Question

我在MYSQL中使用MyISAM引擎。我发现表中的外键已经自动创建了索引。是Sqlalchemy还是MYSQL的行为？我不需要外键的索引。我该怎么办？定义列时添加参数index = False

我的代码：

class A(Base):
    __tablename__ = "a"
    a_id = Column(BigInteger, primary_key=True, autoincrement=True)

class B(Base):
    __tablename__ = "b"
    b_id = Column(BigInteger, primary_key=True, autoincrement=True)
    a_id = Column(BigInteger, ForeignKey(A.a_id))

我发现B中的a_id索引已自动创建。这是对的吗？如果我想删除这样的索引，我可以这样做吗？

class B(Base):
    __tablename__ = "b"
    b_id = Column(BigInteger, primary_key=True, autoincrement=True)
    a_id = Column(BigInteger, ForeignKey(A.a_id), index=False)

Answer 1

我在本地机器上创建了你的模型，但我没有在外键上找到任何索引。

我用

from sqlalchemy import create_engine

engine = create_engine('mysql://test:test@localhost/test1', echo=True)

from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column, Integer, String, Date, Text, BigInteger, ForeignKey
from sqlalchemy.orm.attributes import InstrumentedAttribute


Base = declarative_base()


class A(Base):
    __tablename__ = "a"
    a_id = Column(BigInteger, primary_key=True, autoincrement=True)

class B(Base):
    __tablename__ = "b"
    b_id = Column(BigInteger, primary_key=True, autoincrement=True)
    a_id = Column(BigInteger, ForeignKey(A.a_id))

Base.metadata.create_all(engine)

背景日志是。

2012-02-24 09:39:24,249 INFO sqlalchemy.engine.base.Engine SELECT DATABASE()
2012-02-24 09:39:24,249 INFO sqlalchemy.engine.base.Engine ()
2012-02-24 09:39:24,254 INFO sqlalchemy.engine.base.Engine SHOW VARIABLES LIKE 'character_set%%'
2012-02-24 09:39:24,254 INFO sqlalchemy.engine.base.Engine ()
2012-02-24 09:39:24,277 INFO sqlalchemy.engine.base.Engine SHOW VARIABLES LIKE 'lower_case_table_names'
2012-02-24 09:39:24,277 INFO sqlalchemy.engine.base.Engine ()
2012-02-24 09:39:24,288 INFO sqlalchemy.engine.base.Engine SHOW COLLATION
2012-02-24 09:39:24,289 INFO sqlalchemy.engine.base.Engine ()
2012-02-24 09:39:24,292 INFO sqlalchemy.engine.base.Engine SHOW VARIABLES LIKE 'sql_mode'
2012-02-24 09:39:24,292 INFO sqlalchemy.engine.base.Engine ()
2012-02-24 09:39:24,293 INFO sqlalchemy.engine.base.Engine DESCRIBE `a`
2012-02-24 09:39:24,293 INFO sqlalchemy.engine.base.Engine ()
2012-02-24 09:39:24,321 INFO sqlalchemy.engine.base.Engine ROLLBACK
2012-02-24 09:39:24,322 INFO sqlalchemy.engine.base.Engine DESCRIBE `b`
2012-02-24 09:39:24,322 INFO sqlalchemy.engine.base.Engine ()
2012-02-24 09:39:24,323 INFO sqlalchemy.engine.base.Engine ROLLBACK
2012-02-24 09:39:24,324 INFO sqlalchemy.engine.base.Engine 
CREATE TABLE a (
    a_id BIGINT NOT NULL AUTO_INCREMENT, 
    PRIMARY KEY (a_id)
)


2012-02-24 09:39:24,324 INFO sqlalchemy.engine.base.Engine ()
2012-02-24 09:39:24,590 INFO sqlalchemy.engine.base.Engine COMMIT
2012-02-24 09:39:24,591 INFO sqlalchemy.engine.base.Engine 
CREATE TABLE b (
    b_id BIGINT NOT NULL AUTO_INCREMENT, 
    a_id BIGINT, 
    PRIMARY KEY (b_id), 
    FOREIGN KEY(a_id) REFERENCES a (a_id)
)


2012-02-24 09:39:24,591 INFO sqlalchemy.engine.base.Engine ()
2012-02-24 09:39:24,762 INFO sqlalchemy.engine.base.Engine COMMIT

我还在mysql中检查a和b的create table语句。

mysql> show create table a;
+-------+-------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                  |
+-------+-------------------------------------------------------------------------------------------------------------------------------+
| a     | CREATE TABLE `a` (
  `a_id` bigint(20) NOT NULL AUTO_INCREMENT,
  PRIMARY KEY (`a_id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 |
+-------+-------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> show create table b;
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                                                                                                                                 |
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| b     | CREATE TABLE `b` (
  `b_id` bigint(20) NOT NULL AUTO_INCREMENT,
  `a_id` bigint(20) DEFAULT NULL,
  PRIMARY KEY (`b_id`),
  KEY `a_id` (`a_id`),
  CONSTRAINT `b_ibfk_1` FOREIGN KEY (`a_id`) REFERENCES `a` (`a_id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 |
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

索引仅适用于不在外键中的主键。

请在您的计算机上尝试此操作，如果您发现您的计算机正在表b中创建外键索引，则发布您创建的日志和模型，就像我在此答案中所写的那样。

希望这能解决您的问题。

Answer 2

拉法达，你错了。表b中的KEY a_id（a_id）清楚地显示了对外键字段的索引。这是InnoDB的行为 - 如果此类索引尚不存在，它将自动在外键关系中的字段上创建索引：

http://dev.mysql.com/doc/refman/5.5/en/innodb-foreign-key-constraints.html

Answer 3

这是MySQL的behavior，它对MySQL来说是正确的。你应该同意这个事实。至少我不知道如何摆脱外键的索引。

定义列时添加param index = False？

嗯，也许......我会试试。

Sqlalchemy会自动为外键创建索引吗？

3 个答案: