如果存在使用 django migrations.RunSQL 和 MySQL，我如何删除索引答案

【问题标题】：How I can drop an index if exist using django migrations.RunSQL and MySQL如果存在使用 django migrations.RunSQL 和 MySQL，我如何删除索引
【发布时间】：2017-09-25 07:26:04
【问题描述】：

我正在尝试使用 migrations.RunSQL 删除索引，但我遇到了不存在的问题，有没有办法仅在存在的情况下删除索引？类似于 migrations.RunSQL("DROP INDEX IF EXISTS index_id ON table")。

Django 1.8.18
MySQL 5.6

非常感谢

【问题讨论】：

How can I employ "if exists" for creating or dropping an index in MySQL?的可能重复

标签： mysql django django-migrations

【解决方案1】：

由于 MySQL 不支持 IF EXISTS 索引，您可能需要编写自己的迁移：

def drop_index_if_exists(apps, schema_editor):
    # access to the connection since schema_editor.execute does not return the cursor
    with schema_editor.connection.cursor() as cursor:
       cursor.execute("SHOW INDEX FROM table_name WHERE KEY_NAME = 'index_name'");
       exists = int(cursor.fetchone()) > 0
    # outside with to close the cursor
    if exists:
        schema_editor.execute("CREATE INDEX index_name ON ...")

operations = [
    migrations.RunPython(drop_index_if_exists)
]

为了一致性，您可以编写一个create_index_if_not_exists 方法来取消应用迁移，并调用它：

migrations.RunPython(drop_index_if_exists, create_index_if_not_exists)

【讨论】：

非常感谢你，我已经用你的想法解决了：D
exists = True if cursor.fetchone() else False 将永远是True。您必须检查值
我已经测试过，如果查询不返回行，则 cursor.fetchone() 将返回 None：In [1]: from django.db import connectionIn [2]: with connection.cursor() as cursor:cursor.execute("SHOW INDEX FROM table1 WHERE KEY_NAME = 'index_name'")cursor.execute("SHOW INDEX FROM table1 WHERE KEY_NAME = 'index_name'")print cursor.fetchone()(u'table1', 1L, u'index_name', 1L, u'index_name')In [3]: with connection.cursor() as cursor:@ 987654333@print cursor.fetchone()None

【解决方案2】：

这里有一个解决方案，谢谢@alfonso.kim 的想法

# -*- coding: utf-8 -*-
from __future__ import unicode_literals

from django.db import migrations


def drop_index_if_exists_and_create(apps, schema_editor):
    # access to the connection since schema_editor.execute does not return the cursor
    with schema_editor.connection.cursor() as cursor:
        cursor.execute("SHOW INDEX FROM table_name WHERE KEY_NAME = 'index_name'")
        exists = True if cursor.fetchone() else False
    # outside with to close the cursor
    if exists:
        schema_editor.execute("DROP INDEX index_name ON table_name")

    schema_editor.execute("CREATE INDEX index_name ON table_name(index_name(191))")


class Migration(migrations.Migration):

    dependencies = [
        ('table_name', ''),
    ]

    operations = [
        migrations.RunPython(drop_index_if_exists_and_create)
    ]

【讨论】：

【解决方案3】：

执行您的代码后，我遇到了事务错误：

django.db.transaction.TransactionManagementError: Executing DDL statements while in a transaction on databases that can't perform a rollback is prohibited.

Django 2.5 MySql 5.7

【讨论】：