optimize count(*) (#4555)

src/graph/executor/query/AggregateExecutor.cpp

@@ -17,6 +17,13 @@ folly::Future<Status> AggregateExecutor::execute() {
   auto iter = ectx_->getResult(agg->inputVar()).iter();
   DCHECK(!!iter);
   QueryExpressionContext ctx(ectx_);
+  // We could directly return size of input dataset for `COUNT(*)`
+  if (groupKeys.empty() && groupItems.size() == 1 && groupItems[0]->toString() == "COUNT(*)") {
+    DataSet ds;
+    ds.colNames = agg->colNames();
+    ds.rows.emplace_back(Row({Value(int64_t(iter->size()))}));
+    return finish(ResultBuilder().value(Value(std::move(ds))).build());
+  }
 
   std::unordered_map<List, std::vector<std::unique_ptr<AggData>>, std::hash<nebula::List>> result;
 

tests/tck/features/aggregate/Agg.feature

@@ -47,6 +47,34 @@ Feature: Basic Aggregate and GroupBy
     Then the result should be, in any order, with relax comparison:
       | a |
       | 1 |
+    When executing query:
+      """
+      GO FROM "Tim Duncan" OVER like YIELD like._dst AS dst | YIELD COUNT(*)
+      """
+    Then the result should be, in any order, with relax comparison:
+      | COUNT(*) |
+      | 2        |
+    When executing query:
+      """
+      GO 3 STEPS FROM "Tim Duncan" OVER like YIELD like._dst AS dst | YIELD COUNT(*)
+      """
+    Then the result should be, in any order, with relax comparison:
+      | COUNT(*) |
+      | 5        |
+    When executing query:
+      """
+      GO 1 to 3 STEPS FROM "Tony Parker" OVER serve BIDIRECT YIELD DISTINCT id($$) AS dst | YIELD COUNT(*)
+      """
+    Then the result should be, in any order, with relax comparison:
+      | COUNT(*) |
+      | 41       |
+    When executing query:
+      """
+      MATCH (v:player) RETURN COUNT(*)
+      """
+    Then the result should be, in any order, with relax comparison:
+      | COUNT(*) |
+      | 56       |
 
   Scenario: [1] Basic GroupBy
     When executing query: