[CALCITE-7791] UNNEST of a single-field ROW array loses the field nam…

core/src/main/java/org/apache/calcite/sql/validate/AliasNamespace.java

@@ -111,6 +111,7 @@ protected AliasNamespace(
                 rowType.getFieldList().get(0).getType())
             .build();
         aliasedType = node.getKind() == SqlKind.COLLECTION_TABLE
+                || (node.getKind() == SqlKind.UNNEST && isUnnestOfStructArray((SqlCall) node))
             ? new SingleColumnAliasRelDataType(rowType, singleColumnAlias) : singleColumnAlias;
         // If the sub-query is UNNEST with ordinality
         // and the sub-query has two columns: data column, ordinality column
@@ -162,6 +163,22 @@ protected AliasNamespace(
     }
   }
 
+  /**
+   * Returns whether an UNNEST call's array operand has a struct element type.
+   *
+   * <p>{@link SqlUnnestOperator#inferReturnType} produces a 1-field row for
+   * both struct arrays and scalar arrays. Only the structs have a real field
+   * name worth preserving.
+   */
+  private boolean isUnnestOfStructArray(SqlCall unnestCall) {
+    if (unnestCall.operandCount() != 1) {
+      return false;
+    }
+    final RelDataType operandType = validator.getValidatedNodeType(unnestCall.operand(0));
+    final RelDataType componentType = operandType.getComponentType();
+    return componentType != null && componentType.isStruct();
+  }
+
   private static String getString(RelDataType rowType) {
     StringBuilder buf = new StringBuilder();
     buf.append("(");

core/src/test/java/org/apache/calcite/test/SqlValidatorTest.java

@@ -1201,6 +1201,20 @@ void testDyadicCollateOperator() {
         });
   }
 
+  /** Test case for
+   * <a href="https://issues.apache.org/jira/browse/CALCITE-7791">[CALCITE-7791]
+   * UNNEST of a single-field ROW array loses the field name when aliased</a>. */
+  @Test void testUnnestSingleFieldRow() {
+    sql("select d.a\n"
+        + "from unnest(cast(array[row(1)] as row(a integer) array)) as d")
+        .columnType("INTEGER NOT NULL");
+    // The array's real field name ("A") survives star-expansion too, not just
+    // the synthetic alias ("D").
+    sql("select *\n"
+        + "from unnest(cast(array[row(1)] as row(a integer) array)) as d")
+        .type("RecordType(INTEGER NOT NULL A) NOT NULL");
+  }
+
   @Test void testOverlay() {
     expr("overlay('ABCdef' placing 'abc' from 1)").ok();
     expr("overlay('ABCdef' placing 'abc' from 1 for 3)").ok();