presto-main/src/main/java/io/prestosql/sql/planner/optimizations/HashGenerationOptimizer.java/HashComputation/getHashFunctionCall
class HashComputation:        private Expression getHashFunctionCall(Expression previousHashValue, Symbol symbol, Metadata metadata, TypeProvider types)
        {
            FunctionCall functionCall = new FunctionCallBuilder(metadata)
                    .setName(QualifiedName.of(HASH_CODE))
                    .addArgument(types.get(symbol), symbol.toSymbolReference())
                    .build();

            return new FunctionCallBuilder(metadata)
                    .setName(QualifiedName.of("combine_hash"))
                    .addArgument(BIGINT, previousHashValue)
                    .addArgument(BIGINT, orNullHashCode(functionCall))
                    .build();
        }