Print an warning mesages in '<=>'

Author: maropu

sql/core/src/main/scala/org/apache/spark/sql/Column.scala

@@ -464,7 +464,15 @@ class Column(val expr: Expression) extends Logging {
    * @group expr_ops
    * @since 1.3.0
    */
-  def <=> (other: Any): Column = withExpr { EqualNullSafe(expr, lit(other).expr) }
+  def <=> (other: Any): Column = withExpr {
+    val right = lit(other).expr
+    if (this.expr == right) {
+      logWarning(
+        s"Constructing trivially true equals predicate, '${this.expr} = $right'. " +
+          "Perhaps you need to use aliases.")
+    }
+    EqualNullSafe(expr, right)
+  }
 
   /**
    * Equality test that is safe for null values.