change parser logic

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

Author: xinyual

ppl/src/main/antlr/OpenSearchPPLParser.g4

@@ -388,8 +388,7 @@ statsAggTerm
 statsFunction
    : statsFunctionName LT_PRTHS valueExpression RT_PRTHS        # statsFunctionCall
    | COUNT LT_PRTHS RT_PRTHS                                    # countAllFunctionCall
-   | (DISTINCT_COUNT | DC) LT_PRTHS valueExpression RT_PRTHS    # distinctCountFunctionCall
-   | DISTINCT_COUNT_APPROX LT_PRTHS valueExpression RT_PRTHS    # distinctCountApproxFunctionCall
+   | (DISTINCT_COUNT | DC | DISTINCT_COUNT_APPROX) LT_PRTHS valueExpression RT_PRTHS    # distinctCountFunctionCall
    | takeAggFunction                                            # takeAggFunctionCall
    | percentileApproxFunction                                   # percentileApproxFunctionCall
    ;

ppl/src/main/java/org/opensearch/sql/ppl/parser/AstExpressionBuilder.java

@@ -200,13 +200,8 @@ public UnresolvedExpression visitCountAllFunctionCall(CountAllFunctionCallContex
 
   @Override
   public UnresolvedExpression visitDistinctCountFunctionCall(DistinctCountFunctionCallContext ctx) {
-    return new AggregateFunction("count", visit(ctx.valueExpression()), true);
-  }
-
-  @Override
-  public UnresolvedExpression visitDistinctCountApproxFunctionCall(
-      OpenSearchPPLParser.DistinctCountApproxFunctionCallContext ctx) {
-    return new AggregateFunction("distinct_count_approx", visit(ctx.valueExpression()), true);
+    String funcName = ctx.DISTINCT_COUNT_APPROX()!=null ? "distinct_count_approx" :"count";
+    return new AggregateFunction(funcName, visit(ctx.valueExpression()), true);
   }
 
   @Override