为什么通过ORM加载SQLAlchemy对象比通过MySQLdb游标加载行慢5-8倍？

Question

我注意到，使用SQLAlchemy获取一些数据（以及进行对象关系映射）时速度很慢，而用简单的SQL查询速度却很快。首先，我创建了一个包含一百万条记录的数据库：

mysql> use foo
mysql> describe Foo;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id    | int(11) | NO   | PRI | NULL    |       |
| A     | int(11) | NO   |     | NULL    |       |
| B     | int(11) | NO   |     | NULL    |       |
| C     | int(11) | NO   |     | NULL    |       |
+-------+---------+------+-----+---------+-------+
mysql> SELECT COUNT(*) FROM Foo;
+----------+
| COUNT(*) |
+----------+
|  1000000 |
+----------+
mysql>

作为一个粗略的测试，查询所有的Foo大约需要2秒：

herbert@dev0 ~ $ date; echo 'use foo; select * from Foo;' | mysql -uroot -pxxx > /dev/null; date
zo apr 20 18:48:49 CEST 2014
zo apr 20 18:48:51 CEST 2014

如果我在Python中使用MySQLdb来做这个，整个过程大约需要3秒，包括创建Foo对象：

herbert@dev0 ~ $ python BareORM.py 
query execution time:  0:00:02.198986
total time:  0:00:03.403084

这是输出结果：

#!/usr/bin/python
# -*- coding: utf-8 -*-

import MySQLdb
import sys
import time
import datetime

class Foo:
    def __init__(self, a, b, c):
        self.a=a; self.b=b; self.c=c;

try:
    start = datetime.datetime.now()
    con = MySQLdb.connect('localhost', 'root', 'xxx', 'foo')
    cur = con.cursor();

    cur.execute("""SELECT * FROM Foo LIMIT 1000000""")
    print "query execution time: ", datetime.datetime.now()-start
    foos = [];
    for elem in cur:
        foos.append(Foo(elem[1], elem[2], elem[3]))
    con.commit()

except MySQLdb.Error, e:
    print "Error %d: %s" % (e.args[0], e.args[1])
    sys.exit(1)

finally:
    if con: con.close()
    print "total time: ",  datetime.datetime.now()-start

然而，使用SQLAlchemy来减少重复代码时，完成同样的工作却需要大约25秒：

herbert@dev0 ~ $ python AlchemyORM.py 
total time:  0:00:24.649279

使用的代码如下：

import sqlalchemy
import datetime
import MySQLdb

from sqlalchemy import Column, Integer, create_engine
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker, relationship, backref

Base = declarative_base()

class Foo(Base):
    __tablename__ = 'Foo'
    id = Column(Integer, primary_key=True)
    A  = Column(Integer(unsigned=False), nullable=False)
    B  = Column(Integer(unsigned=False), nullable=False)
    C  = Column(Integer(unsigned=False), nullable=False)

engine  = create_engine('mysql+mysqldb://root:xxx@localhost/foo')
Session = sessionmaker(bind=engine)
session = Session()
start = datetime.datetime.now()
foos  = session.query(Foo).limit(1000000).all()
print "total time: ", datetime.datetime.now()-start

为什么SQLAlchemy的速度比简单SQL解决方案慢大约10倍，假设SQLAlchemy应该做的事情差不多？我能否加快速度？

这是一个更复杂查询的简单示例，它通过急切加载连接了几个表。我考虑过只对单个表进行简单查询，然后使用字典来创建id到对象的映射，并整理一对多的关系。但在这样做之前，我想确认SQLAlchemy是否真的无法表现得更好，因为从软件设计的角度来看，自己写ORM并不是个好主意。在我看来，速度慢2倍是可以接受的（也许）。

如果你知道其他（更快的）Python-SQL ORM，或者类似BigTable的解决方案（已经是ORM），欢迎在评论中提到它们。

编辑：我也尝试用Peewee，这样的结果大约是15秒。

from peewee import *
import datetime;
database = MySQLDatabase("foo", host="localhost", port=3306, user="root", passwd="xxx")

class Foo(Model):
        id = IntegerField()
        A  = IntegerField()
        B  = IntegerField()
        C  = IntegerField()

        class Meta:
                db_table = 'Foo'
                database = database

start = datetime.datetime.now()
foos = Foo.select()
cnt=0;
for i in foos: cnt=cnt+1
print "total time: ", datetime.datetime.now() - start

编辑：作为对Matthias的回应，我尝试在Java中用Hibernate做同样的事情，结果大约是8到10秒，虽然不算快，但比25秒快多了。代码从一些类开始，最后是一些配置：

package herbert.hibernateorm;

import java.util.List;

import org.hibernate.Session; 
import org.hibernate.Transaction;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;

public class App {
   public static void main(String[] args) throws Exception {
      SessionFactory factory = new Configuration().configure().buildSessionFactory();
      Session session = factory.openSession();
      Transaction tx = session.beginTransaction();
      long start = System.currentTimeMillis();
      List foos = session.createQuery("FROM Foo").list(); 
      System.out.println(foos.size());
      System.out.printf("total time: %d\n", System.currentTimeMillis() - start);
      session.close();
   }
}

package herbert.hibernateorm;

public class Foo {
    private int id, a, b, c;
    public Foo() {}
    public Foo(int A, int B, int C) { this.a=A; this.b=B; this.c=C; }

    public int getId() { return id; }
    public void setId(int id) { this.id = id; }
    public int getA() { return a; }
    public void setA(int a) { this.a = a; }
    public int getB() { return b; }
    public void setB(int b) { this.b = b; }
    public int getC() { return c; }
    public void setC(int c) { this.c = c; }
}

配置文件（分别是hibernate.cfg.xml和hibernate.hbm.xml）

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-configuration PUBLIC "-//Hibernate/Hibernate Configuration DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
  <session-factory>
    <property name="hibernate.dialect">org.hibernate.dialect.MySQLDialect</property>
    <property name="hibernate.connection.driver_class">com.mysql.jdbc.Driver</property>
    <property name="hibernate.connection.url">jdbc:mysql://localhost:3306/foo?zeroDateTimeBehavior=convertToNull</property>
    <property name="hibernate.connection.username">root</property>
    <property name="hibernate.connection.password">xxx</property>
    <mapping resource="hibernate.hbm.xml"/>
  </session-factory>
</hibernate-configuration>

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
    <class name="herbert.hibernateorm.Foo" table="Foo" catalog="foo">
        <id name="id" type="int">
            <column name="id" />
            <generator class="assigned" />
        </id>
        <property name="a" type="int">
            <column name="A" not-null="true" />
        </property>
        <property name="b" type="int">
            <column name="B" not-null="true" />
        </property>
        <property name="c" type="int">
            <column name="C" not-null="true" />
        </property>
    </class>
</hibernate-mapping>

最后是运行所有内容的pom文件：

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>herbert</groupId>
    <artifactId>hibernateORM</artifactId>
    <version>1.0-SNAPSHOT</version>
    <packaging>jar</packaging>

    <name>hibernateORM</name>
    <url>http://maven.apache.org</url>
    <repositories>
        <repository>
            <id>unknown-jars-temp-repo</id>
            <name>A temporary repository created by NetBeans for libraries and jars it could not identify. Please replace the dependencies in this repository with correct ones and delete this repository.</name>
            <url>file:${project.basedir}/lib</url>
        </repository>
    </repositories>
    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    </properties>

    <dependencies>
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>3.8.1</version>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>5.1.21</version>
        </dependency>
        <dependency>
            <groupId>org.hibernate</groupId>
            <artifactId>hibernate-core</artifactId>
            <version>4.0.1.Final</version>
        </dependency>
        <dependency>
            <groupId>org.hibernate</groupId>
            <artifactId>hibernate-entitymanager</artifactId>
            <version>4.0.1.Final</version>
        </dependency>
        <dependency>
            <groupId>org.hibernate.common</groupId>
            <artifactId>hibernate-commons-annotations</artifactId>
            <version>4.0.1.Final</version>
        </dependency>   
        <dependency>
            <groupId>nz.ac.waikato.cms.weka</groupId>
            <artifactId>weka-dev</artifactId>
            <version>3.7.10</version>
        </dependency>
        <dependency>
            <groupId>commons-configuration</groupId>
            <artifactId>commons-configuration</artifactId>
            <version>1.9</version>
        </dependency>
        <dependency>
            <groupId>commons-net</groupId>
            <artifactId>commons-net</artifactId>
            <version>3.1</version>
            <classifier>examples</classifier>
        </dependency>
        <dependency>
            <groupId>com.google.code.gson</groupId>
            <artifactId>gson</artifactId>
            <version>2.2.2</version>
        </dependency>
        <dependency>
            <groupId>maven</groupId>
            <artifactId>maven-jetty-plugin</artifactId>
            <version>1.1</version>
            <type>plugin</type>
        </dependency>
        <dependency>
            <groupId>commons-io</groupId>
            <artifactId>commons-io</artifactId>
            <version>2.4</version>
        </dependency>
        <dependency>
                <groupId>com.kenai.nbpwr</groupId>
                <artifactId>org-slf4j-jdk14</artifactId>
                <version>1.6.1-201106101300</version>
                <type>nbm</type>
        </dependency>

    </dependencies>
</project>

mysql orm sqlalchemy 数据库性能对象关系映射查询优化 hibernate 急切加载

为什么通过ORM加载SQLAlchemy对象比通过MySQLdb游标加载行慢5-8倍？

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