PostgreSQL add constraints via NOT VALID / VALIDATE CONSTRAINT

[Adam Johnson]

If you read the ALTER TABLE PostgreSQL docs ( ​https://www.postgresql.org/docs/12/sql-altertable.html ) for ADD CONSTRAINT, you'll see that it supports the NOT VALID option. This prevents the constraint from being checked against all existing data, although it does affect all inserted/updated rows. Adding a constraint this way does not exclusively lock the whole table to check it. To promote it to valid, the VALIDATE CONSTRAINT syntax can be used. This allows a non-exclusive lock on the table to validate all existing rows. This two step techinque allows constraints to be added to very active tables without locking out concurrent access and thus inflicting downtime.

I suggest Django always use this form when adding foreign keys and check constraints.