apache/druid · error · UnsupportedOperationException

Casting to float type is not supported

Error message

Casting to float type is not supported

What it means

The tdigestsketch aggregator's result is a TDigestSketch (a distribution), which cannot be meaningfully cast to a single float. getFloat() therefore always throws UnsupportedOperationException, indicating the requested SQL/output type is incompatible with the aggregation.

Source

Thrown at extensions-contrib/tdigestsketch/src/main/java/org/apache/druid/query/aggregation/tdigestsketch/TDigestSketchAggregator.java:91

      throw new IAE(
          "Expected a number or an instance of MergingDigest, but received [%s] of type [%s]",
          obj,
          obj.getClass()
      );
    }
  }

  @Nullable
  @Override
  public synchronized Object get()
  {
    return histogram;
  }

  @Override
  public float getFloat()
  {
    throw new UnsupportedOperationException("Casting to float type is not supported");
  }

  @Override
  public long getLong()
  {
    throw new UnsupportedOperationException("Casting to long type is not supported");
  }

  @Override
  public synchronized void close()
  {
    histogram = null;
  }
}

View on GitHub (pinned to 9b90983fd2)

Solutions

1. Extract a scalar via TDigestSketchQuantilePostAggregator, or min/max post-aggregators instead of casting the sketch.
2. Remove the cast to FLOAT on the sketch column.
3. If a number is wanted, aggregate the underlying numeric column directly, not the sketch.

Example fix

// SQL before
SELECT CAST(sketch_col AS FLOAT) FROM t
// after
SELECT TDIGEST_QUANTILE(sketch_col, 0.5) FROM t

if (factory instanceof TDigestSketchAggregatorFactory && outputType.equals("FLOAT")) { throw new IllegalArgumentException("tdigestsketch cannot be cast to FLOAT; use quantile/min/max post-aggregators"); }

try { result = agg.getFloat(); } catch (UnsupportedOperationException e) { result = (float) quantilePostAggregator.compute(agg.getObject()); }

When it happens

Trigger: Running a query that requests FLOAT output for a tdigestsketch aggregate, e.g. SQL casting the sketch column to FLOAT, or native query with a float post-aggregator / float output type.

Common situations: SQL like SELECT CAST(tdigest_col AS FLOAT) ...; using the sketch in a query expecting a scalar numeric result instead of going through sketch accessor post-aggregators (quantile, min, max).

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

• Casting to long type is not supported
• Expected a number or an instance of MergingDigest, but recei
• AggregatorFactoryNotMergeableException
• Cannot deserialize type[%s] to an RoaringBitmap64Counter:
• Index[%d] >= size[%d]

AI-assisted analysis of apache/druid@9b90983fd2 (2026-09-07). Data as JSON: /api/errors/5ce3c161590bd042. Report an issue: GitHub.