presto-main/src/main/java/io/prestosql/operator/HashBuilderOperator.java/HashBuilderOperator/buildLookupSource
class HashBuilderOperator:    private LookupSourceSupplier buildLookupSource()
    {
        LookupSourceSupplier partition = index.createLookupSourceSupplier(operatorContext.getSession(), hashChannels, preComputedHashChannel, filterFunctionFactory, sortChannel, searchFunctionFactories, Optional.of(outputChannels));
        hashCollisionsCounter.recordHashCollision(partition.getHashCollisions(), partition.getExpectedHashCollisions());
        checkState(lookupSourceSupplier == null, "lookupSourceSupplier is already set");
        this.lookupSourceSupplier = partition;
        return partition;
    }