Map Calcite ROW to STRUCT in convertSqlTypeNameToExprType (#5737)

convertExprTypeToRelDataType builds STRUCT as MAP<VARCHAR, ANY>, since the v2
path only passes _source JSON through and needs no field types. So the reverse
mapping only had `case MAP -> STRUCT` — a genuine Calcite ROW matched nothing,
fell through to `default -> UNKNOWN`, and surfaced as "type": "unknown" in the
response schema.

That is reachable from the analytics engine, which materializes an `object`
field as a real ROW built from typed columns. Before, `fields city` reported
"unknown"; now "struct", matching what a lucene-only cluster returns for the
same mapping.

Signed-off-by: Marc Handalian <handalm@amazon.com>
Signed-off-by: Marc Handalian <marc.handalian@gmail.com>

Author: mch2

core/src/main/java/org/opensearch/sql/calcite/utils/OpenSearchTypeFactory.java

@@ -252,6 +252,11 @@ public static ExprType convertSqlTypeNameToExprType(SqlTypeName sqlTypeName) {
           INTERVAL;
       case ARRAY -> ARRAY;
       case MAP -> STRUCT;
+      // Calcite spells a struct as ROW. convertExprTypeToRelDataType builds STRUCT as
+      // MAP<VARCHAR, ANY> since the v2 path only passes _source JSON through, so a genuine ROW —
+      // from an engine that builds a struct out of typed columns — matched nothing and fell
+      // through to UNKNOWN.
+      case ROW -> STRUCT;
       case GEOMETRY -> GEO_POINT;
       case NULL, ANY, OTHER -> UNDEFINED;
       default -> UNKNOWN;

core/src/test/java/org/opensearch/sql/calcite/utils/OpenSearchTypeFactoryTest.java

@@ -286,6 +286,12 @@ public void testConvertExprTypeBinaryToNullableVarbinary() {
     assertTrue(result.isNullable());
   }
 
+  @Test
+  public void testConvertRowReturnsStructExprType() {
+    assertEquals(
+        ExprCoreType.STRUCT, OpenSearchTypeFactory.convertSqlTypeNameToExprType(SqlTypeName.ROW));
+  }
+
   // ---------- convertAnalyticsEngineRelDataTypeToExprType ----------
   // UDT-aware variant for the response-schema path. Must agree with the
   // planner-internal convertRelDataTypeToExprType on every non-UDT input.