apache/druid · error · ExpressionValidationException

second argument must be a base64 encoded STRING value but go

Error message

second argument must be a base64 encoded STRING value but got %s instead

What it means

In complex_decode_base64's eval, the second argument must decode to a base64-encoded STRING (or byte[]) of the declared complex type; if it is already a deserialized value of the matching complex type it is passed through, otherwise this validationFailed is thrown describing the actual type of the second argument.

Source

Thrown at processing/src/main/java/org/apache/druid/math/expr/BuiltInExprMacros.java:112

      @Override
      public ExprEval<?> eval(ObjectBinding bindings)
      {
        ExprEval<?> toDecode = args.get(1).eval(bindings);
        if (toDecode.value() == null) {
          return ExprEval.ofComplex(complexType, null);
        }
        final Object serializedValue = toDecode.value();
        final byte[] base64;
        if (serializedValue instanceof String) {
          base64 = StringUtils.decodeBase64String(toDecode.asString());
        } else if (serializedValue instanceof byte[]) {
          base64 = (byte[]) serializedValue;
        } else if (complexType.getComplexTypeName().equals(toDecode.type().getComplexTypeName())) {
          // pass it through, it is already the right thing
          return toDecode;
        } else {
          throw validationFailed(
              "second argument must be a base64 encoded STRING value but got %s instead",
              toDecode.type()
          );
        }

        return ExprEval.ofComplex(complexType, typeStrategy.fromBytes(base64));
      }

      @Nullable
      @Override
      public ExpressionType getOutputType(InputBindingInspector inspector)
      {
        return complexType;
      }

      @Override
      public boolean isLiteral()
      {

View on GitHub (pinned to 9b90983fd2)

Solutions

  1. Pass the column that holds base64-encoded STRING data.
  2. Ensure the declared complex type (arg 1) matches the type the column was serialized with.
  3. Cast the second argument to VARCHAR if it is numeric: complex_decode_base64('type', CAST(col AS VARCHAR)).
  4. Inspect the column's actual type via segment metadata or EXPLAIN.

Example fix

// before
complex_decode_base64('druid.metrics.snapshot', numeric_col)
// after
complex_decode_base64('druid.metrics.snapshot', base64_string_col)
Defensive patterns

Strategy: validation

When it happens

Trigger: Second argument evaluates to a non-string/non-byte[] value of a different type — e.g. a long, double, array, or a complex value of a mismatched complex type name.

Common situations: Pointing the function at the wrong column (raw long/JSON column instead of base64 string); decoding a column serialized with a different complex type than declared in arg 1.

Understand the failure class

Background: "Invalid ... format", "must be in format X", "does not look like a ..." — invalid argument format errors across CLI tools and libraries — this error's family across 17 libraries.

Related errors

AI-assisted analysis of apache/druid@9b90983fd2 (2026-09-07). Data as JSON: /api/errors/45428cd18fbe6b18. Report an issue: GitHub.