Constraints
VALIDATE CONSTRAINT (standalone)
ALTER TABLE orders VALIDATE CONSTRAINT orders_customer_id_fkey;What this does
The entire reason the NOT VALID pattern works. Postgres's own documentation explains it does not need to lock out concurrent updates during validation, since new rows are already enforced; only pre-existing rows need checking. SHARE UPDATE EXCLUSIVE does not conflict with ROW EXCLUSIVE, so ordinary INSERT/UPDATE/DELETE traffic keeps running through a scan that can take minutes on a large table. It does conflict with itself and with SHARE, SHARE ROW EXCLUSIVE, EXCLUSIVE, and ACCESS EXCLUSIVE, so a second VALIDATE CONSTRAINT, CREATE INDEX CONCURRENTLY, or VACUUM on the same table still has to wait.