问为什么session.merge和session.bulk_update_mappings比“查询所有->”要慢得多呢？

EN

create table if not exists testdb.table_datetime
(
    id int auto_increment
        primary key,
    col1 varchar(50) null,
    col2 varchar(50) null,
    col3 varchar(50) null,
    col4 varchar(50) null,
    col5 varchar(50) null,
    col6 varchar(50) null,
    local_modified datetime null
)
collate=utf8mb4_bin;

class DemoClass(Base):
    __tablename__ = 'table_datetime'
    local_modified = Column(DATETIME)

    # __tablename__ = 'table_varchar'
    # local_modified = Column(String)
    id = Column(Integer, primary_key=True)
    col1 = Column(String)
    col2 = Column(String)
    col3 = Column(String)
    col4 = Column(String)
    col5 = Column(String)
    col6 = Column(String)

 # Method 1: bulk update'
# Slow, 6~8s for 500 lines update
# with Session() as session:
#     session.bulk_update_mappings(DemoClass, data)
#     session.commit()

# Method 2: query all once, manully update and commit
# Fast, less than 1s
with Session() as session:
    db_rows = session.query(DemoClass).all() # 500 records in all
    for item in data:
        for db_row in db_rows:
            if item['id'] == db_row.id:
                db_row.col1 = item['col1']
                db_row.col2 = item['col2']
                db_row.col3 = item['col3']
                db_row.col4 = item['col4']
                db_row.col5 = item['col5']
                db_row.col6 = item['col6']
                # db_row.local_modified = item['local_modified']
                db_row.local_modified = datetime.strptime(item['local_modified'], '%Y-%m-%d %H:%M:%S')
                break
    session.commit()

# Method 3 construct ORM class and session merge each, then commit
# Slow, 6~8s for 500 lines update
# try:
#     items = [DemoClass(id=d['id'], col1=d['col1'], col2=d['col2'],col3=d['col3'],
#                             col4=d['col4'],col5=d['col5'],col6=d['col6']
#                             ) for d in data]
# except KeyError as e:
#     return {'status_code': 400, 'error_msg': f"update failed, JSON payload missing key: {e}"}, 400
# try:
#     with Session() as session:
#         for m in items:
#             # print(3) # merge is very slow
#             session.merge(m)
#         session.commit()
#        # print(4)
# except exc.SQLAlchemyError as e:
#     return {'status_code': 500, 'error_msg': 'server error, please contact admin'}, 500
# # print(5)

Flask==2.0.1
PyMySQL==1.0.2
MySQL==1.0.2
SQLAlchemy==1.4.25
Python 3.6.5 Windows
MySQL remote server, Server version 5.7.33-0ubuntu0.18.04.1