From 06909fe084f87a65459a83bd69d7cdbe4fce9a7c Mon Sep 17 00:00:00 2001 From: Caio Ariede Date: Fri, 4 Oct 2019 10:55:41 -0300 Subject: [PATCH] Fixed #28273 -- Doc'd fast nullable column creation with defaults. --- docs/ref/migration-operations.txt | 15 +++++++++++++++ 1 file changed, 15 insertions(+) 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`` ---------------