After tracking down some extreme slowness in loading a one-to-many
relationship (e.g. myobject.foobars), I seem to have isolated the issue to
engine.execute() being much slower with parameterized queries than with
explicit queries. The following is actual code and output for loading
10,971 rows from a database table. (The actual table has more columns than
I'm including here, and is not designed by me.) Note that each explicit
query (where I explicitly set the WHERE clause parameter and pass the
resulting SQL statement to engine.execute()) runs in under 0.1 seconds,
whereas each parameterized query (where I let SQLAlchemy bind the WHERE
clause parameter) takes over 8 seconds.
The difference in runtimes is smaller when the number of rows returned is
smaller, which seems odd since I would have thought that the binding of the
WHERE clause parameters is just done once and would be virtually
instantaneous.
Any thoughts?
Thanks,
Seth
import sqlalchemy as sa
from sqlalchemy.orm import sessionmaker
from sqlalchemy.ext.declarative import declarative_base
import time
engine = sa.create_engine('mssql+pyodbc://Compustat/Compustat')
session = sessionmaker(bind=engine, autoflush=False,
expire_on_commit=False)()
class FooBar(declarative_base()):
__tablename__ = 'sec_dprc'
gvkey = sa.Column(sa.String(6), primary_key=True)
datadate = sa.Column(sa.DateTime, primary_key=True)
value = sa.Column(sa.Float, name='prcod')
def print_timing(func):
def wrapper(*arg):
t1 = time.time()
rows = func(*arg)
t2 = time.time()
print("%30s() len=%d, last=%s, runtime=%0.3fs" % (str(func).split('
at')[0][10:], len(rows), rows[-1], t2 - t1))
return t2 - t1
return wrapper
if __name__ == '__main__':
key = '001045'
query = session.query(FooBar.datadate,
FooBar.value).filter(sa.and_(FooBar.gvkey == key)).order_by(FooBar.datadate)
sql_select_statement_base = str(query)
print(sql_select_statement_base)
@print_timing
def execute_explicit_query():
sql_select_statement_explicit =
sql_select_statement_base.replace(":gvkey_1", "'%s'" % key)
rows =
engine.execute(sa.text(sql_select_statement_explicit)).fetchall()
return rows
@print_timing
def execute_parameterized_query():
rows = engine.execute(sa.text(sql_select_statement_base),
{'gvkey_1':key}).fetchall()
return rows
num_iterations = 5
explicit_runtime = 0.0
parameterized_runtime = 0.0
for i in range(num_iterations):
explicit_runtime += execute_explicit_query()
parameterized_runtime += execute_parameterized_query()
print("Total runtime for %d explicit queries = %0.3fs." %
(num_iterations, explicit_runtime))
print("Total runtime for %d parameterized queries = %0.3fs." %
(num_iterations, parameterized_runtime))
SELECT sec_dprc.datadate AS sec_dprc_datadate, sec_dprc.prcod AS
sec_dprc_prcod
FROM sec_dprc
WHERE sec_dprc.gvkey = :gvkey_1 ORDER BY sec_dprc.datadate
execute_explicit_query() len=10971, last=(datetime.datetime(2014,
5, 9, 0, 0), 37.96), runtime=0.082s
execute_parameterized_query() len=10971, last=(datetime.datetime(2014,
5, 9, 0, 0), 37.96), runtime=8.852s
execute_explicit_query() len=10971, last=(datetime.datetime(2014,
5, 9, 0, 0), 37.96), runtime=0.032s
execute_parameterized_query() len=10971, last=(datetime.datetime(2014,
5, 9, 0, 0), 37.96), runtime=8.754s
execute_explicit_query() len=10971, last=(datetime.datetime(2014,
5, 9, 0, 0), 37.96), runtime=0.039s
execute_parameterized_query() len=10971, last=(datetime.datetime(2014,
5, 9, 0, 0), 37.96), runtime=9.182s
execute_explicit_query() len=10971, last=(datetime.datetime(2014,
5, 9, 0, 0), 37.96), runtime=0.028s
execute_parameterized_query() len=10971, last=(datetime.datetime(2014,
5, 9, 0, 0), 37.96), runtime=9.416s
execute_explicit_query() len=10971, last=(datetime.datetime(2014,
5, 9, 0, 0), 37.96), runtime=0.080s
execute_parameterized_query() len=10971, last=(datetime.datetime(2014,
5, 9, 0, 0), 37.96), runtime=8.425s
Total runtime for 5 explicit queries = 0.260s.
Total runtime for 5 parameterized queries = 44.629s.
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