presto-main/src/main/java/io/prestosql/operator/PartitionedLookupSourceFactory.java/PartitionedLookupSourceFactory/freePartitions
class PartitionedLookupSourceFactory:    private void freePartitions()
    {
        // Let the HashBuilderOperators reduce their accounted memory
        partitionsNoLongerNeeded.set(null);

        lock.writeLock().lock();
        try {
            // Remove out references to partitions to actually free memory
            Arrays.fill(partitions, null);
            lookupSourceSupplier = null;
            closeCachedLookupSources();
        }
        finally {
            lock.writeLock().unlock();
        }
    }