Python MySQLdb的响应时间在silmiliar集上非常不同

Question

我正在使用Python+MySQLdb设计一个crontab作业，用于从MySQL中提取数据，生成XML文件并将其压缩。是的，这是一个归档任务发生在每天中午。

我的代码：

#!/usr/bin/env python
#encoding: utf-8
from dmconfig import DmConf
#from dmdb import Dmdb
import redis
import MySQLdb
import dawnutils


import time
from datetime import datetime, timedelta, date

conf = DmConf().loadConf()

db = MySQLdb.connect(host=conf["DbHost"],user=conf['DbAccount'],passwd=conf['DbPassword'],\
        db=conf['DbName'],charset=conf['DbCharset'])
cache = redis.Redis(host=conf['RedisHost'], port=conf['RedisPort'], 
        db=conf['Redisdbid'], password=conf['RedisPassword'])

#cursor = db.cursor()

def try_reconnect(conn):
    try:
        conn.ping()
    except:
        conn = MySQLdb.connect(host=conf["DbHost"],user=conf['DbAccount'],passwd=conf['DbPassword'],\
            db=conf['DbName'],charset=conf['DbCharset'])


def zip_task(device, start, stop):
    #cursor = db.cursor()
    format = "%Y%m%d%H%M%S"
    begin = time.strftime("%Y-%m-%d %H:%M:%S",time.strptime(start,format))
    end = time.strftime("%Y-%m-%d %H:%M:%S",time.strptime(stop,format))
    print "%s (%s,%s)"%(device, begin, end)
    sql = "SELECT * from `period` WHERE `snrCode` = \"%s\" AND `time` > \"%s\" AND `time` < \"%s\" ORDER BY `recId` DESC"%(device, begin, end)
    print sql
    cursor = db.cursor()

    try_reconnect(db)
    t1 = time.time()
    try:
        cursor.execute(sql)
        results = cursor.fetchall()
    except MySQLdb.Error,e:
        print "Error %s"%(e)

    print ("SQL takes %f seconds"%(time.time()-t1))

    print ("len of reconds, %d"%len(results))

    #for row in results:
        #print row


def dispatcher(devSet, start, stop):
    print "size of set: %d"%len(devSet)
    print devSet
    for dev in devSet:
        zip_task(dev, start, stop)

def archive_task_queue():
    today = datetime.now()
    oneday = timedelta(days=1)
    yesterday = today - oneday
    format = "%Y%m%d%H%M%S"
    begin = time.strftime(format, yesterday.timetuple())[:8] + '120000'
    end = time.strftime(format, today.timetuple())[:8] + '120000'

    sql = "SELECT * from `logbook` WHERE `login` > \"%s\" AND `login` < \"%s\" AND `logout` > \"%s\" AND `logout` < \"%s\""%(begin, end, begin, end)
    print sql

    cursor = db.cursor()
    reclist = []
    try:
        cursor.execute(sql)
        results = cursor.fetchall()

        for row in results:
            #print row
            reclist.append(row[1])
    except MySQLdb.Error,e:
        print "Error %s"%(e)

    #reclist = [u'A2H300001']

    if len(reclist):
        dispatcher(set(reclist), begin, end)

    db.close()

if __name__ == '__main__':
    archive_task_queue()

在我的代码中，我将查询设备活动的日志，并获取当天的活动设备集。并逐个查询每个设备的数据集。问题伴随着第二阶段的查询而来。运行以下命令后查看我的控制台：

SELECT * from `logbook` WHERE `login` > "20160720120000" AND `login` < "20160721                                                                     120000" AND `logout` > "20160720120000" AND `logout` < "20160721120000"
size of set: 4
set([u'B1H700001', u'B1H700002', u'A1E500018', u'A2H300001'])
B1H700001 (2016-07-20 12:00:00,2016-07-21 12:00:00)
SELECT * from `period` WHERE `snrCode` = "B1H700001" AND `time` > "2016-07-20 12                                                                     :00:00" AND `time` < "2016-07-21 12:00:00" ORDER BY `recId` DESC
SQL takes 0.018232 seconds
len of reconds, 597
B1H700002 (2016-07-20 12:00:00,2016-07-21 12:00:00)
SELECT * from `period` WHERE `snrCode` = "B1H700002" AND `time` > "2016-07-20 12                                                                     :00:00" AND `time` < "2016-07-21 12:00:00" ORDER BY `recId` DESC
SQL takes 0.974020 seconds
len of reconds, 4642
A1E500018 (2016-07-20 12:00:00,2016-07-21 12:00:00)
SELECT * from `period` WHERE `snrCode` = "A1E500018" AND `time` > "2016-07-20 12                                                                     :00:00" AND `time` < "2016-07-21 12:00:00" ORDER BY `recId` DESC
SQL takes 0.342373 seconds
len of reconds, 0
A2H300001 (2016-07-20 12:00:00,2016-07-21 12:00:00)
SELECT * from `period` WHERE `snrCode` = "A2H300001" AND `time` > "2016-07-20 12                                                                     :00:00" AND `time` < "2016-07-21 12:00:00" ORDER BY `recId` DESC

SQL takes 68.173677 seconds
len of reconds, 5794

查询时间异常。B1H700002 4642数据点需要0.9s，而A2H300001 5764数据点需要68秒。

然后，我将我的问题缩小到只查询特定的设备ID，这可以在我之前的代码中找到。结果是一样的。该查询需要65秒。

有什么线索吗？