Question

我在python中有一个数据库类，用于查询数据库。

class Database():
  def __init__(self, user, password, host, port, service_name, mode, *args):
    #mode should be 0 if not cx_Oracle.SYSDBA
    self.user = user
    self.password = password
    self.host = host
    self.port = port
    self.user = user
    self.service_name = service_name
    self.logger = logging.getLogger(__name__)
    self.mode = 0
    self.connection = None
    self.connect_string = self.user + '/' + self.password + '@' + dsn
    try:
      self.connection = cx_Oracle.connect(self.connect_string, mode=self.mode, threaded=True)
      self.connection.stmtcachesize = 1000
      self.connection.client_identifier = 'my_app_scheduler'
      self.cursor = self.connection.cursor()
      self.cursor.arraysize = 10000
      self.idVar = self.cursor.var(cx_Oracle.NUMBER)
    except cx_Oracle.DatabaseError, exc:
      error, = exc
      self.logger.exception('Exception occured while trying to create database object : %s', error.message)
      raise exc

  def query(self, q):
    try:
      self.cursor.execute(q)
      return self.cursor.fetchall(), self.cursor.rowcount
    except cx_Oracle.DatabaseError, exc:
      raise exc

这是操作获取的数据并将其转换的代码。

output, rowcount = db_run_query.query(sql_text)
      #self.logger.debug('output : %s, type : %s', output, type(output))
      end_time=time.time()
      time_taken=end_time - start_time
      self.logger.debug('Rowcount : %s, time_taken : %s', rowcount, time_taken)
      column_name = [d[0] for d in db_run_query.cursor.description]
      result = [dict(zip(column_name, row)) for row in output]
      #Convert everything to string : Eg: datetime
      try:
        for each_dict in result:
          for key in each_dict:
            if isinstance(each_dict[key], cx_Oracle.LOB):
              self.logger.debug('%s', each_dict[key].size())
              each_dict[key]=each_dict[key].read()
              #self.logger.debug('%s %s %s %s %s %s %s', key, each_dict, type(key), type(each_dict[key]), type(each_dict), temp_each_dict, type(temp_each_dict))
            else:
              each_dict[key]=str(each_dict[key])
      except Exception as e:
        self.logger.debug(e)

所以没有self.cursor.arraysize = 10000

并且对于像select clob_value from table这样的查询，它能够获取数据并记录Rowcount：4901，time_taken：0.196296930313但是给我一个错误，如

LOB variable no longer valid after subsequent fetch

但是当我提到arraysize参数时，错误消失了。 ( Is arraysize only for lob columns coz it works fine for select other_column from table where rownum<20000 <- other_column in varchar) 为什么会这样？

Answer 1

结果是CLOB和fetchall don't place nice together：

Internally, Oracle uses LOB locators which are allocated based on the
cursor array size. Thus, it is important that the data in the LOB
object be manipulated before another internal fetch takes place. The
safest way to do this is to use the cursor as an iterator. In
particular, do not use the fetchall() method.

避免cursor.fetchall()并像迭代器一样使用它（例如for row in cursor: ...），我就能解决这个问题。

试图了解cx_Oracle的LOB对象

1 个答案: