apache/iceberg · error · UnsupportedOperationException

Wrong number of inputs (expected value)

Error message

Wrong number of inputs (expected value)

What it means

UnaryUnboundFunction is the base class for Iceberg's single-argument Spark transform functions (days, months, hours, years, etc.). Its valueType() checks that exactly one argument was supplied; if zero or multiple arguments are passed, binding fails with this UnsupportedOperationException. This is an argument-count (arity) error raised during query analysis before doBind type checks run.

Source

Thrown at spark/v4.2/spark/src/main/java/org/apache/iceberg/spark/functions/UnaryUnboundFunction.java:39

import org.apache.spark.sql.connector.catalog.functions.BoundFunction;
import org.apache.spark.sql.connector.catalog.functions.UnboundFunction;
import org.apache.spark.sql.types.DataType;
import org.apache.spark.sql.types.StructType;

/** An unbound function that accepts only one argument */
abstract class UnaryUnboundFunction implements UnboundFunction {

  @Override
  public BoundFunction bind(StructType inputType) {
    DataType valueType = valueType(inputType);
    return doBind(valueType);
  }

  protected abstract BoundFunction doBind(DataType valueType);

  private DataType valueType(StructType inputType) {
    if (inputType.size() != 1) {
      throw new UnsupportedOperationException("Wrong number of inputs (expected value)");
    }

    return inputType.fields()[0].dataType();
  }
}

View on GitHub (pinned to 86d9c8fc54)

Solutions

1. Supply exactly one argument: days(ts_col).
2. Remove extra arguments if more than one was passed.
3. Verify the intended function — some Iceberg functions like bucket/truncate need two arguments, unary ones need one.
4. Run DESCRIBE FUNCTION <fn> to confirm the expected arity.

Example fix

// before
SELECT days(ts_col, 'UTC') FROM t
// after
SELECT days(ts_col) FROM t

// Spark Scala — for any unary Iceberg transform (days/months/hours/years)
val args: Seq[Column] = Seq(col)
require(args.size == 1, s"unary transform requires exactly 1 argument, got ${args.size}")

try {
  df.select(expr("days(ts_col)"))
} catch {
  case e: UnsupportedOperationException if e.getMessage.contains("Wrong number of inputs (expected value)") =>
    throw new IllegalArgumentException("This Iceberg transform takes exactly one value argument", e)
}

When it happens

Trigger: Calling a unary Iceberg transform with the wrong number of arguments, e.g. days() with no argument or days(a, b) with two; typo'd function names routing arguments to a unary function.

Common situations: Generated/templated SQL emitting empty argument lists; confusing Spark built-ins that take different arities with Iceberg transform functions; copy-paste errors adding extra arguments.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

• Wrong number of inputs (expected width and value)
• Wrong number of inputs (expected width and value)
• Wrong number of inputs (expected value)
• Wrong number of inputs (expected numBuckets and value)
• Cannot bind: %s does not accept arguments

AI-assisted analysis of apache/iceberg@86d9c8fc54 (2026-09-12). Data as JSON: /api/errors/dc812e18b56c5749. Report an issue: GitHub.