[fix](iceberg) Allow nested decimal precision widening

fe/fe-connector/fe-connector-iceberg/src/main/java/org/apache/doris/connector/iceberg/IcebergComplexTypeDiff.java

@@ -22,6 +22,7 @@
 
 import org.apache.iceberg.UpdateSchema;
 import org.apache.iceberg.types.Type;
+import org.apache.iceberg.types.TypeUtil;
 import org.apache.iceberg.types.Types;
 
 import java.util.HashSet;
@@ -50,7 +51,7 @@
  * {@code UpdateSchema.commit()}, so any guard throwing aborts the whole change atomically.</p>
  *
  * <p><b>Supported shape changes (legacy parity):</b> widen an existing nested field's primitive type (only the
- * iceberg-representable safe promotions int&rarr;long, float&rarr;double, or an exact match), change a nested
+ * iceberg-representable safe promotions, including same-scale decimal precision widening), change a nested
  * field's comment, widen a NOT NULL nested field to nullable, and append new (nullable) STRUCT fields. The
  * category of every nested level must stay the same (struct/array/map); struct fields may not be renamed,
  * reordered, dropped, or narrowed to NOT NULL; a MAP key type may not change.</p>
@@ -310,21 +311,13 @@ private static void requireValueNotNarrowed(Types.MapType oldMap, Types.MapType
 
     /**
      * Whether changing a nested primitive {@code oldType} to {@code newType} is a legal promotion, mirroring
-     * legacy {@code ColumnType.checkSupportSchemaChangeForNestedPrimitive} restricted to the iceberg-representable
-     * cases: an exact match (covers VARCHAR length growth, which both map to iceberg STRING), INT&rarr;BIGINT
-     * (iceberg INTEGER&rarr;LONG), and FLOAT&rarr;DOUBLE. Everything else (e.g. a nested DECIMAL precision change,
-     * any narrowing, a category change) is rejected — matching legacy's restrictive nested rule.
+     * Iceberg's primitive-promotion rules. Besides INTEGER&rarr;LONG and FLOAT&rarr;DOUBLE, Iceberg allows a
+     * DECIMAL precision increase when scale is unchanged. Delegating to Iceberg keeps the connector's validation
+     * aligned with the UpdateSchema operation it is about to commit.
      */
     private static boolean isLegalNestedPrimitivePromotion(Type oldType, Type newType) {
-        if (oldType.equals(newType)) {
-            return true;
-        }
-        Type.TypeID oldId = oldType.typeId();
-        Type.TypeID newId = newType.typeId();
-        if (oldId == Type.TypeID.INTEGER && newId == Type.TypeID.LONG) {
-            return true;
-        }
-        return oldId == Type.TypeID.FLOAT && newId == Type.TypeID.DOUBLE;
+        return newType.isPrimitiveType()
+                && TypeUtil.isPromotionAllowed(oldType, newType.asPrimitiveType());
     }
 
     /** The iceberg type category (struct/list/map) of {@code newType} must equal {@code oldType}'s. */

fe/fe-connector/fe-connector-iceberg/src/test/java/org/apache/doris/connector/iceberg/CatalogBackedIcebergCatalogOpsColumnEvolutionTest.java

@@ -606,17 +606,18 @@ public void testModifyStructNewFieldNotNullableFailsLoud() {
     }
 
     @Test
-    public void testModifyNestedDecimalPrecisionFailsLoud() {
-        // Legacy parity: a nested primitive change is restricted to int->long / float->double / exact; a
-        // DECIMAL precision change inside a struct is rejected (checkSupportSchemaChangeForNestedPrimitive).
+    public void testModifyNestedDecimalPrecisionWidens() {
+        // Iceberg permits a nested DECIMAL precision increase when the scale stays fixed.
         createTable("s_dec", new ConnectorColumn("st",
                 structType(Arrays.asList("a"), Arrays.asList(ConnectorType.of("DECIMALV3", 10, 2)),
                         Arrays.asList(true), Arrays.asList((String) null)), "", true, null, false));
-        DorisConnectorException ex = Assertions.assertThrows(DorisConnectorException.class,
-                () -> modifyComplex("s_dec", "st",
-                        structType(Arrays.asList("a"), Arrays.asList(ConnectorType.of("DECIMALV3", 20, 2)),
-                                Arrays.asList(true), Arrays.asList((String) null)), true));
-        Assertions.assertTrue(ex.getMessage().contains("nested"));
+        modifyComplex("s_dec", "st",
+                structType(Arrays.asList("a"), Arrays.asList(ConnectorType.of("DECIMALV3", 20, 2)),
+                        Arrays.asList(true), Arrays.asList((String) null)), true);
+        Types.DecimalType amount = (Types.DecimalType) reload("s_dec").findField("st")
+                .type().asStructType().field("a").type();
+        Assertions.assertEquals(20, amount.precision());
+        Assertions.assertEquals(2, amount.scale());
     }
 
     @Test

fe/fe-connector/fe-connector-iceberg/src/test/java/org/apache/doris/connector/iceberg/IcebergNestedColumnEvolutionTest.java

@@ -713,6 +713,26 @@ public void testModifyNestedPrimitivePromotionAllowed() {
         Assertions.assertEquals(Type.TypeID.LONG, metric.type().typeId());
     }
 
+    @Test
+    public void testModifyNestedStructDecimalPrecisionPromotionAllowed() {
+        // Iceberg permits DECIMAL precision widening when the scale stays fixed. This is the same complex MODIFY
+        // issued by the Trino product test for STRUCT<field:DECIMAL(5,3)> -> STRUCT<field:DECIMAL(10,3)>.
+        Schema schema = new Schema(Types.NestedField.optional(1, "info", Types.StructType.of(
+                Types.NestedField.optional(2, "amount", Types.DecimalType.of(5, 3)))));
+        createTable("m_decimal_promote", schema);
+
+        ops.modifyColumn("db1", "m_decimal_promote",
+                structModify("info", Collections.singletonList("amount"),
+                        Collections.singletonList(ConnectorType.of("DECIMALV3", 10, 3)),
+                        Collections.singletonList(""), Collections.singletonList(false)),
+                false, null);
+
+        Types.DecimalType amount = (Types.DecimalType) reload("m_decimal_promote").findField("info")
+                .type().asStructType().field("amount").type();
+        Assertions.assertEquals(10, amount.precision());
+        Assertions.assertEquals(3, amount.scale());
+    }
+
     @Test
     public void testModifyNestedPrimitivePromotionDisallowedFailsLoud() {
         // BIGINT -> INT is not an iceberg-representable promotion.

regression-test/data/external_table_p0/iceberg/test_iceberg_nested_schema_evolution_ddl.out

@@ -10,3 +10,9 @@ m_scalar	map<string,bigint(20)>
 -- !query_rows --
 1	\N	10	\N	\N	100	\N	\N	1000	\N	\N	7	70
 2	first	20	after_a	c2	200	202	201	2000	2002	2001	8	80
+
+-- !nested_decimal_schema --
+struct<decimalv3(10, 3)>
+
+-- !nested_decimal_old_row --
+1	12.345

regression-test/suites/external_table_p0/iceberg/test_iceberg_nested_schema_evolution_ddl.groovy

@@ -172,4 +172,34 @@ suite("test_iceberg_nested_schema_evolution_ddl", "p0,external,doris,external_do
         FROM ${tableName}
         ORDER BY id
     """
+
+    // Trino compatibility: an existing nested decimal may widen its precision without changing its scale.
+    sql """DROP TABLE IF EXISTS iceberg_nested_decimal_evolution"""
+    sql """
+    CREATE TABLE iceberg_nested_decimal_evolution (
+        id INT,
+        info STRUCT<amount:DECIMAL(5,3)>
+    )
+    """
+    sql """
+    INSERT INTO iceberg_nested_decimal_evolution VALUES
+        (1, STRUCT(CAST(12.345 AS DECIMAL(5,3))))
+    """
+    sql """
+    ALTER TABLE iceberg_nested_decimal_evolution
+    MODIFY COLUMN info STRUCT<amount:DECIMAL(10,3)>
+    """
+
+    qt_nested_decimal_schema """
+        SELECT COLUMN_TYPE
+        FROM ${catalogName}.information_schema.columns
+        WHERE TABLE_SCHEMA = '${dbName}' AND TABLE_NAME = 'iceberg_nested_decimal_evolution'
+          AND COLUMN_NAME = 'info'
+    """
+
+    order_qt_nested_decimal_old_row """
+        SELECT id, element_at(info, 'amount')
+        FROM iceberg_nested_decimal_evolution
+        ORDER BY id
+    """
 }