Instead of updating a fixed number of rows (based on the amount of rows
available at the start of the update) the method
"update_column_in_batches" will now continue updating rows until it runs
out of rows to process.
For a table with a high rate of inserts this may result in the migration
taking quite some time. However, the alternative is not all rows being
updated or the "change_column_null" method raising an error due to there
being NULL values.
These helpers can be used to perform migrations without taking down the
entire application.
For example, the method "add_column_with_default" can be used to add a
new column with a default value without locking the entire table.
