Function update_hash 

pub fn update_hash(
    on: &[PhysicalExprRef],
    batch: &RecordBatch,
    hash_map: &mut dyn JoinHashMapType,
    offset: usize,
    random_state: &RandomState,
    hashes_buffer: &mut Vec<u64>,
    deleted_offset: usize,
    fifo_hashmap: bool,
) -> Result<()>

Updates hash_map with new entries from batch evaluated against the expressions on using offset as a start value for batch row indices.

fifo_hashmap sets the order of iteration over batch rows while updating hashmap, which allows to keep either first (if set to true) or last (if set to false) row index as a chain head for rows with equal hash values.