SQLAlchemy 偶尔会错误地 returns 一个空结果

Question

SQLAlchemy v1.0.6 
cx_Oracle v5.2

一段时间以来，我们的生产代码一直存在问题，最终将问题缩小到从 SQLAlchemy 返回的数据。

运行多次相同的查询有时会 return 一个空结果。在某些情况下，我们可以在每次执行代码时得到一个空结果return。尽管事实上数据库中的数据根本没有改变，而且同一查询运行的纯 SQL 版本直接在 cx_Oracle 上总是 return 是正确的结果。

这是 SQLAlchemy 的声明代码：

class Database:
    def __init__(self, service_name, database, username, password):
        """
        service_name (str): The service name as defined in tnsnames.ora.
        database (str): The database within the chosen service.
        """
        self.engine = create_engine(
            r'oracle+cx_oracle://{username}:{password}@{service_name}'.format(username=username, password=password,
                                                                              service_name=service_name),
            case_sensitive=False)
        self.session_maker = sessionmaker(bind=self.engine, autoflush=False, autocommit=False)

        # Database name must be injected into every table definition; this is why tables must be procedurally generated.
        self.Base = declarative_base()  # base class for all database tables
        self.build_tables(database)

    def make_session(self):
        """Create a read-only session for the database."""
        def readonly_abort():
            raise Exception('writing is prohibited; db is read-only')

        session = self.session_maker()
        session.flush = readonly_abort
        return session

    def build_tables(self, database):
        class Lot(self.Base):
            __tablename__ = 'lot'
            __table_args__ = {'schema': database}
            lot_key = Column(Integer, primary_key=True)
            lot_id = Column(String, name='lot_id')

        self.lot = Lot

这里是测试代码：

def sqlalchemy_test():
    db = dp_orm.Database(service_name, database)
    session = db.make_session()
    cursor = session.query(db.lot)
    results = cursor.first()
    if results is None:
        raise Exception

def cx_oracle_test():
    import cx_Oracle
    import set_environment_variables
    conn = cx_Oracle.Connection(username, password, service_name)
    cursor = conn.cursor()

    c = cursor.execute('SELECT * FROM {}.lot WHERE rownum <= 1'.format(database))
    results = list(c)
    if len(results) != 1:
        raise Exception

第一个函数 sqlalchemy_test 大约有 50% 的时间会出错。第二个函数cx_oracle_test还没有出错。现在有趣的是：如果我们在 cursor = session.query(db.lot) 和 results = cursor.first() 之间引入几秒钟的停顿，问题就会消失。所以它看起来像是某种时间问题。

知道这里发生了什么吗？

编辑：我已经简化了创建错误所需的代码。在这里：

from sqlalchemy import create_engine, Column, Integer, String
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker

# Fix environment variables
import os
try:
    del os.environ['ORACLE_HOME']
except KeyError:
    pass
os.environ['TNS_ADMIN'] = r'C:\product.1.0\client_1\network\admin'
os.environ['PATH'] = r'C:\product.1.0\client_1\BIN;' + os.environ['PATH']

engine = create_engine(r'oracle+cx_oracle://{username}:{password}@{service_name}'.format(username='USER', password='PASSWORD', service_name='SERVICE'))
session_maker = sessionmaker(bind=engine)
base_class = declarative_base()

class Lot(base_class):
    __tablename__ = 'lot'
    __table_args__ = {'schema': 'SCHEMA_NAME'}
    lot_key = Column(Integer, primary_key=True)
    lot_id = Column(String)

session = session_maker()
cursor = session.query(Lot)
result = cursor.first()
if result is None:
    raise Exception

Answer 1

找到答案：问题是cx_Oracle 5.2。重新安装 cx_Oracle 5.1.3 解决了问题。

SQLAlchemy 偶尔会错误地 returns 一个空结果

SQLAlchemy occasionally erroneously returns an empty result

python

sqlalchemy

cx-oracle