apache/druid · error · IAE

operator ' ' in expression ( ) is not supported on type…

Error message

operator '%s' in expression (%s %s %s) is not supported on type STRING.

What it means

Binary operator expressions (e.g. '+', '&&', comparisons) in Druid expressions cannot be applied to STRING-typed operands. When the binary op's eval path dispatches to evalString, it unconditionally throws IllegalArgumentException because the operator has no defined string semantics.

Solutions

1. Add an explicit cast in the expression, e.g. CAST(a AS DOUBLE) + CAST(b AS DOUBLE) or use numeric parse functions like ParseLong/ParseDouble
2. Verify the actual column types in the datasource; fix ingestion specs so the column is ingested as long/double instead of string
3. Rewrite the expression to use string-appropriate operators/functions instead of arithmetic/comparison ops

Example fix

// before: expression "dim1 + dim2" with string columns
// after
"CAST(dim1 AS DOUBLE) + CAST(dim2 AS DOUBLE)"

// Java-side guard before evaluating
if (leftType.is(ExpressionType.STRING) || rightType.is(ExpressionType.STRING)) {
  throw new IllegalArgumentException("string operands need CAST before binary operator");
}

try {
  result = expr.eval(bindings);
} catch (IllegalArgumentException e) {
  if (e.getMessage() != null && e.getMessage().contains("not supported on type STRING")) {
    // re-evaluate with casts or fail gracefully
  } else { throw e; }
}

When it happens

Trigger: Evaluating a Druid native expression such as 'a + b' or 'a < b' where left or right operand evaluates to type STRING instead of a numeric/boolean type.

Common situations: Ingestion spec transforms/filters or SQL-level virtual columns referencing a string column with arithmetic; a column that was expected to be numeric is actually stored as string (schema drift); missing a CAST in the expression.

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 cast [ ] to [ ] (Types.InvalidCastException from…
• Cannot create type [ ]
• Cannot implicitly cast
• Expected key [ ] to be a String, but got [ ]
• needs a LONG as its second argument but got

AI-assisted analysis of apache/druid@9b90983fd2 (2026-09-07). Data as JSON: /api/errors/5a2eed38e2ba72e8. Report an issue: GitHub.

Appendix: source

Thrown at processing/src/main/java/org/apache/druid/math/expr/BinaryEvalOpExprBase.java:155

    ExpressionType type = ExpressionTypeConversion.autoDetect(leftVal, rightVal);
    switch (type.getType()) {
      case STRING:
        return evalString(leftVal.asString(), rightVal.asString());
      case LONG:
        return ExprEval.of(evalLong(leftVal.asLong(), rightVal.asLong()));
      case DOUBLE:
      default:
        if (leftVal.isNumericNull() || rightVal.isNumericNull()) {
          return ExprEval.ofMissing();
        }
        return ExprEval.of(evalDouble(leftVal.asDouble(), rightVal.asDouble()));
    }
  }

  protected ExprEval evalString(@Nullable String left, @Nullable String right)
  {
    throw new IAE(
        "operator '%s' in expression (%s %s %s) is not supported on type STRING.",
        this.op,
        this.left.stringify(),
        this.op,
        this.right.stringify()
    );
  }

  protected abstract long evalLong(long left, long right);

  protected abstract double evalDouble(double left, double right);
}

@SuppressWarnings("ClassName")
abstract class BinaryBooleanOpExprBase extends BinaryOpExprBase
{
  BinaryBooleanOpExprBase(String op, Expr left, Expr right)
  {

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