Refactor the Alias node to avoid ambiguity

ppl-spark-integration/src/main/java/org/opensearch/sql/ast/expression/Alias.java

@@ -13,27 +13,23 @@
 import org.opensearch.sql.ast.AbstractNodeVisitor;
 
 /**
- * Alias abstraction that associate an unnamed expression with a name and an optional alias. The
- * name and alias information preserved is useful for semantic analysis and response formatting
+ * Alias abstraction that associate an unnamed expression with a name.
+ * The name information preserved is useful for semantic analysis and response formatting
  * eventually. This can avoid restoring the info in toString() method which is inaccurate because
  * original info is already lost.
  */
-@AllArgsConstructor
 @EqualsAndHashCode(callSuper = false)
 @Getter
 @RequiredArgsConstructor
 @ToString
 public class Alias extends UnresolvedExpression {
 
-  /** Original field name. */
+  /** The name to be associated with the result of computing delegated expression. */
   private final String name;
 
   /** Expression aliased. */
   private final UnresolvedExpression delegated;
 
-  /** Optional field alias. */
-  private String alias;
-
   @Override
   public <T, C> T accept(AbstractNodeVisitor<T, C> nodeVisitor, C context) {
     return nodeVisitor.visitAlias(this, context);

ppl-spark-integration/src/main/java/org/opensearch/sql/ppl/CatalystExpressionVisitor.java

@@ -257,7 +257,7 @@ public Expression visitAlias(Alias node, CatalystPlanContext context) {
         Expression arg = context.popNamedParseExpressions().get();
         return context.getNamedParseExpressions().push(
                 org.apache.spark.sql.catalyst.expressions.Alias$.MODULE$.apply(arg,
-                        node.getAlias() != null ? node.getAlias() : node.getName(),
+                        node.getName(),
                         NamedExpression.newExprId(),
                         seq(new java.util.ArrayList<String>()),
                         Option.empty(),

ppl-spark-integration/src/main/java/org/opensearch/sql/ppl/parser/AstExpressionBuilder.java

@@ -383,8 +383,7 @@ public UnresolvedExpression visitBooleanLiteral(OpenSearchPPLParser.BooleanLiter
     public UnresolvedExpression visitBySpanClause(OpenSearchPPLParser.BySpanClauseContext ctx) {
         String name = ctx.spanClause().getText();
         return ctx.alias != null
-                ? new Alias(
-                name, visit(ctx.spanClause()), StringUtils.unquoteIdentifier(ctx.alias.getText()))
+                ? new Alias(StringUtils.unquoteIdentifier(ctx.alias.getText()), visit(ctx.spanClause()))
                 : new Alias(name, visit(ctx.spanClause()));
     }