apache/druid · error · IllegalArgumentException

Unknown type [ ]

Error message

Unknown type [%s]

What it means

Thrown by the @JsonCreator ColumnTypeMergePolicy.fromString in AbstractSegmentMetadataCache when a configured column type merge policy string is not one of the supported names ('leastRestrictive' or 'first'). The merge policy controls how column types are unified across segments when computing a table's RowSignature.

Solutions

1. Set the merge policy value to exactly 'leastRestrictive' or 'first' (case-insensitive).
2. Check the Druid version's supported policies; names may differ across versions.
3. Search runtime properties/JSON config for the merge policy key and correct the typo.
4. Restart the broker/historical after fixing the config.

Example fix

// before
druid.segment.metadata.columnNameTyping=least-restrictive
// after
druid.segment.metadata.columnNameTyping=leastRestrictive

boolean valid = "leastRestrictive".equalsIgnoreCase(v) || "first".equalsIgnoreCase(v);
if (!valid) throw new IllegalArgumentException("merge policy must be leastRestrictive or first: " + v);

try { startMetadataCache(config); } catch (IAE e) { if (e.getMessage().startsWith("Unknown type")) { log.error("Fix druid.segment.metadata merge policy config"); } }

When it happens

Trigger: Deserializing a ColumnTypeMergePolicy from config (e.g. druid.segment.metadata columnNameTyping or equivalent metadata cache config) with a value other than 'leastRestrictive' or 'first' (case-insensitive).

Common situations: Typo in the config value (e.g. 'least-restrictive', 'firsttype'); using a policy name from a different Druid version; YAML/JSON config quoting mistakes; copy-pasted config from a fork.

Understand the failure class

Background: Invalid enum value errors: "Unknown type", "Invalid scope", "must be one of" — when a string is not on the library's allowed list — this error's family across 23 libraries.

Related errors

• Invalid druid.storage.transfer.asyncHttpClientType
• Invalid value[ ] for[ ]
• Null worker assignment strategy
• Ambiguous directions
• At least one of baseDir or files should be specified

AI-assisted analysis of apache/druid@9b90983fd2 (2026-09-07). Data as JSON: /api/errors/0957877836a18c7d. Report an issue: GitHub.

Appendix: source

Thrown at server/src/main/java/org/apache/druid/segment/metadata/AbstractSegmentMetadataCache.java:1126

   * for the same column from segment to segment. It is used to help compute a {@link RowSignature} for a table in
   * Druid based on the segment metadata of all segments, merging the types of each column encountered to end up with
   * a single type to represent it globally.
   */
  @FunctionalInterface
  public interface ColumnTypeMergePolicy
  {
    ColumnType merge(ColumnType existingType, ColumnType newType);

    @JsonCreator
    static ColumnTypeMergePolicy fromString(String type)
    {
      if (LeastRestrictiveTypeMergePolicy.NAME.equalsIgnoreCase(type)) {
        return LeastRestrictiveTypeMergePolicy.INSTANCE;
      }
      if (FirstTypeMergePolicy.NAME.equalsIgnoreCase(type)) {
        return FirstTypeMergePolicy.INSTANCE;
      }
      throw new IAE("Unknown type [%s]", type);
    }
  }

  /**
   * Classic logic, we use the first type we encounter. This policy is effectively 'newest first' because we iterated
   * segments starting from the most recent time chunk, so this typically results in the most recently used type being
   * chosen, at least for systems that are continuously updated with 'current' data.
   *
   * Since {@link ColumnTypeMergePolicy} are used to compute the SQL schema, at least in systems using SQL schemas which
   * are partially or fully computed by this cache, this merge policy can result in query time errors if incompatible
   * types are mixed if the chosen type is more restrictive than the types of some segments. If data is likely to vary
   * in type across segments, consider using {@link LeastRestrictiveTypeMergePolicy} instead.
   */
  public static class FirstTypeMergePolicy implements ColumnTypeMergePolicy
  {
    public static final String NAME = "latestInterval";
    private static final FirstTypeMergePolicy INSTANCE = new FirstTypeMergePolicy();

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