apache/druid · error · RE

Failed to parse array: element

Error message

Failed to parse array: element %s is not a string

What it means

Raised in ExprListenerImpl.exitStringArray when an element of a STRING array literal is not a recognized STRING token (and not NULL), so the listener cannot build the VARCHAR array's value. The array literal grammar expected string elements but found some other token type, meaning malformed array syntax or wrong element types in the expression.

Solutions

1. Ensure every element of the array literal is a quoted string or NULL, e.g. ARRAY['a', 'b', NULL].
2. Quote numeric or identifier elements if a string array is intended.
3. Fix unbalanced quotes or stray tokens that break the string-element parse.

Example fix

// before
ARRAY<STRING>[foo, 123]
// after
ARRAY<STRING>['foo', '123']

// Java: ensure every element is a quoted string or NULL
static boolean isStringArrayLiteral(java.util.List<String> elems) {
  return elems.stream().allMatch(e -> e.equals("NULL")
      || (e.startsWith("'") && e.endsWith("'") && e.length() >= 2));
}

When it happens

Trigger: Parsing ARRAY<STRING>[1, 2] or ARRAY<STRING>[abc] — elements that are not quoted string literals in the string-array grammar path.

Common situations: Unquoted identifiers or numbers in string arrays; generated expressions that skip quoting; copying JSON arrays (unquoted numbers) into expression string arrays.

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.

• Parsing and encoding errors: unexpected token, malformed input — why parsers reject input and how to find the real culprit.

Related errors

• Failed to parse array element
• Failed to convert array type
• Failed to parse array element
• Failed to parse array element
• Failed to parse array element

AI-assisted analysis of apache/druid@9b90983fd2 (2026-09-07). Data as JSON: /api/errors/d92f7c8417db3197. Report an issue: GitHub.

Appendix: source

Thrown at processing/src/main/java/org/apache/druid/math/expr/ExprListenerImpl.java:491

        values[i] = Numbers.parseLongObject(ctx.numericElement(i).DOUBLE().getText());
      } else {
        throw new RE("Failed to parse array element %s as a long", ctx.numericElement(i).getText());
      }
    }
    nodes.put(ctx, new ArrayExpr(ExpressionType.LONG_ARRAY, values));
  }

  @Override
  public void exitStringArray(ExprParser.StringArrayContext ctx)
  {
    Object[] values = new Object[ctx.stringElement().size()];
    for (int i = 0; i < values.length; i++) {
      if (ctx.stringElement(i).NULL() != null) {
        values[i] = null;
      } else if (ctx.stringElement(i).STRING() != null) {
        values[i] = escapeStringLiteral(ctx.stringElement(i).STRING().getText());
      } else {
        throw new RE("Failed to parse array: element %s is not a string", ctx.stringElement(i).getText());
      }
    }
    nodes.put(ctx, new ArrayExpr(ExpressionType.STRING_ARRAY, values));
  }

  @Override
  public void exitExplicitStringArray(ExprParser.ExplicitStringArrayContext ctx)
  {
    Object[] values = new Object[ctx.literalElement().size()];
    for (int i = 0; i < values.length; i++) {
      if (ctx.literalElement(i).NULL() != null) {
        values[i] = null;
      } else if (ctx.literalElement(i).STRING() != null) {
        values[i] = escapeStringLiteral(ctx.literalElement(i).STRING().getText());
      } else if (ctx.literalElement(i).DOUBLE() != null) {
        values[i] = ctx.literalElement(i).DOUBLE().getText();
      } else if (ctx.literalElement(i).LONG() != null) {
        values[i] = ctx.literalElement(i).LONG().getText();

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