选择语句上的SQLAlchemy内存占用

Question

根据SQLAlchemy，在for循环中，将select语句视为可迭代。结果是将返回大量行的select语句不会使用过多的内存。

我在MySQL表上发现以下语句：

for row in my_connections.execute(MyTable.__table__.select()):
    yield row

似乎不遵循此要求，因为我溢出了可用内存并在产生第一行之前开始崩溃。我究竟做错了什么？

Answer 1

基本MySQLdb游标立即从服务器获取整个查询结果。这会消耗大量内存和时间。当您要进行大型查询并一次从服务器提取结果时，请使用MySQLdb.cursors.SSCursor。

因此，connect_args={'cursorclass': MySQLdb.cursors.SSCursor} 在创建时尝试传递engine：

   from sqlalchemy import create_engine, MetaData
   import MySQLdb.cursors
   engine = create_engine('mysql://root:zenoss@localhost/e2', connect_args={'cursorclass': MySQLdb.cursors.SSCursor})
   meta = MetaData(engine, reflect=True)
   conn = engine.connect()
   rs = s.execution_options(stream_results=True).execute()

参见http://www.sqlalchemy.org/trac/ticket/1089

---

请注意，使用SSCursor会锁定表，直到提取完成。这会影响使用同一连接的其他游标：来自同一连接的两个游标不能同时从表中读取。

但是，来自不同连接的游标可以同时从同一表中读取。

这是一些演示问题的代码：

import MySQLdb
import MySQLdb.cursors as cursors
import threading
import logging
import config

logger = logging.getLogger(__name__)
query = 'SELECT * FROM huge_table LIMIT 200'

def oursql_conn():
    import oursql
    conn = oursql.connect(
        host=config.HOST, user=config.USER, passwd=config.PASS,
        db=config.MYDB)
    return conn

def mysqldb_conn():
    conn = MySQLdb.connect(
        host=config.HOST, user=config.USER,
        passwd=config.PASS, db=config.MYDB,
        cursorclass=cursors.SSCursor) 
    return conn

def two_cursors_one_conn():
    """Two SSCursors can not use one connection concurrently"""
    def worker(conn):
        cursor = conn.cursor()
        cursor.execute(query)
        for row in cursor:
            logger.info(row)

    conn = mysqldb_conn()
    threads = [threading.Thread(target=worker, args=(conn, ))
               for n in range(2)]
    for t in threads:
        t.daemon = True
        t.start()
        # Second thread may hang or raise OperationalError:
        # File "/usr/lib/pymodules/python2.7/MySQLdb/cursors.py", line 289, in _fetch_row
        #   return self._result.fetch_row(size, self._fetch_type)
        # OperationalError: (2013, 'Lost connection to MySQL server during query')

    for t in threads:
        t.join()

def two_cursors_two_conn():
    """Two SSCursors from independent connections can use the same table concurrently"""    
    def worker():
        conn = mysqldb_conn()        
        cursor = conn.cursor()
        cursor.execute(query)
        for row in cursor:
            logger.info(row)

    threads = [threading.Thread(target=worker) for n in range(2)]
    for t in threads:
        t.daemon = True
        t.start()
    for t in threads:
        t.join()


logging.basicConfig(level=logging.DEBUG,
                    format='[%(asctime)s %(threadName)s] %(message)s',
                    datefmt='%H:%M:%S')
two_cursors_one_conn()
two_cursors_two_conn()

请注意，oursql是Python的另一套MySQL绑定。oursql游标是真正的服务器端游标，默认情况下会延迟获取行。随着oursql安装，如果你改变

conn = mysqldb_conn()

至

conn = oursql_conn()

然后two_cursors_one_conn()运行而不会挂起或引发异常。