What causes "must have at least two arguments" in apache/druid?

Add the missing second argument to the expression call so it has at least two

apache/druid · error · DruidException

must have at least two arguments

Error message

must have at least two arguments

What it means

Druid's PARSE_JSON-like nested-data expression macro requires at least two arguments when applied. The macro's apply() validates arity up front and throws a validation failure (ExpressionValidationException) if fewer than two argument expressions are supplied, because the function cannot be meaningfully evaluated without them.

Source

Thrown at processing/src/main/java/org/apache/druid/query/expression/NestedDataExpressions.java:129

    @Inject
    public JsonMergeExprMacro(
        @Json ObjectMapper jsonMapper
    )
    {
      this.jsonMapper = jsonMapper;
    }

    @Override
    public String name()
    {
      return NAME;
    }

    @Override
    public Expr apply(List<Expr> args)
    {
      if (args.size() < 2) {
        throw validationFailed("must have at least two arguments");
      }

      final class ParseJsonExpr extends ExprMacroTable.BaseScalarMacroFunctionExpr
      {
        public ParseJsonExpr(List<Expr> args)
        {
          super(JsonMergeExprMacro.this, args);
        }

        @Override
        public ExprEval<?> eval(ObjectBinding bindings)
        {
          final Object retVal = getArgAsObject(args.get(0).eval(bindings), true);

          if (retVal == null) {
            // If any argument is null, the return of the entire function is null.
            return ExprEval.ofComplex(ExpressionType.NESTED_DATA, null);
          }

View on GitHub (pinned to 9b90983fd2)

Solutions

Add the missing second argument to the expression call so it has at least two
Check the macro's documented signature in NestedDataExpressions and match argument count
If building args programmatically, assert args.size() >= 2 before calling Expr.apply

Example fix

// before
Expr expr = macro.apply(Collections.singletonList(jsonColumnExpr));
// after
Expr expr = macro.apply(Arrays.asList(jsonColumnExpr, pathExpr));

Defensive patterns

Strategy: validation

Validate before calling

if (args == null || args.size() < 2) {
  throw new IllegalArgumentException("expression macro requires at least two arguments");
}

Try / catch

try {
  Expr expr = macro.apply(args);
} catch (ExpressionValidationException e) {
  log.error("Invalid expression arity: %s", e.getMessage());
}

Prevention

Count arguments against the macro signature before calling apply
Validate expression strings with Parser.parse before submission
Cover macro arity in unit tests

When it happens

Trigger: Calling the expression macro (e.g. via JSON_QUERY/JSON_VALUE style native expressions or the ExprMacroTable) with 0 or 1 arguments, e.g. writing PARSE_JSON(x) when the macro's signature requires two, or an SQL planner lowering a function call that dropped an argument.

Common situations: Hand-written native JSON expressions with missing arguments; copy-pasted expression strings from older Druid versions where the macro accepted fewer args; programmatically building Expr argument lists and forgetting an element.

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

AI-assisted analysis of apache/druid@9b90983fd2 (2026-09-07). Data as JSON: /api/errors/b738c4f44b4180fe. Report an issue: GitHub.