create type find_index_result as (indexname text, indexdef text);

CREATE OR REPLACE FUNCTION find_index(_relname text, _fields text[])
  RETURNS setof find_index_result AS $_$
DECLARE
  _rec RECORD;
  _res find_index_result;
  _relid oid;
  _tmp text;
  _f text;
  _field_cnt int;
  _cnt int;
BEGIN

  _field_cnt := array_upper(_fields,  1 );

  for _rec in execute('select '''||_relname||'''::regclass::oid') loop
    _relid := _rec.oid;
  end loop;
  
  for _res in (
    select c.relname,
      pg_get_indexdef(i.indexrelid)
    from pg_index i
      join pg_class c on c.oid=i.indexrelid
    where i.indrelid=_relid
  ) loop

    _tmp := substring(_res.indexdef from '\\((.*)\\)$');
    _cnt :=  0 ;
    for _i in  1 .. 32  loop
      _f := trim(split_part(_tmp, ',', _i));
      exit when _f is null;
      if _f = any (_fields) then
        _cnt := _cnt +  1 ;
        if _cnt = _field_cnt then
          return next _res;
          exit;
        end if;
      end if;
    end loop; --index field

  end loop; --index
  
  return;
END;
$_$ LANGUAGE plpgsql VOLATILE;


----------------------------------------------проверка
create temp table ind (a int, b text, c date);
create index ind_a on ind(a);
create index ind_ab on ind(a,b);
create index ind_abc on ind(a,b,c);
create index ind_cb on ind(c,b);

select * from find_index('ind', '{a}');
 indexname |                     indexdef
-----------+---------------------------------------------------
 ind_abc   | CREATE INDEX ind_abc ON ind USING btree (a, b, c)
 ind_ab    | CREATE INDEX ind_ab ON ind USING btree (a, b)
 ind_a     | CREATE INDEX ind_a ON ind USING btree (a)

select * from find_index('ind', '{a,b}');
 indexname |                     indexdef
-----------+---------------------------------------------------
 ind_abc   | CREATE INDEX ind_abc ON ind USING btree (a, b, c)
 ind_ab    | CREATE INDEX ind_ab ON ind USING btree (a, b)

select * from find_index('ind', '{c,a,b}');
 indexname |                     indexdef
-----------+---------------------------------------------------
 ind_abc   | CREATE INDEX ind_abc ON ind USING btree (a, b, c)