我注意到使用SQLAlchemy获取（和ORM）某些数据速度较慢，但使用纯SQL语句则可以更快地获取。首先，我创建了一个拥有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>
作为一个简单的测试，查询所有Foo需要大约2秒钟：
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
如果我使用MySQLdb在Python中执行此操作，则需要大约3秒钟的时间，包括Foo对象的构建：
herbert@dev0 ~ $ python BareORM.py
query execution time: 0:00:02.198986
total time: 0:00:03.403084
以下哪个是输出结果：
#!/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来减少样板代码的话，执行相同的任务需要大约25秒钟：
herbert@dev0 ~ $ python AlchemyORM.py
total time: 0:00:24.649279
使用以下代码：
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
为什么假设SQLAlchemy应该做同样的事情，它的操作速度比裸的SQL慢大约10倍？我能用什么方法加快速度吗？
以下是更复杂的查询的最简工作示例，使用贪婪加载连接多个表。 我考虑只在单个表上进行简单的查询，然后使用字典创建id ->对象映射，并整理一对N关系。 但在这样做之前，我想确保SQLAlchemy无法表现得更好，因为从软件设计的角度来看，编写自己的ORM是一个糟糕的主意。 在我看来，2倍的减速是可以接受的（也许）。
如果您了解其他（更快的）Python-SQL ORM或类似BigTable的解决方案（已经是ORM），请随时在评论中提及。
编辑：也尝试了Peewee，结果是约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：作为对马蒂亚斯的回应，我尝试使用Hibernate在Java中做同样的事情，结果大约是8到10秒，虽然不算快，但比25秒要快得多。代码从一些类开始，以一些配置结束：
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; }
}
配置文件分别为 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Maven 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>