add UT

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

Author: xinyual

core/src/main/java/org/opensearch/sql/ast/dsl/AstDSL.java

@@ -49,6 +49,7 @@
 import org.opensearch.sql.ast.expression.WindowFunction;
 import org.opensearch.sql.ast.expression.Xor;
 import org.opensearch.sql.ast.tree.Aggregation;
+import org.opensearch.sql.ast.tree.AppendPipe;
 import org.opensearch.sql.ast.tree.Bin;
 import org.opensearch.sql.ast.tree.CountBin;
 import org.opensearch.sql.ast.tree.Dedupe;
@@ -550,6 +551,11 @@ public static Trendline trendline(
     return new Trendline(sortField, Arrays.asList(computations)).attach(input);
   }
 
+  public static AppendPipe appendPipe(UnresolvedPlan input, UnresolvedPlan subquery) {
+
+    return new AppendPipe(subquery).attach(input);
+  }
+
   public static Trendline.TrendlineComputation computation(
       Integer numDataPoints, Field dataField, String alias, Trendline.TrendlineType type) {
     return new Trendline.TrendlineComputation(numDataPoints, dataField, alias, type);

ppl/src/test/java/org/opensearch/sql/ppl/parser/AstBuilderTest.java

@@ -12,6 +12,7 @@
 import static org.opensearch.sql.ast.dsl.AstDSL.agg;
 import static org.opensearch.sql.ast.dsl.AstDSL.aggregate;
 import static org.opensearch.sql.ast.dsl.AstDSL.alias;
+import static org.opensearch.sql.ast.dsl.AstDSL.appendPipe;
 import static org.opensearch.sql.ast.dsl.AstDSL.argument;
 import static org.opensearch.sql.ast.dsl.AstDSL.booleanLiteral;
 import static org.opensearch.sql.ast.dsl.AstDSL.compare;
@@ -870,6 +871,20 @@ public void testFillNullValueWithFields() {
         fillNull(relation("t"), intLiteral(0), true, field("a"), field("b"), field("c")));
   }
 
+  @Test
+  public void testAppendPipe() {
+    assertEqual(
+        "source=t | appendpipe [ stats COUNT() ]",
+        appendPipe(
+            relation("t"),
+            agg(
+                null,
+                exprList(alias("COUNT()", aggregate("count", AstDSL.allFields()))),
+                emptyList(),
+                emptyList(),
+                defaultStatsArgs())));
+  }
+
   public void testTrendline() {
     assertEqual(
         "source=t | trendline sma(5, test_field) as test_field_alias sma(1, test_field_2) as"