postgresql – 功能获得正确的一周数

我想创建一个函数来获得正确的一周数.
我已经发布了 here以找到“原生”解决方案,但显然没有.

我试图根据这个mysql example创建功能

这是转换为postgresql的代码：

CREATE OR REPLACE FUNCTION week_num_year(_date date)    
RETURNS integer AS
$BODY$declare 
_year integer;
begin


select date_part('year',_date) into _year; 
return ceil((to_char(_date,'DDD')::integer+(to_char(('01-01-'||_year)::date,'D')::integer%7-7))/7);           


end;$BODY$
  LANGUAGE plpgsql VOLATILE
  COST 100;

但它给出了错误的结果,有人可以帮助我吗？

我的配置：Postgresql 9.2

create or replace function week_num_year(_date date)
returns integer as
$body$
declare
_year date;
_week_number integer;
begin
select date_trunc('year',_date)::date into _year
;
with first_friday as (
    select extract(doy from a::date) ff
    from generate_series(_year,_year + 6,'1 day') s(a)
    where extract(dow from a) = 5
)
select floor(
        (extract(doy from _date) - (select ff from first_friday) - 1) / 7
    ) + 2 into _week_number
;
return _week_number
;
end;
$body$
language plpgsql immutable