apache/druid · error · UnsupportedOperationException

Cannot convert object to float

Error message

Cannot convert object to float

What it means

Runtime type guard in the unnest cursor's column value selector: when the unnested cell value is neither null nor a Number (e.g. it is a String or other object), the selector cannot fulfill a primitive-float read and raises this error. It fires when a query reads an unnested column as FLOAT while the actual unnested values are non-numeric; fix by unnesting/coercing values to numbers or selecting the column with a compatible type (string/object).

Source

Thrown at processing/src/main/java/org/apache/druid/segment/UnnestColumnValueSelectorCursor.java:159

              return 0;
            }
            if (value instanceof Number) {
              return ((Number) value).doubleValue();
            }
            throw new UOE("Cannot convert object to double");
          }

          @Override
          public float getFloat()
          {
            Object value = getObject();
            if (value == null) {
              return 0;
            }
            if (value instanceof Number) {
              return ((Number) value).floatValue();
            }
            throw new UOE("Cannot convert object to float");
          }

          @Override
          public long getLong()
          {
            Object value = getObject();
            if (value == null) {
              return 0;
            }
            if (value instanceof Number) {
              return ((Number) value).longValue();
            }
            throw new UOE("Cannot convert object to long");
          }

          @Override
          public void inspectRuntimeShape(RuntimeShapeInspector inspector)
          {

View on GitHub (pinned to 9b90983fd2)

Solutions

1. Coerce column values to numeric before unnesting
2. Verify segment schema with segment metadata query and fix the column type at ingestion
3. Change the query to use string handling (e.g. CAST in SQL) instead of float selectors

Object v = selector.getObject();
if (!(v instanceof Number) && v != null) { /* route to string/cast handling */ }

static boolean isFloatSafe(Object o) { return o == null || o instanceof Number; }

try { float f = selector.getFloat(); } catch (UOE e) { /* fallback: parse Object as float */ }

When it happens

Trigger: getFloat() called on an UnnestColumnValueSelectorCursor whose underlying getObject() returns a non-Number object (string, map, list, complex).

Common situations: FLOAT aggregations over an unnested column that actually holds strings or objects; schema drift where a segment was written with a different type than the query assumes.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

• Cannot convert object to double
• Cannot convert object to long
• Cannot deserialize type[%s] to an RoaringBitmap64Counter:
• Expected a number or an instance of MergingDigest, but recei
• Object is not of a type that can be deserialized to a quanti

AI-assisted analysis of apache/druid@9b90983fd2 (2026-09-07). Data as JSON: /api/errors/084ca707828f8533. Report an issue: GitHub.