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Diffstat (limited to 'docs/ref')
| -rw-r--r-- | docs/ref/migration-operations.txt | 15 |
1 files changed, 15 insertions, 0 deletions
diff --git a/docs/ref/migration-operations.txt b/docs/ref/migration-operations.txt index cbb76f79f7..f082452a31 100644 --- a/docs/ref/migration-operations.txt +++ b/docs/ref/migration-operations.txt @@ -150,6 +150,21 @@ a default value to put into existing rows. It does not affect the behavior of setting defaults in the database directly - Django never sets database defaults and always applies them in the Django ORM code. +.. warning:: + + On older databases, adding a field with a default value may cause a full + rewrite of the table. This happens even for nullable fields and may have a + negative performance impact. To avoid that, the following steps should be + taken. + + * Add the nullable field without the default value and run the + :djadmin:`makemigrations` command. This should generate a migration with + an ``AddField`` operation. + + * Add the default value to your field and run the :djadmin:`makemigrations` + command. This should generate a migration with an ``AlterField`` + operation. + ``RemoveField`` --------------- |