Delete duplicates with using most recent "last created" timestamps for remaining records

1_boilerplate.sql

drop table if exists moscow_weather;

create table moscow_weather (
  id bigserial not null primary key,
  year int2 not null,
  month int2 not null,
  created timestamptz not null default clock_timestamp(),
  weather_is_fine boolean
);

insert into moscow_weather (year, month, weather_is_fine)
select gen_y, gen_m, true
from 
  (select generate_series as gen_y from generate_series(2000, 2017)) as y,
  (select generate_series as gen_m from generate_series(1, 12)) as m
where gen_y < 2017 or gen_m <= 6
;

insert into moscow_weather (year, month, weather_is_fine)
values (2017, 5, false);

insert into moscow_weather (year, month, weather_is_fine)
values (2017, 5, false);

insert into moscow_weather (year, month, weather_is_fine)
values (2017, 6, false);

select * from moscow_weather order by year desc, month desc;

2_remove_duplicates.sql

with candidates as (
  select    year,
            month,
            max(id) as keep_id,
            array_agg(id) as all_ids,
            array_remove(                -- Postgres 9.3+
              array_agg(id), max(id)
            ) as discard_ids,
            min(created) as first_created
  from      moscow_weather
  group by  1, 2
  having    count(*) > 1
), patch_rows as (
  update     moscow_weather
  set        created = c.first_created
  from       candidates as c
  where      moscow_weather.id = c.keep_id
  returning  *
), delete_rows as (
  delete from   moscow_weather
  where         id in (select unnest(discard_ids) from candidates)
  returning     *
)
select
  (select count(*) from patch_rows) as "patched / left",
  (select count(*) from delete_rows) as deleted
;

3_create_UK.sql

create unique index u_moscow_weather_year_month on moscow_weather using btree(year, month);