Delete, foreign key, index usage

Publication ( has a foreign key (ut) and more than 50million records) that references the top of the chain of references. This field (ut) is also the primary key of publication.

In the case of belongs_to (about 231393000 records)  which references the same table (article) ut has an index.

All other tables in this dependency chain reports 100% or near 100% usage of the indexes e.g.

citation_2010_2014

0

0

226

1882

2510

0

1910

0

816

1910

4420

The indexes are on a ssd and we have set the random_page_cost to 1 for those queries.

The definition of belongs_to:

CREATE TABLE wos_2017_1.belongs_to
(
  suborg_id uuid,
  organisation_id uuid,
  address_id uuid,
  ut citext,
  uuid uuid NOT NULL,
  id integer NOT NULL DEFAULT nextval('wos_2017_1.belongs2_id_seq'::regclass),
  pref_name_id uuid,
  addr_no smallint,
  reprint_addr_no smallint,
  CONSTRAINT belongs2_pkey PRIMARY KEY (uuid),
  CONSTRAINT belongs_to_address_id_fkey FOREIGN KEY (address_id)
      REFERENCES wos_2017_1.address (uuid) MATCH SIMPLE
      ON UPDATE NO ACTION ON DELETE SET NULL,
  CONSTRAINT belongs_to_pref_name_id_fkey FOREIGN KEY (pref_name_id)
      REFERENCES wos_2017_1.org_pref_name (uuid) MATCH SIMPLE
      ON UPDATE NO ACTION ON DELETE SET NULL,
  CONSTRAINT belongs_to_suborg_id_fkey FOREIGN KEY (suborg_id)
      REFERENCES wos_2017_1.suborg (uuid) MATCH SIMPLE
      ON UPDATE NO ACTION ON DELETE SET NULL,
  CONSTRAINT belongs_to_ut_fkey FOREIGN KEY (ut)
      REFERENCES wos_2017_1.article (ut) MATCH SIMPLE
      ON UPDATE NO ACTION ON DELETE CASCADE,
  CONSTRAINT belongs2_id_key UNIQUE (id),
  CONSTRAINT belongs2_ut_suborg_id_organisation_id_address_id_addr_no_re_key UNIQUE (ut, suborg_id, organisation_id, address_id, addr_no, reprint_addr_no, pref_name_id)
)
WITH (
  OIDS=FALSE
);
with indexes on address_id, organisation_id, pref_name_id, ut

I have also tried to set enable_seqscan to false for these queries, but still no usage of the indexes.

Why would that be?

Regards

Johann

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