apache/iceberg · error · UnsupportedOperationException

Cannot apply unknown modify-column-position change:

Error message

Cannot apply unknown modify-column-position change: 

What it means

applyModifyColumnPosition supports only TableChange.First and TableChange.After positions; any other ModifyColumnPosition subtype throws UnsupportedOperationException. It maps Flink column-position changes onto Iceberg moveFirst/moveAfter operations.

Source

Thrown at flink/v2.3/flink/src/main/java/org/apache/iceberg/flink/util/FlinkAlterTableUtil.java:241

          (TableChange.ModifyColumnComment) modifyColumn;
      pendingUpdate.updateColumnDoc(
          modifyComment.getOldColumn().getName(), modifyComment.getNewComment());
    } else {
      throw new UnsupportedOperationException(
          "Cannot apply unknown modify-column change: " + modifyColumn);
    }
  }

  private static void applyModifyColumnPosition(
      UpdateSchema pendingUpdate, TableChange.ModifyColumnPosition modifyColumnPosition) {
    TableChange.ColumnPosition newPosition = modifyColumnPosition.getNewPosition();
    if (newPosition instanceof TableChange.First) {
      pendingUpdate.moveFirst(modifyColumnPosition.getOldColumn().getName());
    } else if (newPosition instanceof TableChange.After) {
      TableChange.After after = (TableChange.After) newPosition;
      pendingUpdate.moveAfter(modifyColumnPosition.getOldColumn().getName(), after.column());
    } else {
      throw new UnsupportedOperationException(
          "Cannot apply unknown modify-column-position change: " + modifyColumnPosition);
    }
  }

  private static void applyUniqueConstraint(
      UpdateSchema pendingUpdate, UniqueConstraint constraint) {
    switch (constraint.getType()) {
      case PRIMARY_KEY:
        pendingUpdate.setIdentifierFields(constraint.getColumns());
        break;
      case UNIQUE_KEY:
        throw new UnsupportedOperationException(
            "Unsupported table change: setting unique key constraints.");
      default:
        throw new UnsupportedOperationException(
            "Cannot apply unknown unique constraint: " + constraint.getType().name());
    }
  }

View on GitHub (pinned to 86d9c8fc54)

Solutions

1. Express the position as TableChange.First or TableChange.After.
2. Rewrite the DDL to use AFTER with the preceding column name instead of BEFORE.
3. Apply the move directly: updateSchema().moveBefore(...) on the Table API if BEFORE semantics are needed.

Example fix

// before
new TableChange.ModifyColumnPosition(new TableChange.Before("other"), column);
// after
new TableChange.ModifyColumnPosition(new TableChange.After("other"), column);

// java
if (!(pos instanceof TableChange.First) && !(pos instanceof TableChange.After)) {
  throw new IllegalArgumentException("only First/After positions supported; rewrite BEFORE as AFTER");
}

boolean isSupportedPosition(TableChange.ColumnPosition pos) {
  return pos instanceof TableChange.First || pos instanceof TableChange.After;
}

try {
  FlinkAlterTableUtil.applyModifyColumnPosition(update, change);
} catch (UnsupportedOperationException e) {
  if (e.getMessage().startsWith("Cannot apply unknown modify-column-position")) {
    // convert Before to After with preceding column and retry
  } else { throw e; }
}

When it happens

Trigger: Issuing a column-position change using TableChange.Before (or another position subtype) through the Flink alter-table path.

Common situations: Using `ALTER TABLE ... MODIFY COLUMN c AFTER/BY ...` DDL variants that Flink translates to a position subtype Iceberg does not handle.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

• Cannot apply unknown modify-column-position change:
• Altering schema is not supported in the old alterTable API.
• Altering partition keys is not supported yet.
• Creating table with computed columns is not supported yet.
• Creating table with watermark specs is not supported yet.

AI-assisted analysis of apache/iceberg@86d9c8fc54 (2026-09-12). Data as JSON: /api/errors/b73722dcbe04f0f2. Report an issue: GitHub.