@Override
    protected void onMerge(AggregationResult result) {
        ExpressionCountAggregationResult other = (ExpressionCountAggregationResult) result;
        sketch = sketchMerger.merge(sketch, other.sketch);
        // Any cached result should be invalidated.
        estimatedUniqueCount = UNDEFINED;
    }
@return The unique count estimated by the HyperLogLog algorithm.
searchlib/src/main/java/com/yahoo/searchlib/aggregation/ExpressionCountAggregationResult.java