apache/druid · error · IllegalStateException

Transform name ' ' cannot be used twice

Error message

Transform name '%s' cannot be used twice

What it means

TransformSpec's constructor validates that all configured Transform objects have unique names, since transformed columns would otherwise collide ambiguously. If two transforms share a name, this ISE is thrown at spec construction — typically when deserializing an ingestion spec's transformSpec or a query's transformSpec from JSON.

Solutions

1. Rename one of the duplicate transforms so each name in the transforms array is unique.
2. If both transforms compute the same thing, delete the redundant entry and keep one.
3. If you need both intermediate and final values, give the second transform a distinct name and reference the first by column name in its expression.
4. Validate the spec JSON before submission (e.g. assert distinct transform names in your tooling).

Example fix

// before
"transforms": [
  {"name": "ts", "expression": "timestamp_parse(raw_ts)"},
  {"name": "ts", "expression": "format(raw_ts)"}
]
// after
"transforms": [
  {"name": "ts", "expression": "timestamp_parse(raw_ts)"},
  {"name": "ts_formatted", "expression": "format(raw_ts)"}
]

Set<String> names = new HashSet<>();
for (Transform t : transformSpec.getTransforms()) {
    if (!names.add(t.getName())) {
        throw new IllegalArgumentException("Duplicate transform name: " + t.getName());
    }
}

When it happens

Trigger: Building/submitting an ingestion task or native query whose transformSpec.transforms list contains two Transform entries with the same name.

Common situations: Copy-pasting a transform block in the ingestion spec and forgetting to rename it; programmatically generating transforms where a loop appends duplicate names; merging specs from templates that both define the same transform name.

Understand the failure class

Background: Schema validation failed / invalid input schema: payload rejected because its shape doesn't match the expected schema — this error's family across 28 libraries.

Related errors

• Cannot simultaneously replace and append to existing…
• Could not transform dimension value for
• Could not transform value for
• Attempt to add row to swapped-out sink for segment
• Bloom filter aggregators are query-time only

AI-assisted analysis of apache/druid@9b90983fd2 (2026-09-07). Data as JSON: /api/errors/c6a5c32dd714f875. Report an issue: GitHub.

Appendix: source

Thrown at processing/src/main/java/org/apache/druid/segment/transform/TransformSpec.java:62

  public static final TransformSpec NONE = new TransformSpec(null, null);

  private final DimFilter filter;
  private final List<Transform> transforms;

  @JsonCreator
  public TransformSpec(
      @JsonProperty("filter") final DimFilter filter,
      @JsonProperty("transforms") final List<Transform> transforms
  )
  {
    this.filter = filter;
    this.transforms = transforms == null ? ImmutableList.of() : transforms;

    // Check for name collisions.
    final Set<String> seen = new HashSet<>();
    for (Transform transform : this.transforms) {
      if (!seen.add(transform.getName())) {
        throw new ISE("Transform name '%s' cannot be used twice", transform.getName());
      }
    }
  }

  @JsonProperty
  @Nullable
  public DimFilter getFilter()
  {
    return filter;
  }

  @JsonProperty
  public List<Transform> getTransforms()
  {
    return transforms;
  }

  public InputSourceReader decorate(InputSourceReader reader)

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