Postgres FK constraint and index are independent.
Recently I was looked for unused indexes and found trick to eliminate indexes needed for constraint enforcing by:
WHERE NOT EXISTS (SELECT 1 FROM pg_constraint c WHERE c.conindid = pg_stat_user_indexes.indexrelid)
We created FK constraints independently from indexes.
Is it possible to somehow associate index to FK constraint so
pg_constraint.conindid will point from FK to index?
ALTER TABLE my ALTER CONSTRAINT my_your_fk USING INDEX my_your_fk_i;
UPDATE I see something related in Postgres docs:
ALTER TABLE distributors DROP CONSTRAINT distributors_pkey, ADD CONSTRAINT distributors_pkey PRIMARY KEY USING INDEX dist_id_temp_idx;