Release: 2.0.48 | Release Date: March 2, 2026

# SQLAlchemy 2.0 Documentation

# Source code for examples.performance.large_resultsets

    """In this series of tests, we are looking at time to load a large number
    of very small and simple rows.

    A special test here illustrates the difference between fetching the
    rows from the raw DBAPI and throwing them away, vs. assembling each
    row into a completely basic Python object and appending to a list. The
    time spent typically more than doubles.  The point is that while
    DBAPIs will give you raw rows very fast if they are written in C, the
    moment you do anything with those rows, even something trivial,
    overhead grows extremely fast in cPython. SQLAlchemy's Core and
    lighter-weight ORM options add absolutely minimal overhead, and the
    full blown ORM doesn't do terribly either even though mapped objects
    provide a huge amount of functionality.

    """

    from sqlalchemy import Column
    from sqlalchemy import create_engine
    from sqlalchemy import Identity
    from sqlalchemy import Integer
    from sqlalchemy import String
    from sqlalchemy.ext.declarative import declarative_base
    from sqlalchemy.orm import Bundle
    from sqlalchemy.orm import Session
    from . import Profiler

    Base = declarative_base()
    engine = None

    class Customer(Base):
        __tablename__ = "customer"
        id = Column(Integer, Identity(), primary_key=True)
        name = Column(String(255))
        description = Column(String(255))

    Profiler.init("large_resultsets", num=500000)

    @Profiler.setup_once
    def setup_database(dburl, echo, num):
        global engine
        engine = create_engine(dburl, echo=echo)
        Base.metadata.drop_all(engine)
        Base.metadata.create_all(engine)

        s = Session(engine)
        for chunk in range(0, num, 10000):
            s.execute(
                Customer.__table__.insert(),
                params=[
                    {
                        "name": "customer name %d" % i,
                        "description": "customer description %d" % i,
                    }
                    for i in range(chunk, chunk + 10000)
                ],
            )
        s.commit()

    @Profiler.profile
    def test_orm_full_objects_list(n):
        """Load fully tracked ORM objects into one big list()."""

        sess = Session(engine)
        list(sess.query(Customer).limit(n))

    @Profiler.profile
    def test_orm_full_objects_chunks(n):
        """Load fully tracked ORM objects a chunk at a time using yield_per()."""

        sess = Session(engine)
        for obj in sess.query(Customer).yield_per(1000).limit(n):
            pass

    @Profiler.profile
    def test_orm_bundles(n):
        """Load lightweight "bundle" objects using the ORM."""

        sess = Session(engine)
        bundle = Bundle(
            "customer", Customer.id, Customer.name, Customer.description
        )
        for row in sess.query(bundle).yield_per(10000).limit(n):
            pass

    @Profiler.profile
    def test_orm_columns(n):
        """Load individual columns into named tuples using the ORM."""

        sess = Session(engine)
        for row in (
            sess.query(Customer.id, Customer.name, Customer.description)
            .yield_per(10000)
            .limit(n)
        ):
            pass

    @Profiler.profile
    def test_core_fetchall(n):
        """Load Core result rows using fetchall."""

        with engine.connect() as conn:
            result = conn.execute(Customer.__table__.select().limit(n)).fetchall()
            for row in result:
                row.id, row.name, row.description

    @Profiler.profile
    def test_core_fetchall_mapping(n):
        """Load Core result rows using fetchall."""

        with engine.connect() as conn:
            result = (
                conn.execute(Customer.__table__.select().limit(n))
                .mappings()
                .fetchall()
            )
            for row in result:
                row["id"], row["name"], row["description"]

    @Profiler.profile
    def test_core_fetchmany_w_streaming(n):
        """Load Core result rows using fetchmany/streaming."""

        with engine.connect() as conn:
            result = conn.execution_options(stream_results=True).execute(
                Customer.__table__.select().limit(n)
            )
            while True:
                chunk = result.fetchmany(10000)
                if not chunk:
                    break
                for row in chunk:
                    row.id, row.name, row.description

    @Profiler.profile
    def test_core_fetchmany(n):
        """Load Core result rows using Core / fetchmany."""

        with engine.connect() as conn:
            result = conn.execute(Customer.__table__.select().limit(n))
            while True:
                chunk = result.fetchmany(10000)
                if not chunk:
                    break
                for row in chunk:
                    row.id, row.name, row.description

    @Profiler.profile
    def test_dbapi_fetchall_plus_append_objects(n):
        """Load rows using DBAPI fetchall(), generate an object for each row."""

        _test_dbapi_raw(n, True)

    @Profiler.profile
    def test_dbapi_fetchall_no_object(n):
        """Load rows using DBAPI fetchall(), don't make any objects."""

        _test_dbapi_raw(n, False)

    def _test_dbapi_raw(n, make_objects):
        compiled = (
            Customer.__table__.select()
            .limit(n)
            .compile(
                dialect=engine.dialect, compile_kwargs={"literal_binds": True}
            )
        )

        if make_objects:
            # because if you're going to roll your own, you're probably
            # going to do this, so see how this pushes you right back into
            # ORM land anyway :)
            class SimpleCustomer:
                def __init__(self, id_, name, description):
                    self.id_ = id_
                    self.name = name
                    self.description = description

        sql = str(compiled)

        conn = engine.raw_connection()
        cursor = conn.cursor()
        cursor.execute(sql)

        if make_objects:
            for row in cursor.fetchall():
                # ensure that we fully fetch!
                SimpleCustomer(id_=row[0], name=row[1], description=row[2])
        else:
            for row in cursor.fetchall():
                # ensure that we fully fetch!
                row[0], row[1], row[2]

        conn.close()

    if __name__ == "__main__":
        Profiler.main()