Error "is not a table or materialized view" when creating a unique index on amaterialized view on PostgreSQL 9.5.10 - Mailing list pgsql-admin

Hello

I have PostgreSQL 9.5.10 and when I receive the error: <table> is not a table or materialized view when I try to create a unique index on a materialized view.

CREATE MATERIALIZED VIEW public.focg_uni AS
SELECT f.nrfo,
   to_char(min(fd.validfrom), 'YYYY'::text) AS anul_internarii
  FROM focg f
    JOIN focgdepartment fd ON fd.idfocg = f.id
 GROUP BY f.nrfo
WITH DATA;

REFRESH MATERIALIZED VIEW public.focg_uni

CREATE UNIQUE INDEX focg_uni_check_index ON focg_uni_check(nrfo, an);

Thank you