRule.Config.DEFAULT.toRule();
+  private static final OpenSearchAggregateIndexScanRule COUNT_STAR_INDEX_SCAN =
+      OpenSearchAggregateIndexScanRule.Config.COUNT_STAR.toRule();
   private static final OpenSearchLimitIndexScanRule LIMIT_INDEX_SCAN =
       OpenSearchLimitIndexScanRule.Config.DEFAULT.toRule();
   private static final OpenSearchSortIndexScanRule SORT_INDEX_SCAN =
@@ -26,6 +28,7 @@ public class OpenSearchIndexRules {
           PROJECT_INDEX_SCAN,
           FILTER_INDEX_SCAN,
           AGGREGATE_INDEX_SCAN,
+          COUNT_STAR_INDEX_SCAN,
           LIMIT_INDEX_SCAN,
           SORT_INDEX_SCAN);