試圖瞭解cx_Oracle的LOB對象

Question

我有一個用於查詢數據庫的python數據庫類。

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 

而這是操縱提取的數據並將其轉換的代碼。

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) 

因此，沒有self.cursor.arraysize = 10000

，並像select clob_value from table查詢它能夠獲取數據，並記錄行數：4901，TIME_TAKEN：0.196296930313但給我這樣的錯誤

LOB variable no longer valid after subsequent fetch 

但是當我提到arraysize參數時，錯誤消失了。 (Is arraysize only for lob columns coz it works fine for select other_column from table where rownum<20000 <- other_column in varchar) 這是爲什麼發生？

Answer 1

原來的CLOB和fetchalldon'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()，並使用它像一個迭代器（例如for row in cursor: ...），我能解決這個問題。