SQLite's ALTER TABLE functionality leaves a lot to be desired, so
in practice the way you alter SQLite schema is to create a new
table, migrate the data into it, then drop the old table. However,
this is potentially complicated by the presence of foreign key
constraints.
In order to resolve the foreign key constraint issue we add a
`maintenance()` method to the `DataSource` abstraction layer. This
operates similarly to the existing `transaction()` method, but the
SQLite implementation of this method defers enforcement of foreign
keys until the transaction is about to commit.
Tau
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