apache/druid · error · UnsupportedOperationException

Cannot convert object to double

Error message

Cannot convert object to double

What it means

During UNNEST processing, the UnnestColumnValueSelectorCursor's primitive double accessor encounters a non-numeric, non-null object from the underlying column and throws UOE. The cursor only knows how to coerce Numbers (and null to 0); arbitrary objects (e.g. strings, complex objects) have no defined double conversion here.

Solutions

1. Ensure the unnest input column contains numeric values (fix ingestion typing or cast values upstream)
2. Cast/coerce values to numbers before unnesting (e.g. try_cast expressions, JSON value extraction with a numeric type)
3. Check the column's reported ValueType in the segment metadata; reingest or re-index with the correct type

Example fix

// before
// unnest on column with string values, then SUM over it
// after
// add a virtual column: CAST(JSON_VALUE(col, '$.v') AS DOUBLE) and unnest/aggregate that

Object v = valueSelector.getObject();
if (v instanceof Number) { double d = ((Number) v).doubleValue(); }

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

try { double d = selector.getDouble(); } catch (UOE e) { /* treat as null/coerce via string parse */ }

When it happens

Trigger: Unnesting a column whose values are objects that are neither null nor Number, then reading doubles via getDouble() (e.g. a numeric-typed query expecting double output over a column containing strings or complex values).

Common situations: Unnest input column mis-typed in schema (string values where numbers expected); using UNNEST on a nested-data/array column whose element objects are not numeric; expression output typed as complex fed into numeric aggregation.

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 float
• Cannot convert object to long
• attempt to get boolean[] null vector from string[] only…
• attempt to get double[] from string[] only scalar binding
• attempt to get long[] from string[] only scalar binding

AI-assisted analysis of apache/druid@9b90983fd2 (2026-09-07). Data as JSON: /api/errors/8049ed0bab151ab7. Report an issue: GitHub.

Appendix: source

Thrown at processing/src/main/java/org/apache/druid/segment/UnnestColumnValueSelectorCursor.java:146

      @Override
      public ColumnValueSelector makeColumnValueSelector(String columnName)
      {
        if (!outputName.equals(columnName)) {
          return baseColumnSelectorFactory.makeColumnValueSelector(columnName);
        }
        return new ColumnValueSelector()
        {
          @Override
          public double getDouble()
          {
            Object value = getObject();
            if (value == null) {
              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()
          {

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