[Proposal]: unsafe change_column_null default value

[twonegatives]

Rails change_column_null method accepts four arguments, with the fourth one replacing existing NULLs with some other value. Looks kinda useful feature although the implementation of such a replacement might be pretty dangerous on large tables (postgres, mysql):

UPDATE #{quote_table_name(table_name)}
SET #{quote_column_name(column_name)}=#{quote_default_expression(default, column)}
WHERE #{quote_column_name(column_name)} IS NULL

What we're doing here is a WHERE query on a table which might be lacking of needed index on column_name and UPDATE for (potentially) millions of records at once.
A little more safe way might be to require user to make this UPDATE in batches manually before firing change_column_null up.
WDYT?

[ankane]

Hey @twonegatives, makes sense to me. Do you want to try adding it?

[twonegatives]

hey @ankane sure, I'll give it a try.

[ankane]

Merged!!