presto-main/src/main/java/io/prestosql/operator/HashAggregationOperator.java/HashAggregationOperator/closeAggregationBuilder
class HashAggregationOperator:    private void closeAggregationBuilder()
    {
        outputPages = null;
        if (aggregationBuilder != null) {
            aggregationBuilder.recordHashCollisions(hashCollisionsCounter);
            aggregationBuilder.close();
            // aggregationBuilder.close() will release all memory reserved in memory accounting.
            // The reference must be set to null afterwards to avoid unaccounted memory.
            aggregationBuilder = null;
        }
        memoryContext.setBytes(0);
    }