apache/druid · error · ExpressionValidationException

argument should be an identifier expression. Use array()…

Error message

argument %s should be an identifier expression. Use array() instead

What it means

Thrown during argument validation when a Druid expression function requires its first (only) argument to be an identifier expression — i.e. a bare column reference — but receives an arbitrary expression. The message hints that if an array value was intended, the array() constructor should be used instead.

Solutions

1. Pass a bare column identifier as the first argument, e.g. f(myColumn).
2. If you meant to supply an array value, use the array() constructor: f(array(1, 2, 3)) as appropriate for the function.
3. Materialize the intermediate expression into a new column (e.g. via a subquery/ingestion expression) and reference that column.
4. Check the function's documentation to confirm it operates on identifiers (e.g. for special planning/index handling).

Example fix

// before
some_fn(CONCAT(col, '_x'))
// after
some_fn(col)

// only pass bare column references
assert (!(arg instanceof Expr)) || arg instanceof Column; // API users: pass "col", not expressions

// ensure first arg is a plain identifier, not a call or literal
if (arg.startsWith("(") || arg.contains("(")) reject();

When it happens

Trigger: Calling the function with a computed expression, literal, or nested function call as the first argument, e.g. f(COALESCE(col, 'x')) or f('literal'), instead of a plain column name like f(col).

Common situations: Rewriting queries where the original wrapped the column in a function; users trying to pass an inline array literal with legacy syntax instead of array(1,2,3); query builders that auto-wrap columns.

Understand the failure class

Background: "must be a positive integer", "cannot be empty", "invalid argument": how invalid-argument errors work across open-source libraries — this error's family across 33 libraries.

Related errors

• needs a positive integer as the second argument
• needs an integer as the second argument
• Cannot cast [ ] to [ ] (Types.InvalidCastException from…
• Cannot find strategy for type
• Cannot implicitly cast

AI-assisted analysis of apache/druid@9b90983fd2 (2026-09-07). Data as JSON: /api/errors/7f781880523941dc. Report an issue: GitHub.

Appendix: source

Thrown at processing/src/main/java/org/apache/druid/math/expr/Function.java:3318

    public String name()
    {
      return "mv_to_array";
    }

    @Override
    public ExprEval apply(List<Expr> args, Expr.ObjectBinding bindings)
    {
      return args.get(0).eval(bindings).castTo(ExpressionType.STRING_ARRAY);
    }

    @Override
    public void validateArguments(List<Expr> args)
    {
      validationHelperCheckArgumentCount(args, 1);
      IdentifierExpr expr = args.get(0).getIdentifierExprIfIdentifierExpr();

      if (expr == null) {
        throw validationFailed(
            "argument %s should be an identifier expression. Use array() instead",
            args.get(0).toString()
        );
      }
    }

    @Nullable
    @Override
    public ExpressionType getOutputType(Expr.InputBindingInspector inspector, List<Expr> args)
    {
      return ExpressionType.STRING_ARRAY;
    }

    @Override
    public boolean hasArrayInputs()
    {
      return true;
    }

View on GitHub (pinned to 9b90983fd2)