apache/druid · error · IllegalArgumentException

Unknown format [ ]

Error message

Unknown format [%s]

What it means

Thrown when deserializing a search-query result object from cache: the pullObject map function expects the cached input to be a Map or a SearchHit, and any other type fails with IAE('Unknown format [%s]'). This means the cached payload has an unexpected shape for the search toolchest. It usually signals cache corruption or a cache written by a different Druid version.

Solutions

1. Clear the Druid cache (segment and query caches) after upgrading versions
2. Verify cache configuration (type, key serialization) matches expected search-result formats
3. Reproduce without cache (set useCache/populateCache=false) to confirm the cache is the source

Example fix

// before
query context: {"useCache": true} // with stale cache entries
// after
flush cache, then: {"useCache": true, "populateCache": true} // with regenerated entries

if (!(input instanceof Map) && !(input instanceof SearchHit)) { throw new IllegalStateException("bad cache entry: " + input.getClass()); }

boolean isDecodableSearchResult(Object o) { return o instanceof Map || o instanceof SearchHit; }

try { hit = decoder.apply(input); } catch (IAE e) { log.warn("invalid cached search result, skipping cache", e); fallbackToSegmentScan(); }

When it happens

Trigger: Reading search-query results from cache where the entry is neither a Map nor a SearchHit; a stale/corrupted cache entry or cache written by an incompatible Druid version; cache level misconfiguration delivering raw/non-cached objects.

Common situations: Upgrading Druid without flushing an old segment-result cache; cache-population bugs in custom caching layers; local cache files corrupted on disk.

Related errors

• Can only handle [ ], got [ ]
• Cannot add / to a Spectator Histogram
• Cannot create type [ ]
• Cannot deserialize type
• Failed to retrieve results from cache for query ID

AI-assisted analysis of apache/druid@9b90983fd2 (2026-09-07). Data as JSON: /api/errors/773bf221cbc9433f. Report an issue: GitHub.

Appendix: source

Thrown at processing/src/main/java/org/apache/druid/query/search/SearchQueryQueryToolChest.java:243

                          new Function<Object, SearchHit>()
                          {
                            @Override
                            public SearchHit apply(@Nullable Object input)
                            {
                              String dim;
                              String val;
                              Integer count;
                              if (input instanceof Map) {
                                dim = outputNameMap.get((String) ((Map) input).get("dimension"));
                                val = (String) ((Map) input).get("value");
                                count = (Integer) ((Map) input).get("count");
                              } else if (input instanceof SearchHit) {
                                SearchHit cached = (SearchHit) input;
                                dim = outputNameMap.get(cached.getDimension());
                                val = cached.getValue();
                                count = cached.getCount();
                              } else {
                                throw new IAE("Unknown format [%s]", input.getClass());
                              }
                              return new SearchHit(dim, val, count);
                            }
                          }
                      )
                  )
              );
            } else {
              return new Result<>(
                  DateTimes.utc(((Number) result.get(0)).longValue()),
                  new SearchResultValue(
                      Lists.transform(
                          (List) result.get(1),
                          new Function<Object, SearchHit>()
                          {
                            @Override
                            public SearchHit apply(@Nullable Object input)
                            {

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