MySQL並鎖定表，讀取，然後截斷

Question

我在python中使用mysqldb。

我需要爲表格執行以下操作。

1) Lock 
2) Read 
3) Truncate the table 
4) Unlock 

當我運行下面的代碼時，我得到了下面的錯誤。所以，我不確定如何鎖定表格以便讀取它，然後截斷表格。我需要確保沒有其他連接讀取數據。

asin_list = [] 
    conn = MySQLdb.connect(host=parms['database']['operations']['host'],user=parms['database']['operations']['username'],passwd=parms['database']['operations']['password'],db=parms['database']['operations']['database']) 
    cursor = conn.cursor() 

    query = "LOCK TABLES asin_one_time_only READ" 
    cursor.execute(query) 
    print 'fu1' 

    query = """select asin FROM asin_one_time_only""" 
    cursor.execute(query) 
    rows = cursor.fetchall() 
    for row in rows: 
     asin_list.append(row[0]) 

    print asin_list 
    print 'fu2' 
    query = "UNLOCK TABLES;" 
    cursor.execute(query) 
    conn.commit() 


    print 'fu3' 
    query = "LOCK TABLES asin_one_time_only WRITE" 
    cursor.execute(query) 


    query = """truncate table amz_one_time_only""" 
    cursor.execute(query) 
    conn.commit() 

    print 'fu3' 
    query = "UNLOCK TABLES;" 
    cursor.execute(query) 
    conn.commit() 

    cursor.close() 
    conn.close() 

Traceback (most recent call last): 
    File "/home/ubuntu/workspace/Amazon-Products-Crawler-1/threaded_crawl.py", line 1086, in <module> 
    onetime = getOneTimeOnlyAsins(parms) 
    File "/home/ubuntu/workspace/Amazon-Products-Crawler-1/threaded_crawl.py", line 109, in getOneTimeOnlyAsins 
    cursor.execute(query) 
    File "/usr/lib/pymodules/python2.7/MySQLdb/cursors.py", line 166, in execute 
    self.errorhandler(self, exc, value) 
    File "/usr/lib/pymodules/python2.7/MySQLdb/connections.py", line 35, in defaulterrorhandler 
    raise errorclass, errorvalue 
_mysql_exceptions.OperationalError: (1192, "Can't execute the given command because you have active locked tables or an active transaction") 

Answer 1

您不能截斷爲寫入而鎖定的表。這是因爲「截斷」意思是「銷燬表格，並用相同的模式重新創建一個新表格。」

然而你可以，空表中。而不是TRUNCATE TABLE asin_one_time_only使用DELETE FROM asin_one_time_only。請注意，這不會重置自動增量編號。如果你想重置爲好，使用ALTER TABLE asin_one_time_only auto_increment=1

我建議這樣做：

LOCK TABLES asin_one_time_only READ; 
SELECT asin FROM asin_one_time_only; 
-- minimize the possibility of someone writing to the table in-between 
-- an "UNLOCK TABLES" and a "LOCK TABLES" by just issuing a new LOCK TABLES 
-- I am not 100% sure that MySQL will do this atomically, so there is a 
-- possibility that you may delete a row that was not read. 
-- If this is unacceptable, then use a "LOCK TABLES asin_one_time_only WRITE" 
-- from the very beginning. 
LOCK TABLES asin_one_time_only WRITE; 
DELETE FROM asin_one_time_only; 
ALTER TABLE asin_one_time_only auto_increment=1; 
UNLOCK TABLES; 

Answer 2

您不能截斷爲寫入而鎖定的表。

有一個在這裏的問題一整個辯論：http://bugs.mysql.com/bug.php?id=20667

你能做什麼，不過是刪除該表並重新創建它。